View source for Philosophy:Intelligence quotient

{{Short description|Score from a test designed to assess intelligence}}
{{Infobox diagnostic
| name    = Intelligence quotient
| image   = Raven Matrix.svg
| alt     = [picture of an example IQ test item]
| caption = One kind of IQ test item, modelled after items in the Raven's Progressive Matrices test
| ICD10   = {{ICD10|Z|01|8}}
| ICD9    = {{ICD9proc|94.01}}
}}
{{Psychology sidebar|all}}
An '''intelligence quotient''' ('''IQ''') is a total score derived from a set of standardised tests or subtests designed to assess [[Philosophy:Human intelligence|human intelligence]].<ref>{{cite journal |last1=Braaten |first1=Ellen B. |last2=Norman |first2=Dennis |title=Intelligence (IQ) Testing |journal=Pediatrics in Review |date=1 November 2006 |volume=27 |issue=11 |pages=403–408 |doi=10.1542/pir.27-11-403 |pmid=17079505 |issn=0191-9601 |url=https://pedsinreview.aappublications.org/content/27/11/403 |access-date=22 January 2020}}</ref> The abbreviation "IQ" was coined by the [[Social:Psychologist|psychologist]] William Stern for the [[Social:German language|German]] term {{lang|de|Intelligenzquotient}}, his term for a scoring method for [[Philosophy:Intelligence|intelligence]] tests at University of Breslau he advocated in a 1912 book.{{sfn|Stern|1914|pp=70–84 (1914 English translation); pp. 48–58 (1912 original German edition)}}

Historically, IQ was a score obtained by dividing a person's [[Philosophy:Mental age|mental age]] score, obtained by administering an intelligence test, by the person's chronological age, both expressed in terms of years and months. The resulting fraction ([[Quotient|quotient]]) was multiplied by 100 to obtain the IQ score.<ref>{{Cite encyclopedia |title=Glossary of Important Assessment and Measurement Terms|publisher=National Council on Measurement in Education |date=2016 |location=Philadelphia, PA |url=http://www.ncme.org/ncme/NCME/Resource_Center/Glossary/NCME/Resource_Center/Glossary1.aspx?hkey=4bb87415-44dc-4088-9ed9-e8515326a061#anchorI |access-date=1 July 2017 |entry=intelligence quotient (IQ) |archive-url=https://web.archive.org/web/20170722194028/http://www.ncme.org/ncme/NCME/Resource_Center/Glossary/NCME/Resource_Center/Glossary1.aspx?hkey=4bb87415-44dc-4088-9ed9-e8515326a061#anchorI |archive-date=22 July 2017|url-status=dead}}</ref> For modern [[Philosophy:Intelligence quotient#Current tests|IQ tests]], the [[Social:Test score|raw score]] is [[Data transformation (statistics)|transformed]] to a [[Normal distribution|normal distribution]] with mean 100 and [[Standard deviation|standard deviation]] 15.<ref name="Gottfredson2009pp31–32" /> This results in approximately two-thirds of the population scoring between IQ 85 and IQ 115 and about 2 percent each above 130 and below 70.<ref name="Neisser97">{{cite journal |last=Neisser |first=Ulrich |title=Rising Scores on Intelligence Tests |journal=American Scientist |volume=85 |issue=5 |pages=440–447 |year=1997 |url=http://www.americanscientist.org/issues/feature/rising-scores-on-intelligence-tests/ |archive-url=https://web.archive.org/web/20161104214157/http://www.americanscientist.org/issues/feature/rising-scores-on-intelligence-tests/99999 |archive-date=4 November 2016 |access-date=1 December 2017 |bibcode=1997AmSci..85..440N}}</ref><ref name="Hunt2011p5" />

Scores from intelligence tests are estimates of intelligence. Unlike, for example, distance and mass, a concrete measure of intelligence cannot be achieved given the abstract nature of the concept of "intelligence".<ref>{{cite book|last1=Haier|first1=Richard|title=The Neuroscience of Intelligence |publisher=Cambridge University Press|isbn=9781107461437|pages=18–19|date=28 December 2016}}</ref> IQ scores have been shown to be associated with such factors as [[Medicine:Human nutrition|nutrition]],<ref>{{Cite journal |last1=Cusick |first1=Sarah E. |last2=Georgieff |first2=Michael K. |date=1 August 2017 |title=The Role of Nutrition in Brain Development: The Golden Opportunity of the 'First 1000 Days' |journal=The Journal of Pediatrics |volume=175 |pages=16–21 |doi=10.1016/j.jpeds.2016.05.013 |pmc=4981537 |pmid=27266965}}</ref><ref>{{Cite journal|last1=Saloojee|first1=Haroon|last2=Pettifor|first2=John M|date=15 December 2001|title=Iron deficiency and impaired child development|journal=British Medical Journal|volume=323|issue=7326|pages=1377–1378|doi=10.1136/bmj.323.7326.1377|issn=0959-8138|pmc=1121846|pmid=11744547}}</ref><ref>{{Cite journal|last1=Qian|first1=Ming|last2=Wang|first2=Dong|last3=Watkins|first3=William E.|last4=Gebski|first4=Val|last5=Yan|first5=Yu Qin|last6=Li|first6=Mu|last7=Chen|first7=Zu Pei|date=2005|title=The effects of iodine on intelligence in children: a meta-analysis of studies conducted in China|url=https://pubmed.ncbi.nlm.nih.gov/15734706/|journal=Asia Pacific Journal of Clinical Nutrition|volume=14|issue=1|pages=32–42|issn=0964-7058|pmid=15734706}}</ref> parental [[Social:Socioeconomic status|socioeconomic status]],<ref>{{Cite journal|last1=Poh|first1=Bee Koon|last2=Lee|first2=Shoo Thien|last3=Yeo|first3=Giin Shang|last4=Tang|first4=Kean Choon|last5=Noor Afifah|first5=Ab Rahim|last6=Siti Hanisa|first6=Awal|last7=Parikh|first7=Panam|last8=Wong|first8=Jyh Eiin|last9=Ng|first9=Alvin Lai Oon|last10=SEANUTS Study Group|date=13 June 2019|title=Low socioeconomic status and severe obesity are linked to poor cognitive performance in Malaysian children|journal=BMC Public Health|volume=19|issue=Suppl 4|pages=541|doi=10.1186/s12889-019-6856-4|issn=1471-2458|pmc=6565598|pmid=31196019 |doi-access=free }}</ref><ref>{{Cite journal|last1=Galván|first1=Marcos|last2=Uauy|first2=Ricardo|last3=Corvalán|first3=Camila|last4=López-Rodríguez|first4=Guadalupe|last5=Kain|first5=Juliana|date=September 2013|title=Determinants of cognitive development of low SES children in Chile: a post-transitional country with rising childhood obesity rates|url=https://pubmed.ncbi.nlm.nih.gov/22915146/|journal=Maternal and Child Health Journal|volume=17|issue=7|pages=1243–1251|doi=10.1007/s10995-012-1121-9|issn=1573-6628|pmid=22915146|s2cid=19767926}}</ref> [[Disease#Morbidity|morbidity]] and [[Mortality rate|mortality]],<ref>{{cite journal |author1=Markus Jokela |author2=G. David Batty |author3=Ian J. Deary |author4=Catharine R. Gale |author5=Mika Kivimäki |year=2009 |title=Low Childhood IQ and Early Adult Mortality: The Role of Explanatory Factors in the 1958 British Birth Cohort |journal=Pediatrics |volume=124 |issue=3 |pages=e380 – e388 |doi=10.1542/peds.2009-0334 |pmid=19706576 |s2cid=25256969}}</ref>{{sfn|Deary|Batty|2007}} parental [[Social:Social status|social status]],{{sfn|Neisser et al.|1995}} and [[Medicine:Prenatal and perinatal psychology|perinatal environment]].<ref>{{Cite journal|last1=Ronfani|first1=Luca|last2=Vecchi Brumatti|first2=Liza|last3=Mariuz|first3=Marika|last4=Tognin|first4=Veronica|date=2015|title=The Complex Interaction between Home Environment, Socioeconomic Status, Maternal IQ and Early Child Neurocognitive Development: A Multivariate Analysis of Data Collected in a Newborn Cohort Study|journal=PLOS ONE|volume=10|issue=5|pages=e0127052|doi=10.1371/journal.pone.0127052|pmid=25996934|pmc=4440732|bibcode=2015PLoSO..1027052R|doi-access=free}}</ref> While the [[Biology:Heritability of IQ|heritability of IQ]] has been investigated for nearly a century, there is still debate about the significance of heritability estimates<ref>{{cite journal |last1=Johnson |first1=Wendy |last2=Turkheimer |first2=Eric |last3=Gottesman |first3=Irving I. |last4=Bouchard |first4=Thomas J. |title=Beyond Heritability |journal=Current Directions in Psychological Science |date=August 2009 |volume=18 |issue=4 |pages=217–220 |doi=10.1111/j.1467-8721.2009.01639.x |pmid=20625474 |pmc=2899491}}</ref>{{sfn|Turkheimer|2008}} and the mechanisms of inheritance.<ref name=Devlin97>{{cite journal |pages=468–71 |issue=6641 |volume=388 |year=1997 |pmid=9242404 |journal=Nature |doi=10.1038/41319 |last1=Devlin |first1=B. |last2=Daniels |first2=Michael |last3=Roeder |first3=Kathryn |title=The heritability of IQ |bibcode=1997Natur.388..468D |s2cid=4313884|doi-access=free }}</ref>

IQ scores are used for educational placement, assessment of [[Medicine:Intellectual disability|intellectual disability]], and evaluating job applicants. In research contexts, they have been studied as predictors of [[Engineering:Performance rating (work measurement)|job performance]]<ref name="Schmidt98"/> and [[Finance:Income|income]].<ref name="Strenze2007"/> They are also used to study distributions of psychometric intelligence in populations and the [[Correlation and dependence|correlations]] between it and other variables. Raw scores on IQ tests for many populations have been rising at an average rate that scales to three IQ points per decade since the early 20th century, a phenomenon called the [[Philosophy:Flynn effect|Flynn effect]]. Investigation of different patterns of increases in subtest scores can also inform current research on human intelligence.

==History==

===Precursors to IQ testing===
Historically, even before IQ tests were devised, there were attempts to classify people into [[Philosophy:Human intelligence|intelligence]] categories by observing their behavior in daily life.<ref name="TermanOldClasses" /><ref name="WechslerOldClasses" /> Those other forms of behavioral observation are still important for validating classifications based primarily on IQ test scores. Both intelligence classification by observation of behavior outside the testing room and classification by IQ testing depend on the definition of "intelligence" used in a particular case and on the reliability and error of estimation in the classification procedure.

The English statistician [[Biography:Francis Galton|Francis Galton]] (1822–1911) made the first attempt at creating a standardized test for rating a person's intelligence. A pioneer of [[Psychometrics|psychometrics]] and the application of statistical methods to the study of human diversity and the study of inheritance of human traits, he believed that intelligence was largely a product of heredity (by which he did not mean [[Biology:History of genetics#Post-Mendel, pre-re-discovery|genes]], although he did develop several [[Biology:Mendelian inheritance|pre-Mendelian]] theories of particulate inheritance).<ref>{{cite journal |last1=Bulmer |first1=M |year=1999 |title=The development of Francis Galton's ideas on the mechanism of heredity |journal=Journal of the History of Biology |volume=32 |issue=3 |pages=263–292 | doi = 10.1023/a:1004608217247 |pmid=11624207 |s2cid=10451997}}</ref><ref>{{cite journal |last1=Cowan |first1=R. S. |year=1972 |title=Francis Galton's contribution to genetics |journal=Journal of the History of Biology |volume=5 |issue=2 |pages=389–412 |doi=10.1007/bf00346665|pmid=11610126 |s2cid=30206332}}</ref><ref>{{cite journal |last1=Burbridge |first1=D |year=2001 |title=Francis Galton on twins, heredity and social class |journal=British Journal for the History of Science |volume=34 |issue=3 |pages=323–340 |doi=10.1017/s0007087401004332|pmid=11700679}}</ref> He hypothesized that there should exist a correlation between intelligence and other observable traits such as [[Biology:Reflex|reflex]]es, muscle grip, and [[Medicine:Craniometry#Bertillon, Galton and criminology|head size]].<ref>{{cite journal |last1=Fancher |first1=R. E. |year=1983 |title=Biographical origins of Francis Galton's psychology |journal=Isis |volume=74 |issue=2 |pages=227–233 |doi=10.1086/353245|pmid=6347965|s2cid=40565053 }}</ref> He set up the first mental testing center in the world in 1882 and he published "Inquiries into Human Faculty and Its Development" in 1883, in which he set out his theories. After gathering data on a variety of physical variables, he was unable to show any such correlation, and he eventually abandoned this research.<ref name="Kaufman2009p21" /><ref>{{cite journal |last1=Gillham |first1=Nicholas W. |title=Sir Francis Galton and the birth of eugenics |journal=Annual Review of Genetics |volume=35 |issue=1 |pages=83–101 |year=2001 |pmid=11700278 |doi=10.1146/annurev.genet.35.102401.090055}}</ref>
[[File:Alfred Binet.jpg|thumb|upright|Psychologist Alfred Binet, co-developer of the Stanford–Binet test]]
French psychologist Alfred Binet, together with Victor Henri and Théodore Simon, had more success in 1905, when they published the [[Social:Stanford–Binet Intelligence Scales|Binet–Simon test]], which focused on verbal abilities. It was intended to identify "mental retardation" in school children,<ref name=Kaufman2009/> but in specific contradistinction to claims made by psychiatrists that these children were "sick" (not "slow") and should therefore be removed from school and cared for in asylums.<ref>{{cite journal |last1=Nicolas |first1=S. |last2=Andrieu |first2=B. |last3=Croizet |first3=J.-C. |last4=Sanitioso |first4=R. B. |last5=Burman |first5=J. T. |year=2013 |title=Sick? Or slow? On the origins of intelligence as a psychological object |journal=Intelligence |volume=41 |issue=5 |pages=699–711 |doi=10.1016/j.intell.2013.08.006 |doi-access=free}} (This is an [[Open access|open access]] article, made freely available by [[Company:Elsevier|Elsevier]].)</ref> The score on the Binet–Simon scale would reveal the child's [[Philosophy:Mental age|mental age]]. For example, a six-year-old child who passed all the tasks usually passed by six-year-olds—but nothing beyond—would have a mental age that matched his chronological age, 6.0. (Fancher, 1985). Binet thought that intelligence was multifaceted, but came under the control of practical judgment.

In Binet's view, there were limitations with the scale and he stressed what he saw as the remarkable diversity of intelligence and the subsequent need to study it using qualitative, as opposed to quantitative, measures (White, 2000). American psychologist Henry H. Goddard published a translation of it in 1910. American psychologist [[Biography:Lewis Terman|Lewis Terman]] at [[Organization:Stanford University|Stanford University]] revised the Binet–Simon scale, which resulted in the [[Social:Stanford–Binet Intelligence Scales|Stanford–Binet Intelligence Scales]] (1916). It became the most popular test in the United States for decades.<ref name=Kaufman2009/>{{sfn|Terman et al.|1915}}<ref>{{cite journal |last1=Wallin |first1=J. E. W. |title=The new clinical psychology and the psycho-clinicist |journal=Journal of Educational Psychology |volume=2 |issue=3 |pages=121–32 |year=1911 |doi=10.1037/h0075544 |url=https://zenodo.org/record/1429171}}</ref><ref>{{cite journal |last1=Richardson |first1=John T. E. |title=Howard Andrew Knox and the origins of performance testing on Ellis Island, 1912-1916 |journal=History of Psychology |volume=6 |issue=2 |pages=143–70 |year=2003 |pmid=12822554 |doi=10.1037/1093-4510.6.2.143}}</ref>

===General factor (''g'')===
The many different kinds of IQ tests include a wide variety of item content. Some test items are visual, while many are verbal. Test items vary from being based on abstract-reasoning problems to concentrating on arithmetic, vocabulary, or general knowledge.

The British psychologist Charles Spearman in 1904 made the first formal [[Factor analysis|factor analysis]] of [[Correlation|correlation]]s between the tests. He observed that children's school grades across seemingly unrelated school subjects were positively correlated, and reasoned that these correlations reflected the influence of an underlying general mental ability that entered into performance on all kinds of mental tests. He suggested that all mental performance could be conceptualized in terms of a single general ability factor and a large number of narrow task-specific ability factors. Spearman named it ''g'' for "general factor" and labeled the specific factors or abilities for specific tasks ''s''.{{sfn|Deary|2001|pp=6–12}} In any collection of test items that make up an IQ test, the score that best measures ''g'' is the composite score that has the highest correlations with all the item scores. Typically, the "''g''-loaded" composite score of an IQ test battery appears to involve a common strength in abstract reasoning across the test's item content.{{Citation needed|date=June 2020}}

===United States military selection in World War I===
During World War I, the Army needed a way to evaluate and assign recruits to appropriate tasks. This led to the development of several mental tests by Robert Yerkes, who worked with major hereditarians of American psychometrics—including Terman, Goddard—to write the test.<ref name=Gould>{{harvnb|Gould|1996}}</ref> The testing generated controversy and much public debate in the United States. Nonverbal or "performance" tests were developed for those who could not speak English or were suspected of malingering.<ref name=Kaufman2009/> Based on Goddard's translation of the Binet–Simon test, the tests had an impact in screening men for officer training:

<blockquote>...the tests did have a strong impact in some areas, particularly in screening men for officer training. At the start of the war, the army and national guard maintained nine thousand officers. By the end, two hundred thousand officers presided, and two- thirds of them had started their careers in training camps where the tests were applied. In some camps, no man scoring below C could be considered for officer training.<ref name=Gould/></blockquote>

In total 1.75 million men were tested, making the results the first mass-produced written tests of intelligence, though considered dubious and non-usable, for reasons including high variability of test implementation throughout different camps and questions testing for familiarity with American culture rather than intelligence.<ref name=Gould/> After the war, positive publicity promoted by army psychologists helped to make psychology a respected field.<ref>{{cite book |first1=Carrie H. |last1=Kennedy |first2=Jeffrey A. |last2=McNeil |editor1-first=Carrie H. |editor1-last=Kennedy |editor2-first=Eric |editor2-last=Zillmer |year=2006 |chapter=A history of military psychology |chapter-url= https://books.google.com/books?id=rytCzdXGgXkC&pg=PA1 |title=Military Psychology: Clinical and Operational Applications |pages=1–17 |location= New York |publisher=Guilford Press |isbn=978-1-57230-724-7}}</ref> Subsequently, there was an increase in jobs and funding in psychology in the United States.<ref>{{cite journal |last1=Katzell |first1=Raymond A. |last2=Austin |first2=James T. |year=1992 |title=From then to now: The development of industrial-organizational psychology in the United States |journal=Journal of Applied Psychology |volume=77 |issue=6 |pages=803–35 |doi=10.1037/0021-9010.77.6.803}}</ref> Group intelligence tests were developed and became widely used in schools and industry.<ref name="Kevles, D. J. 1968">{{cite journal |last1=Kevles |first1=D. J. |title=Testing the Army's Intelligence: Psychologists and the Military in World War I |journal=The Journal of American History |volume=55 |issue=3 |pages=565–81 |year=1968 |doi=10.2307/1891014 |jstor=1891014}}</ref>

The results of these tests, which at the time reaffirmed contemporary racism and nationalism, are considered controversial and dubious, having rested on certain contested assumptions: that intelligence was heritable, innate, and could be relegated to a single number, the tests were enacted systematically, and test questions actually tested for innate intelligence rather than subsuming environmental factors.<ref name=Gould/> The tests also allowed for the bolstering of jingoist narratives in the context of increased immigration, which may have influenced the passing of the Immigration Restriction Act of 1924.<ref name=Gould/>

L.L. Thurstone argued for a model of intelligence that included seven unrelated factors (verbal comprehension, word fluency, number facility, spatial visualization, associative memory, perceptual speed, reasoning, and induction). While not widely used, Thurstone's model influenced later theories.<ref name=Kaufman2009/>

David Wechsler produced the first version of his test in 1939. It gradually became more popular and overtook the Stanford–Binet in the 1960s. It has been revised several times, as is common for IQ tests, to incorporate new research. One explanation is that psychologists and educators wanted more information than the single score from the Binet. Wechsler's ten or more subtests provided this. Another is that the Stanford–Binet test reflected mostly verbal abilities, while the Wechsler test also reflected nonverbal abilities. The Stanford–Binet has also been revised several times and is now similar to the Wechsler in several aspects, but the Wechsler continues to be the most popular test in the United States.<ref name=Kaufman2009/>

===IQ testing and the eugenics movement in the United States===
[[Biology:Eugenics|Eugenics]], a set of beliefs and practices aimed at improving the [[Biology:Genetics|genetic]] quality of the human population by excluding people and groups judged to be inferior and promoting those judged to be superior,<ref name="Spektorowski">{{cite book|last1=Spektorowski|first1=Alberto|url=https://books.google.com/books?id=zdkdAAAAQBAJ&q=Historically,+the+term+has+referred+to+everything+from+prenatal+care+for+mothers+to+forced+sterilization+and+euthanasia&pg=PA24|title=Politics of Eugenics: Productionism, Population, and National Welfare|last2=Ireni-Saban|first2=Liza|date=2013|publisher=Routledge|isbn=978-0-203-74023-1|location=London|page=24|quote=As an applied science, thus, the practice of eugenics referred to everything from prenatal care for mothers to forced sterilization and euthanasia. Galton divided the practice of eugenics into two types—positive and negative—both aimed at improving the human race through selective breeding.|access-date=16 January 2017}}</ref><ref>{{cite web|date=26 September 2010|title=Eugenics|url=http://ghr.nlm.nih.gov/glossary=eugenics|work=Unified Medical Language System (Psychological Index Terms)|publisher=National Library of Medicine}}</ref><ref name="Galton1904">{{cite journal|author=Galton, Francis|date=July 1904|title=Eugenics: Its Definition, Scope, and Aims|url=http://www.mugu.com/galton/essays/1900-1911/galton-1904-am-journ-soc-eugenics-scope-aims.htm|journal=The American Journal of Sociology|volume=X|issue=1|pages=82, 1st paragraph|bibcode=1904Natur..70...82.|doi=10.1038/070082a0|archive-url=https://web.archive.org/web/20071103082723/http://galton.org/essays/1900-1911/galton-1904-am-journ-soc-eugenics-scope-aims.htm|archive-date=3 November 2007|access-date=27 December 2010|quote=Eugenics is the science which deals with all influences that improve the inborn qualities of a race; also with those that develop them to the utmost advantage.|doi-access=free}}</ref> played a significant role in the history and culture of the {{wipe|United States}} during the [[Social:Progressive Era|Progressive Era]], from the late 19th century until US involvement in [[Social:World War II|World War II]].<ref name="SusanCurrell">{{cite book|author1=Susan Currell|author2=Christina Cogdell|title=Popular Eugenics: National Efficiency and American Mass Culture in the 1930s|url=https://books.google.com/books?id=WRL6MbBO024C&pg=PA86|year=2006|publisher=Ohio University Press|isbn=978-0-8214-1691-4|pages=2–3}}</ref><ref>{{Cite web|url=http://www.princeton.edu/~tleonard/papers/retrospectives.pdf|title=Eugenics and Economics in the Progressive Era|language=en}}</ref>

The American eugenics movement was rooted in the [[Philosophy:Biological determinism|biological determinist]] ideas of the British Scientist Sir Francis Galton. In 1883, Galton first used the word eugenics to describe the biological improvement of human genes and the concept of being "well-born".<ref>{{Cite web|url=http://exhibits.hsl.virginia.edu/eugenics/2-origins/|title=Origins of Eugenics: From Sir Francis Galton to Virginia's Racial Integrity Act of 1924|website=University of Virginia: Historical Collections at the Claude Moore Health Sciences Library|access-date=25 October 2019}}</ref><ref>{{Cite journal|last=Norrgard|first=K.|date=2008|title=Human testing, the eugenics movement, and IRBs|journal=Nature Education|volume=1|pages=170}}</ref> He believed that differences in a person's ability were acquired primarily through genetics and that eugenics could be implemented through [[Biology:Selective breeding|selective breeding]] in order for the human race to improve in its overall quality, therefore allowing for humans to direct their own evolution.<ref>{{Cite web|url=http://galton.org/books/hereditary-genius/text/pdf/galton-1869-genius-v3.pdf|title=Hereditary Genius|last=Galton|first=Francis|date=1869|page=64|access-date=1 October 2019}}</ref>

Henry H. Goddard was a eugenicist. In 1908, he published his own version, ''The Binet and Simon Test of Intellectual Capacity'', and cordially promoted the test. He quickly extended the use of the scale to the public schools (1913), to immigration (Ellis Island, 1914) and to a court of law (1914).<ref name=":3">{{Cite web|url=http://www.apa.org/monitor/2009/01/assessment.aspx|title=The birth of American intelligence testing|access-date=11 November 2017}}</ref>

Unlike Galton, who promoted eugenics through selective breeding for positive traits, Goddard went with the US eugenics movement to eliminate "undesirable" traits.<ref>{{Cite web|url=https://www.nature.com/scitable/forums/genetics-generation/america-s-hidden-history-the-eugenics-movement-123919444|title=America's Hidden History: The Eugenics Movement {{!}} Learn Science at Scitable|website=www.nature.com|language=en|access-date=11 November 2017}}</ref> Goddard used the term "feeble-minded" to refer to people who did not perform well on the test. He argued that "feeble-mindedness" was caused by heredity, and thus feeble-minded people should be prevented from giving birth, either by institutional isolation or sterilization surgeries.<ref name=":3" /> At first, sterilization targeted the disabled, but was later extended to poor people. Goddard's intelligence test was endorsed by the eugenicists to push for laws for forced sterilization. Different states adopted the sterilization laws at different paces. These laws, whose constitutionality was upheld by the Supreme Court in their 1927 ruling [[Social:Buck v. Bell|Buck v. Bell]], forced over 60,000 people to go through sterilization in the United States.<ref>{{Cite web|url=http://www.eugenicsarchive.org/html/eugenics/essay8text.html|title=Social Origins of Eugenics|website=www.eugenicsarchive.org|access-date=11 November 2017}}</ref>

California's sterilization program was so effective that the Nazis turned to the government for advice on how to prevent the birth of the "unfit".<ref name="HARNE">{{Cite web|url=http://hnn.us/article/1796 |title=The Horrifying American Roots of Nazi Eugenics |website=hnn.us |date=September 2003 |access-date=11 November 2017}}</ref> While the US eugenics movement lost much of its momentum in the 1940s in view of the horrors of Nazi Germany, advocates of eugenics (including Nazi geneticist Otmar Freiherr von Verschuer) continued to work and promote their ideas in the United States.<ref name="HARNE" /> In later decades, some eugenic principles have made a resurgence as a voluntary means of selective reproduction, with some calling them "[[Philosophy:New eugenics|new eugenics]]".<ref>{{cite journal |last1=Vizcarrondo |first1=Felipe E. |title=Human Enhancement: The New Eugenics |journal=The Linacre Quarterly |date=August 2014 |volume=81 |issue=3 |pages=239–243 |doi=10.1179/2050854914Y.0000000021 |pmid=25249705 |pmc=4135459}}</ref> As it becomes possible to test for and correlate genes with IQ (and its proxies),<ref>{{cite web |last1=Regalado |first1=Antonio |title=Eugenics 2.0: We're at the Dawn of Choosing Embryos by Health, Height, and More |url=https://www.technologyreview.com/s/609204/eugenics-20-were-at-the-dawn-of-choosing-embryos-by-health-height-and-more/ |website=Technology Review |access-date=20 November 2019}}</ref> ethicists and embryonic [[Medicine:Genetic testing|genetic testing]] companies are attempting to understand the ways in which the technology can be ethically deployed.<ref>{{cite web |last1=LeMieux |first1=Julianna |title=Polygenic Risk Scores and Genomic Prediction: Q&A with Stephen Hsu |url=https://www.genengnews.com/insights/polygenic-risk-scores-and-genomic-prediction-qa-with-stephen-hsu/ |website=Genetic Engineering and Biotechnology News |access-date=20 November 2019 |date=1 April 2019}}</ref>

===Cattell–Horn–Carroll theory===
{{main|Philosophy:Cattell–Horn–Carroll theory}}
[[File:Raymond Cattell.jpg|thumb|upright|right|Psychologist Raymond Cattell defined fluid and crystallized intelligence and authored the Cattell Culture Fair III IQ test.]]
Raymond Cattell (1941) proposed two types of cognitive abilities in a revision of Spearman's concept of general intelligence. Fluid intelligence (Gf) was hypothesized as the ability to solve novel problems by using reasoning, and crystallized intelligence (Gc) was hypothesized as a knowledge-based ability that was very dependent on education and experience. In addition, fluid intelligence was hypothesized to decline with age, while crystallized intelligence was largely resistant to the effects of aging. The theory was almost forgotten, but was revived by his student John L. Horn (1966) who later argued Gf and Gc were only two among several factors, and who eventually identified nine or ten broad abilities. The theory continued to be called Gf-Gc theory.<ref name=Kaufman2009/>

[[Biography:John Bissell Carroll|John B. Carroll]] (1993), after a comprehensive reanalysis of earlier data, proposed the three stratum theory, which is a hierarchical model with three levels. The bottom stratum consists of narrow abilities that are highly specialized (e.g., induction, spelling ability). The second stratum consists of broad abilities. Carroll identified eight second-stratum abilities. Carroll accepted Spearman's concept of general intelligence, for the most part, as a representation of the uppermost, third stratum.<ref name=Lubinski2004>{{cite journal |last1=Lubinski |first1=David |title=Introduction to the Special Section on Cognitive Abilities: 100 Years After Spearman's (1904) "'General Intelligence,' Objectively Determined and Measured" |journal=Journal of Personality and Social Psychology |date=2004 |volume=86 |issue=1 |pages=96–111 |doi=10.1037/0022-3514.86.1.96 |pmid=14717630 |s2cid=6024297}}</ref>{{sfn|Carroll|1993|p={{page needed|date=October 2020}}}}

In 1999, a merging of the Gf-Gc theory of Cattell and Horn with Carroll's Three-Stratum theory has led to the Cattell–Horn–Carroll theory (CHC Theory), with ''g'' as the top of the hierarchy, ten broad abilities below, and further subdivided into seventy narrow abilities on the third stratum. CHC Theory has greatly influenced many of the current broad IQ tests.<ref name=Kaufman2009/>

Modern tests do not necessarily measure all of these broad abilities. For example, ''quantitative knowledge'' and ''reading & writing ability'' may be seen as measures of school achievement and not IQ.<ref name=Kaufman2009/> ''Decision speed'' may be difficult to measure without special equipment. ''g'' was earlier often subdivided into only Gf and Gc, which were thought to correspond to the nonverbal or performance subtests and verbal subtests in earlier versions of the popular Wechsler IQ test. More recent research has shown the situation to be more complex.<ref name="Kaufman2009" /> Modern comprehensive IQ tests do not stop at reporting a single IQ score. Although they still give an overall score, they now also give scores for many of these more restricted abilities, identifying particular strengths and weaknesses of an individual.<ref name="Kaufman2009" />

===Other theories===
An alternative to standard IQ tests, meant to test the proximal development of children, originated in the writings of psychologist [[Biography:Lev Vygotsky|Lev Vygotsky]] (1896–1934) during his last two years of his life.<ref>{{cite book|last1=Mindes|first1=Gayle|title=Assessing Young Children |date=2003 |publisher= Merrill/Prentice Hall |isbn=9780130929082 |page=158 |url=https://books.google.com/books?id=x41LAAAAYAAJ}}</ref><ref>{{cite book |last1=Haywood|first1=H. Carl |url=https://books.google.com/books?id=xQekS_oqGzoC|title=Dynamic Assessment in Practice: Clinical and Educational Applications |last2=Lidz |first2=Carol S.|date=2006|publisher=Cambridge University Press |isbn=9781139462075 |page=1}}</ref> According to Vygotsky, the maximum level of complexity and difficulty of problems that a child is capable to solve under some guidance indicates their level of potential development. The difference between this level of potential and the lower level of unassisted performance indicates the child's zone of proximal development.<ref>{{cite book |url=http://www.marxists.org/archive/vygotsky/works/1934/problem-age.htm |last=Vygotsky |first=L.S. |year=1934 |chapter=The Problem of Age |title=The Collected Works of L. S. Vygotsky, Volume 5 |publication-date=1998 |pages=187–205}}</ref> Combination of the two indexes{{--}}the level of actual and the zone of the proximal development{{--}}according to Vygotsky, provides a significantly more informative indicator of psychological development than the assessment of the level of actual development alone.<ref>{{cite book |last=Chaiklin |first=S. |year=2003 |chapter=The Zone of Proximal Development in Vygotsky's analysis of learning and instruction |editor-last1=Kozulin |editor-first1=A. |editor-last2=Gindis |editor-first2=B. |editor-last3=Ageyev |editor-first3=V. |editor-last4=Miller |editor-first4=S. |title=Vygotsky's educational theory and practice in cultural context |pages=39–64 |location=Cambridge |publisher=Cambridge University Press}}</ref><ref>{{cite journal |last=Zaretskii |first=V.K. |title=The Zone of Proximal Development What Vygotsky Did Not Have Time to Write |journal=Journal of Russian and East European Psychology |volume=47 |issue=6 |date=November–December 2009 |pages=70–93|doi=10.2753/RPO1061-0405470604 |s2cid=146894219 }}</ref> His ideas on the zone of development were later developed in a number of psychological and educational theories and practices, most notably under the banner of dynamic assessment, which seeks to measure developmental potential<ref>{{cite journal|last1=Sternberg|first1=R.S. |last2=Grigorenko|first2=E.L.|year=2001|title=All testing is dynamic testing|journal=Issues in Education |volume=7 |issue=2|pages=137–170}}</ref><ref>Sternberg, R.J. & Grigorenko, E.L. (2002). Dynamic testing: The nature and measurement of learning potential. Cambridge: University of Cambridge</ref>{{sfn|Haywood|Lidz |2006|p={{page needed|date=October 2020}}}} (for instance, in the work of Reuven Feuerstein and his associates,<ref>Feuerstein, R., Feuerstein, S., Falik, L & Rand, Y. (1979; 2002). Dynamic assessments of cognitive modifiability. ICELP Press, Jerusalem: Israel</ref> who has criticized standard IQ testing for its putative assumption or acceptance of "fixed and immutable" characteristics of intelligence or cognitive functioning). Dynamic assessment has been further elaborated in the work of Ann Brown, and John D. Bransford and in theories of multiple intelligences authored by [[Biography:Howard Gardner|Howard Gardner]] and [[Biography:Robert Sternberg|Robert Sternberg]].<ref>{{cite book |contributor=Dodge, Kenneth A. |contribution=Foreword |pages=xiii–xv |last1=Haywood |first1=H. Carl |last2=Lidz |first2=Carol S. |title=Dynamic Assessment in Practice: Clinical And Educational Applications |publisher=Cambridge University Press |year=2006}}</ref><ref>{{cite book |last=Kozulin |first=A. |year=2014 |chapter=Dynamic assessment in search of its identity |editor-last1=Yasnitsky |editor-first1=A. |editor-last2=van der Veer |editor-first2=R. |editor-last3=Ferrari |editor-first3=M. |title=The Cambridge Handbook of Cultural-Historical Psychology |publisher=Cambridge University Press |pages=126–147}}</ref>

J.P. Guilford's Structure of Intellect (1967) model of intelligence used three dimensions, which, when combined, yielded a total of 120 types of intelligence. It was popular in the 1970s and early 1980s, but faded owing to both practical problems and theoretical criticisms.<ref name="Kaufman2009" />

[[Biography:Alexander Luria|Alexander Luria]]'s earlier work on neuropsychological processes led to the PASS theory (1997). It argued that only looking at one general factor was inadequate for researchers and clinicians who worked with learning disabilities, attention disorders, intellectual disability, and interventions for such disabilities. The PASS model covers four kinds of processes (planning process, attention/arousal process, simultaneous processing, and successive processing). The planning processes involve decision making, problem solving, and performing activities and require goal setting and self-monitoring.

The attention/arousal process involves selectively attending to a particular stimulus, ignoring distractions, and maintaining vigilance. Simultaneous processing involves the integration of stimuli into a group and requires the observation of relationships. Successive processing involves the integration of stimuli into serial order. The planning and attention/arousal components comes from structures located in the frontal lobe, and the simultaneous and successive processes come from structures located in the posterior region of the cortex.<ref name=Das1975>{{Cite journal |author1=Das, J.P. |author2=Kirby, J. |author3=Jarman, R.F. |year=1975 |title=Simultaneous and successive synthesis: An alternative model for cognitive abilities |journal=Psychological Bulletin |volume=82 |pages=87–103 |doi=10.1037/h0076163}}</ref><ref name=Das2002>{{Cite journal |author=Das, J.P. |year=2000 |title=A better look at intelligence |journal=Current Directions in Psychological Science |volume=11 |pages=28–33 |doi=10.1111/1467-8721.00162|s2cid=146129242}}</ref><ref name= Naglieri1990>{{Cite journal |author1=Naglieri, J.A. |author2=Das, J.P. |year=2002 |title=Planning, attention, simultaneous, and successive cognitive processes as a model for assessment |journal=School Psychology Review |volume=19 |issue=4 |pages=423–442|doi=10.1080/02796015.1990.12087349 }}</ref> It has influenced some recent IQ tests, and been seen as a complement to the Cattell–Horn–Carroll theory described above.<ref name=Kaufman2009/>

==Current tests==
[[File:IQ distribution.svg|thumb|upright=1.2|Normalized IQ distribution with mean 100 and standard deviation 15]]
There are a variety of individually administered IQ tests in use in the English-speaking world.<ref name="Urbina2011Table2.1" /><ref name="FlanaganHarrison2012chs8-16" /><ref>{{Citation |last1=Stanek |first1=Kevin C. |title=Taxonomies and Compendia of Cognitive Ability and Personality Constructs and Measures Relevant to Industrial, Work and Organizational Psychology |date=2018 |url=http://sk.sagepub.com/reference/the-sage-handbook-of-industrial-work-and-org-psychology-vol1/i3345.xml |work=The SAGE Handbook of Industrial, Work and Organizational Psychology: Personnel Psychology and Employee Performance |pages=366–407 |access-date=2024-01-08 |place=1 Oliver's Yard, 55 City Road London EC1Y 1SP |publisher=SAGE Publications Ltd |doi=10.4135/9781473914940.n14 |isbn=978-1-4462-0721-5 |last2=Ones |first2=Deniz S.}}</ref> The most commonly used individual IQ test series is the Wechsler Adult Intelligence Scale (WAIS) for adults and the Wechsler Intelligence Scale for Children (WISC) for school-age test-takers. Other commonly used individual IQ tests (some of which do not label their standard scores as "IQ" scores) include the current versions of the [[Social:Stanford–Binet Intelligence Scales|Stanford–Binet Intelligence Scales]], [[Social:Woodcock–Johnson Tests of Cognitive Abilities|Woodcock–Johnson Tests of Cognitive Abilities]], the Kaufman Assessment Battery for Children, the Cognitive Assessment System, and the Differential Ability Scales.

There are various other IQ tests, including:
# Raven's Progressive Matrices
# Cattell Culture Fair III
# Reynolds Intellectual Assessment Scales
# Thurstone's Primary Mental Abilities<ref>{{Cite web|title = Primary Mental Abilities Test {{!}} psychological test|url = https://www.britannica.com/topic/Primary-Mental-Abilities-Test|website = Encyclopædia Britannica|access-date = 26 November 2015}}</ref><ref>{{Cite web|title = Defining and Measuring Psychological Attributes|url = http://homepages.rpi.edu/~verwyc/TESTOH2.htm|website = homepages.rpi.edu|access-date = 26 November 2015|archive-date = 15 October 2018 |archive-url = https://web.archive.org/web/20181015132922/http://homepages.rpi.edu/~verwyc/TESTOH2.htm|url-status = dead}}</ref>
# Kaufman Brief Intelligence Test<ref>{{Cite journal|title = Test Review: Review of Kaufman Brief Intelligence Test, Second Edition Kaufman, A. S., & Kaufman, N. L. (2004). Kaufman Brief Intelligence Test, Second Edition. Bloomington, MN: Pearson, Inc|journal = Journal of Psychoeducational Assessment|date = 1 April 2010|issn = 0734-2829|pages = 167–174|volume = 28|issue = 2|doi = 10.1177/0734282909348217|first1 = Sherry K.|last1 = Bain|first2 = Kathryn E.|last2 = Jaspers|s2cid = 143961429}}</ref>
# Multidimensional Aptitude Battery II
# [[Philosophy:Das–Naglieri cognitive assessment system|Das–Naglieri cognitive assessment system]]
# Naglieri Nonverbal Ability Test
#[[Social:Wide Range Intelligence Test|Wide Range Intelligence Test]]

IQ scales are ordinally scaled.<ref>{{cite book|title=Psychology: An Introduction |last=Mussen |first=Paul Henry |year=1973 |publisher=Heath |location=Lexington, MA |isbn=978-0-669-61382-7 |page=[https://archive.org/details/psychologyintrod00muss/page/363 363] |quote=The I.Q. is essentially a rank; there are no true "units" of intellectual ability. |url=https://archive.org/details/psychologyintrod00muss/page/363}}</ref><ref>{{cite book |title=The WISC-III Companion: A Guide to Interpretation and Educational Intervention |last=Truch |first=Steve |year=1993 |publisher=Pro-Ed |location=Austin, TX |isbn=978-0-89079-585-9 |page=35 |quote=An IQ score is not an equal-interval score, as is evident in Table A.4 in the WISC-III manual.}}</ref><ref>{{cite book |title=Measuring Intelligence: Facts and Fallacies |url=https://archive.org/details/measuringintelli00bart |url-access=limited |last=Bartholomew |first=David J. |year=2004 |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-54478-8 |quote=When we come to quantities like IQ or g, as we are presently able to measure them, we shall see later that we have an even lower level of measurement—an ordinal level. This means that the numbers we assign to individuals can only be used to rank them—the number tells us where the individual comes in the rank order and nothing else. |page=[https://archive.org/details/measuringintelli00bart/page/n65 50]}}</ref><ref>{{harvnb|Mackintosh|1998|pp=[https://archive.org/details/iqhumanintellige00mack/page/30 30–31]}} "In the jargon of psychological measurement theory, IQ is an ordinal scale, where we are simply rank-ordering people. ... It is not even appropriate to claim that the 10-point difference between IQ scores of 110 and 100 is the same as the 10-point difference between IQs of 160 and 150"</ref><ref>{{cite journal |last=Stevens |first=S. S. |author-link=Stanley Smith Stevens |title=On the Theory of Scales of Measurement |journal=Science |volume=103 |issue=2684 |pages=677–680 |year=1946 |pmid=17750512 |doi=10.1126/science.103.2684.677 |bibcode=1946Sci...103..677S |s2cid=4667599}}</ref> The [[Social:Test score|raw score]] of the norming [[Sample (statistics)|sample]] is usually (rank order) [[Data transformation (statistics)|transformed]] to a [[Normal distribution|normal distribution]] with mean 100 and [[Standard deviation|standard deviation]] 15.<ref name="Gottfredson2009pp31–32" /> While one [[Standard deviation|standard deviation]] is 15 points, and two SDs are 30 points, and so on, this does not imply that mental ability is linearly related to IQ, such that IQ 50 would mean half the cognitive ability of IQ 100. In particular, IQ points are not percentage points.

==Reliability and validity==
{|class="wikitable sortable" style="font-size:small; float:right; text-align:center; margin:0 0 0.5em 1em" summary="Sortable table showing actual I.Q. scores of twelve students on three different I.Q. tests, with students identified by pseudonyms in cited data source."
|+ IQ scores can differ to some degree for the same person on different IQ tests, so a person does not always belong to the same IQ score range each time the person is tested. (IQ score table data and pupil pseudonyms adapted from description of KABC-II norming study cited in {{harvp|Kaufman|2009}}.<ref name="Kaufman2009Fig5.1" /><ref name="KaufmanSB2013Fig3.1" />)
|-
! class="unsortable" |Pupil!!KABC-II!!WISC-III!!WJ-III
|-
|A||90||95||111
|-
|B||125||110||105
|-
|C||100||93||101
|-
|D||116||127||118
|-
|E||93||105||93
|-
|F||106||105||105
|-
|G||95||100||90
|-
|H||112||113||103
|-
|I||104||96||97
|-
|J||101||99||86
|-
|K||81||78||75
|-
|L||116||124||102
|}

===Reliability===
Psychometricians generally regard IQ tests as having high statistical reliability.{{sfn|Neisser et al.|1995}}<ref name="Mackintosh2011p169">{{Harvnb |Mackintosh|2011|page=169}} "after the age of 8–10, IQ scores remain relatively stable: the correlation between IQ scores from age 8 to 18 and IQ at age 40 is over 0.70."</ref> Reliability represents the measurement consistency of a test.<ref name="Weiten">{{cite book|vauthors= Weiten W|title=Psychology: Themes and Variations |publisher=Cengage Learning|year=2016|page=281|isbn=978-1305856127 |url=https://books.google.com/books?id=ALkaCgAAQBAJ&pg=PT331}}</ref> A reliable test produces similar scores upon repetition.<ref name="Weiten"/> On aggregate, IQ tests exhibit high reliability, although test-takers may have varying scores when taking the same test on differing occasions, and may have varying scores when taking different IQ tests at the same age. Like all statistical quantities, any particular estimate of IQ has an associated standard error that measures uncertainty about the estimate. For modern tests, the confidence interval can be approximately 10 points and reported standard error of measurement can be as low as about three points.<ref>{{cite web |title=WISC-V Interpretive Report Sample |website=Pearson |url=https://images.pearsonclinical.com/images/assets/wisc-v/WISC-VInterpretiveReportSample-1.pdf |access-date=29 September 2020 |pages=18}}</ref> Reported standard error may be an underestimate, as it does not account for all sources of error.<ref>{{cite book |last1=Kaufman |first1=Alan S. |last2=Raiford |first2=Susan Engi |last3=Coalson |first3=Diane L. |year=2016 |title=Intelligent testing with the WISC-V |publisher=Wiley |location=Hoboken, NJ |isbn=978-1-118-58923-6 |pages=683–702 |quote=Reliability estimates in Table 4.1 and standard errors of measurement in Table 4.4 should be considered best-case estimates because they do not consider other major sources of error, such as transient error, administration error, or scoring error (Hanna, Bradley, & Holen, 1981), which influence test scores in clinical assessments. Another factor that must be considered is the extent to which subtest scores reflect portions of true score variance due to a hierarchical general intelligence factor and variance due to specific group factors because these sources of true score variance are conflated.}}</ref>

Outside influences such as low motivation or high anxiety can occasionally lower a person's IQ test score.<ref name="Weiten" /> For individuals with very low scores, the 95% confidence interval may be greater than 40 points, potentially complicating the accuracy of diagnoses of intellectual disability.<ref>{{cite journal |last1=Whitaker |first1=Simon |title=Error in the estimation of intellectual ability in the low range using the WISC-IV and WAIS-III |journal=Personality and Individual Differences |date=April 2010 |volume=48 |issue=5 |pages=517–521 |url=https://www.researchgate.net/publication/222824571 |access-date=22 January 2020 |doi=10.1016/j.paid.2009.11.017}}</ref> By the same token, high IQ scores are also significantly less reliable than those near to the population median.<ref>{{harvnb|Lohman|Foley Nicpon|2012|p={{page needed|date=October 2020}}}}. "The concerns associated with SEMs [standard errors of measurement] are actually substantially worse for scores at the extremes of the distribution, especially when scores approach the maximum possible on a test ... when students answer most of the items correctly. In these cases, errors of measurement for scale scores will increase substantially at the extremes of the distribution. Commonly the SEM is from two to four times larger for very high scores than for scores near the mean (Lord, 1980)."</ref> Reports of IQ scores much higher than 160 are considered dubious.<ref>{{harvnb|Urbina|2011|p=20}} "[Curve-fitting] is just one of the reasons to be suspicious of reported IQ scores much higher than 160"</ref>

===Validity as a measure of intelligence===
Reliability and validity are very different concepts. While reliability reflects reproducibility, validity refers to whether the test measures what it purports to measure.<ref name="Weiten" /> While IQ tests are generally considered to measure some forms of intelligence, they may fail to serve as an accurate measure of broader definitions of [[Philosophy:Human intelligence|human intelligence]] inclusive of, for example, [[Philosophy:Creativity|creativity]] and [[Social:Social intelligence|social intelligence]]. For this reason, psychologist Wayne Weiten argues that their [[Construct validity|construct validity]] must be carefully qualified, and not be overstated.<ref name="Weiten" /> According to Weiten, "IQ tests are valid measures of the kind of intelligence necessary to do well in academic work. But if the purpose is to assess intelligence in a broader sense, the validity of IQ tests is questionable."<ref name="Weiten" />

Some scientists have disputed the value of IQ as a measure of intelligence altogether. In ''[[Biology:The Mismeasure of Man|The Mismeasure of Man]]'' (1981, expanded edition 1996), [[Biology:Evolutionary biology|evolutionary biologist]] [[Biography:Stephen Jay Gould|Stephen Jay Gould]] compared IQ testing with the now-discredited practice of determining intelligence via [[Medicine:Craniometry|craniometry]], arguing that both are based on the fallacy of [[Philosophy:Reification (fallacy)|reification]], "our tendency to convert abstract concepts into entities".<ref name="TMoMp24">{{harvnb|Gould|1981|p=24}}. {{harvnb|Gould|1996|p=[https://books.google.com/books?id=WTtTiG4eda0C&pg=PA56 56]}}.</ref> Gould's argument sparked a great deal of debate,<ref name="Kaplan et al">{{cite journal|last1=Kaplan|first1=Jonathan Michael |last2=Pigliucci|first2=Massimo|last3=Banta|first3=Joshua Alexander|year=2015|title=Gould on Morton, Redux: What can the debate reveal about the limits of data? |journal=Studies in History and Philosophy of Biological and Biomedical Sciences |url=http://philpapers.org/archive/KAPGOM.pdf |volume=30|pages=1–10}}</ref><ref>{{Cite journal|last1=Weisberg|first1=Michael|last2=Paul|first2=Diane B.|date=19 April 2016 |title=Morton, Gould, and Bias: A Comment on "The Mismeasure of Science" |journal=PLOS Biology |volume=14 |issue=4 |at=e1002444 |doi=10.1371/journal.pbio.1002444 |issn=1544-9173 |pmc=4836680 |pmid=27092558 |doi-access=free }}</ref> and the book is listed as one of ''Discover Magazine''{{'}}s "25 Greatest Science Books of All Time".<ref>{{cite magazine |date=7 December 2006 |url=https://www.discovermagazine.com/the-sciences/25-greatest-science-books-of-all-time |title=25 Greatest Science Books of All Time |magazine=Discover}}</ref>

Along these same lines, critics such as [[Biography:Keith Stanovich|Keith Stanovich]] do not dispute the capacity of IQ test scores to predict some kinds of achievement, but argue that basing a concept of intelligence on IQ test scores alone neglects other important aspects of mental ability.{{sfn|Neisser et al.|1995}}<ref>Brooks, David (14 September 2007). [https://www.nytimes.com/2007/09/14/opinion/14brooks.html "The Waning of I.Q."]. ''{{wipe|The New York Times}}''.</ref> [[Biography:Robert Sternberg|Robert Sternberg]], another significant critic of IQ as the main measure of human cognitive abilities, argued that reducing the concept of intelligence to the measure of ''g'' does not fully account for the different skills and knowledge types that produce success in human society.<ref>Sternberg, Robert J., and Richard K. Wagner. "The g-ocentric view of intelligence and job performance is wrong." Current directions in psychological science (1993): 1–5.</ref>

Despite these objections, clinical psychologists generally regard IQ scores as having sufficient [[Validity (statistics)|statistical validity]] for many clinical purposes.{{Specify |reason=Quick summary of which clinical purposes it's used for, and ideally some of the limitations. Is this sentence better suited for the reliability section?|date=October 2020}}<ref name="Kaufman2009"/>{{sfn|Anastasi|Urbina|1997|pp=326–327}}

===Test bias or differential item functioning===
Differential item functioning (DIF), sometimes referred to as measurement bias, is a phenomenon when participants from different groups (e.g. gender, race, disability) with the same latent abilities give different answers to specific questions on the same IQ test.<ref>Embretson, S. E., Reise, S. P. (2000).''Item Response Theory for Psychologists''. New Jersey: Lawrence Erlbaum.</ref> DIF analysis measures such specific items on a test alongside measuring participants' latent abilities on other similar questions. A consistent different group response to a specific question among similar types of questions can indicate an effect of DIF. It does not count as differential item functioning if both groups have an equally valid chance of giving different responses to the same questions. Such bias can be a result of culture, educational level and other factors that are independent of group traits. DIF is only considered if test-takers from different groups ''with the same underlying [[Social:Latent variable|latent]] ability level'' have a different chance of giving specific responses.<ref name=":1">{{cite journal |last1=Zumbo|first1=B.D. |year=2007|title=Three generations of differential item functioning (DIF) analyses: Considering where it has been, where it is now, and where it is going |journal=Language Assessment Quarterly |volume=4 |issue=2 |pages=223–233|doi=10.1080/15434300701375832|s2cid=17426415}}</ref> Such questions are usually removed in order to make the test equally fair for both groups. Common techniques for analyzing DIF are [[Item response theory|item response theory]] (IRT) based methods, Mantel-Haenszel, and [[Logistic regression|logistic regression]].<ref name=":1" />

A 2005 study found that "differential validity in prediction suggests that the WAIS-R test may contain cultural influences that reduce the validity of the WAIS-R as a measure of cognitive ability for Mexican American students,"<ref>{{cite journal|last1=Verney|first1=SP|last2=Granholm |first2=E|last3=Marshall|first3=SP|last4=Malcarne|first4=VL|last5=Saccuzzo|first5=DP|year=2005 |title=Culture-Fair Cognitive Ability Assessment: Information Processing and Psychophysiological Approaches |journal=Assessment|volume=12|issue=3|pages=303–19|doi=10.1177/1073191105276674|pmid=16123251 |s2cid=31024437}}</ref> indicating a weaker positive correlation relative to sampled white students. Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.<ref>{{cite journal|last1=Shuttleworth-Edwards|first1=Ann|last2=Kemp|first2=Ryan|last3=Rust |first3=Annegret |last4=Muirhead|first4=Joanne|last5=Hartman|first5=Nigel|last6=Radloff|first6=Sarah|year=2004|title=Cross-cultural Effects on IQ Test Performance: AReview and Preliminary Normative Indications on WAIS-III Test Performance |journal=Journal of Clinical and Experimental Neuropsychology|volume=26|issue=7|pages=903–20 |doi=10.1080/13803390490510824 |pmid=15742541 |s2cid=16060622}}</ref><ref>{{cite journal|last1=Cronshaw |first1=Steven F. |last2=Hamilton|first2=Leah K.|last3=Onyura|first3=Betty R. |last4=Winston |first4=Andrew S. |year=2006|title=Case for Non-Biased Intelligence Testing Against Black Africans Has Not Been Made: A Comment on Rushton, Skuy, and Bons (2004)|journal=International Journal of Selection and Assessment |volume=14|issue=3|pages=278–87|doi=10.1111/j.1468-2389.2006.00346.x |s2cid=91179275}}</ref> Standard intelligence tests, such as the Stanford–Binet, are often inappropriate for autistic children; the alternative of using developmental or adaptive skills measures are relatively poor measures of intelligence in autistic children, and may have resulted in incorrect claims that a majority of autistic children are of low intelligence.<ref>{{cite journal|last1=Edelson|first1=M. G. |year=2006|title=Are the Majority of Children With Autism Mentally Retarded?: A Systematic Evaluation of the Data|journal=Focus on Autism and Other Developmental Disabilities |volume=21|issue=2|pages=66–83 |doi=10.1177/10883576060210020301|s2cid=145809356}}</ref>

===Flynn effect===
{{Main|Philosophy:Flynn effect}}

Since the early 20th century, raw scores on IQ tests have increased in most parts of the world.<ref name="Neisser1998">{{Cite book|title=The Rising Curve: Long-Term Gains in IQ and Related Measures |editor-last=Neisser |editor-first=Ulric |display-authors=8 |author1=Ulric Neisser |author2=James R. Flynn |author3=Carmi Schooler |author4=Patricia M. Greenfield |author5=Wendy M. Williams |author6=Marian Sigman |author7=Shannon E. Whaley |author8=Reynaldo Martorell |author9=Richard Lynn |author10=Robert M. Hauser |author11=David W. Grissmer |author12=Stephanie Williamson |author13=Sheila Nataraj Kirby |author14=Mark Berends |author15=Stephen J. Ceci |author16=Tina B. Rosenblum |author17=Matthew Kumpf |author18=Min-Hsiung Huang |author19=Irwin D. Waldman |author20=Samuel H. Preston |author21=John C. Loehlin |year=1998 |publisher=American Psychological Association |location=Washington, DC |isbn=978-1-55798-503-3 |series=APA Science Volume Series |url=https://archive.org/details/risingcurvelongt00neis}}{{Page needed|date=January 2011}}</ref>{{sfn|Mackintosh|1998|p={{Page needed|date=January 2011}}}}{{sfn|Flynn|2009|p={{Page needed|date=January 2011}}}} When a new version of an IQ test is normed, the standard scoring is set so performance at the population median results in a score of IQ 100. The phenomenon of rising raw score performance means if test-takers are scored by a constant standard scoring rule, IQ test scores have been rising at an average rate of around three IQ points per decade. This phenomenon was named the Flynn effect in the book ''The Bell Curve'' after James R. Flynn, the author who did the most to bring this phenomenon to the attention of psychologists.<ref name="Flynn1984">{{cite journal |last1=Flynn |first1=James R. |title=The mean IQ of Americans: Massive gains 1932 to 1978. |journal=Psychological Bulletin |volume=95 |issue=1 |pages=29–51 |year=1984 |doi=10.1037/0033-2909.95.1.29 |s2cid=51999517}}</ref><ref name="Flynn1987">{{cite journal |last1=Flynn |pages=171–91 |first1=James R. |issue=2 |volume=101 |year=1987 |title=Massive IQ gains in 14 nations: What IQ tests really measure. |doi=10.1037/0033-2909.101.2.171 |journal=Psychological Bulletin}}</ref>

Researchers have been exploring the issue of whether the Flynn effect is equally strong on performance of all kinds of IQ test items, whether the effect may have ended in some developed nations, whether there are social subgroup differences in the effect, and what possible causes of the effect might be.<ref>{{Cite book |last1=Zhou |first1=Xiaobin |last2=Grégoire |first2=Jacques |last3=Zhu |first3=Jianjin |title=WAIS-IV Clinical Use and Interpretation: Scientist-Practitioner Perspectives |editor1-last=Weiss |editor1-first=Lawrence G. |editor2-last=Saklofske |editor2-first=Donald H. |editor3-last=Coalson |editor3-first=Diane |editor4-last=Raiford |editor4-first=Susan |chapter=The Flynn Effect and the Wechsler Scales |year=2010 |publisher=Academic Press |location=Amsterdam |series=Practical Resources for the Mental Health Professional |isbn=978-0-12-375035-8}}{{Page needed|date=January 2011}}</ref> A 2011 textbook, ''IQ and Human Intelligence'', by N. J. Mackintosh, noted the Flynn effect demolishes the fears that IQ would be decreased. He also asks whether it represents a real increase in intelligence beyond IQ scores.{{sfn|Mackintosh|2011|pp=25–27}} A 2011 psychology textbook, lead authored by Harvard Psychologist Professor [[Biography:Daniel Schacter|Daniel Schacter]], noted that humans' inherited intelligence could be [[Biology:Dysgenics|going down]] while acquired intelligence goes up.<ref>{{cite book |first1=Daniel L. |last1=Schacter |first2=Daniel T. |last2=Gilbert |first3=Daniel M. |last3=Wegner |title=Psychology|date=2011 |publisher=Palgrave Macmillan |location=Basingstoke |isbn=978-0230579835 |page=384}}</ref>

Research has suggested that the Flynn effect has slowed or reversed course in some Western countries beginning in the late 20th century. The phenomenon has been termed the ''negative Flynn effect''.<ref name=":4">{{Cite journal|last1=Bratsberg|first1=Bernt|last2=Rogeberg|first2=Ole|date=26 June 2018|title=Flynn effect and its reversal are both environmentally caused |journal=Proceedings of the National Academy of Sciences|volume=115 |issue=26|pages=6674–6678|doi=10.1073/pnas.1718793115|pmid=29891660|pmc=6042097|bibcode=2018PNAS..115.6674B |doi-access=free}}</ref> A study of Norwegian military conscripts' test records found that IQ scores have been falling for generations born after the year 1975, and that the underlying cause of both initial increasing and subsequent falling trends appears to be environmental rather than genetic.<ref name=":4" />

===Age===
IQ can change to some degree over the course of childhood.{{sfn|Kaufman|2009|pp=[https://archive.org/details/iqtestingpsych00phdd/page/n234 220]–222}} In one longitudinal study, the mean IQ scores of tests at ages 17 and 18 were correlated at {{nowrap|1=''r'' = 0.86}} with the mean scores of tests at ages five, six, and seven and at {{nowrap|1=''r'' = 0.96}}{{Explain|date=October 2020|reason=Please provide context to r correlation values. Are 0.86 and 0.96 good? How do they compare with correlation at older ages?}} with the mean scores of tests at ages 11, 12, and 13.{{sfn|Neisser et al.|1995}}

For decades, practitioners' handbooks and textbooks on IQ testing have reported IQ declines with age after the beginning of adulthood. However, later researchers pointed out this phenomenon is related to the [[Philosophy:Flynn effect|Flynn effect]] and is in part a [[Cohort (statistics)|cohort]] effect rather than a true aging effect. A variety of studies of IQ and aging have been conducted since the norming of the first Wechsler Intelligence Scale drew attention to IQ differences in different age groups of adults. The current consensus is that fluid intelligence generally declines with age after early adulthood, while crystallized intelligence remains intact. Both cohort effects (the birth year of the test-takers) and practice effects (test-takers taking the same form of IQ test more than once) must be controlled to gain accurate data.{{Inconsistent|date=October 2020|reason=Resolve the distinction between IQ (which, by definition, is age-normalized) and intelligence (which IQ attempts to measure) in this section.}} It is unclear whether any lifestyle intervention can preserve fluid intelligence into older ages.{{sfn|Kaufman|2009|loc="Chapter 8"|p={{Page needed|date=January 2011}}}}

The exact peak age of fluid intelligence or crystallized intelligence remains elusive. Cross-sectional studies usually show that especially fluid intelligence peaks at a relatively young age (often in the early adulthood) while longitudinal data mostly show that intelligence is stable until mid-adulthood or later. Subsequently, intelligence seems to decline slowly.<ref name="DesjardinsWarnke2012">{{cite journal |last1=Desjardins |first1=Richard |last2=Warnke |first2=Arne Jonas |year=2012 |doi=10.1787/5k9csvw87ckh-en |url=http://www.oecd-ilibrary.org/education/ageing-and-skills_5k9csvw87ckh-en |title=Ageing and Skills |journal=OECD Education Working Papers |hdl=10419/57089 |doi-access=free|hdl-access=free }}</ref>

==Genetics and environment==
Environmental and [[Biology:Heredity|genetic]] factors play a role in determining IQ. Their relative importance has been the subject of much research and debate.<ref>{{citation|title=Continuity of Genetic and Environmental Influences on Cognition across the Life Span: A Meta-Analysis of Longitudinal Twin and Adoption Studies|first1=Elliot M|last1=Tucker-Drob|first2=Daniel A|last2=Briley|journal=Psychological Bulletin|year=2014|volume=140|issue=4|pages=949–979|doi=10.1037/a0035893|pmc=4069230|pmid=24611582}}</ref>

===Heritability===

The general figure for the [[Philosophy:Heritability|heritability]] of IQ, according to an [[Organization:American Psychological Association|American Psychological Association]] report, is 0.45 for children, and rises to around 0.75 for late adolescents and adults.{{sfn|Neisser et al.|1995}} Heritability measures for [[Philosophy:G factor (psychometrics)|''g'' factor]] in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.9 in adulthood.<ref name="BouchardWilson2013">{{cite journal |last1=Bouchard |first1=Thomas J. |title=The Wilson Effect: The Increase in Heritability of IQ With Age |journal=Twin Research and Human Genetics |date=7 August 2013 |volume=16 |issue=5 |pages=923–930 |doi=10.1017/thg.2013.54 |pmid=23919982 |s2cid=13747480|doi-access=free }}</ref><ref name="PanizzonVuoksimaa2014">{{cite journal |last1=Panizzon |first1=Matthew S. |last2=Vuoksimaa |first2=Eero |last3=Spoon |first3=Kelly M. |last4=Jacobson |first4=Kristen C. |last5=Lyons |first5=Michael J. |last6=Franz |first6=Carol E. |last7=Xian |first7=Hong |last8=Vasilopoulos |first8=Terrie |last9=Kremen |first9=William S. |date=March 2014 |title=Genetic and environmental influences on general cognitive ability: Is ''g'' a valid latent construct? |journal=Intelligence |volume=43 |pages=65–76 |doi=10.1016/j.intell.2014.01.008 |pmc=4002017 |pmid=24791031}}</ref> One proposed explanation is that people with different genes tend to reinforce the effects of those genes, for example by seeking out different environments.{{sfn|Neisser et al.|1995}}<ref>{{cite journal |display-authors=8 |last1=Huguet |first1=Guillaume |last2=Schramm |first2=Catherine |last3=Douard |first3=Elise |last4=Jiang |first4=Lai |last5=Labbe |first5=Aurélie |last6=Tihy |first6=Frédérique |last7=Mathonnet |first7=Géraldine |last8=Nizard |first8=Sonia |last9=Lemyre |first9=Emmanuelle |last10=Mathieu |first10=Alexandre |last11=Poline |first11=Jean-Baptiste |last12=Loth |first12=Eva |last13=Toro |first13=Roberto |last14=Schumann |first14=Gunter |last15=Conrod |first15=Patricia |last16=Pausova |first16=Zdenka |last17=Greenwood |first17=Celia |last18=Paus |first18=Tomas |last19=Bourgeron |first19=Thomas |last20=Jacquemont |first20=Sébastien |author21=IMAGEN Consortium |title=Measuring and Estimating the Effect Sizes of Copy Number Variants on General Intelligence in Community-Based Samples |journal=JAMA Psychiatry |date=May 2018 |volume=75 |issue=5 |pages=447–457 |doi=10.1001/jamapsychiatry.2018.0039 |pmid=29562078 |pmc=5875373}}</ref>

===Shared family environment===
Family members have aspects of environments in common (for example, characteristics of the home). This shared family environment accounts for 0.25–0.35 of the variation in IQ in childhood. By late adolescence, it is quite low (zero in some studies). The effect for several other psychological traits is similar. These studies have not looked at the effects of extreme environments, such as in abusive families.{{sfn|Neisser et al.|1995}}<ref>{{cite journal |last1=Bouchard |first1=TJ Jr. |title=Genetic and environmental influences on adult intelligence and special mental abilities |journal=Human Biology; an International Record of Research |volume=70 |issue=2 |pages=257–79 |year=1998 |pmid=9549239}}</ref><ref name="Plomin0103">{{cite journal |last1=Plomin |first1=R |last2=Asbury |first2=K |last3=Dunn |first3=J |title=Why are children in the same family so different? Nonshared environment a decade later |journal=Canadian Journal of Psychiatry |volume=46 |issue=3 |pages=225–33 |year=2001 |pmid=11320676|doi=10.1177/070674370104600302 |doi-access=free}}</ref>{{sfn|Harris|2009|p={{page needed|date=October 2020}}}}

===Non-shared family environment and environment outside the family===
Although parents treat their children differently, such differential treatment explains only a small amount of nonshared environmental influence. One suggestion is that children react differently to the same environment because of different genes. More likely influences may be the impact of peers and other experiences outside the family.{{sfn|Neisser et al.|1995}}<ref name="Plomin0103"/>

===Individual genes===
A very large proportion of the over 17,000 human genes are thought to have an effect on the development and functionality of the brain.<ref>{{cite journal |last1=Pietropaolo |first1=S. |last2=Crusio |first2=W. E. |doi=10.1002/wcs.135 |pmid=26302082 |title=Genes and cognition |journal=Wiley Interdisciplinary Reviews: Cognitive Science |volume=2 |issue=3 |pages=345–352 |year=2010}}</ref> While a number of individual genes have been reported to be associated with IQ, none have a strong effect. Deary and colleagues (2009) reported that no finding of a strong single gene effect on IQ has been replicated.{{sfn|Deary|Johnson|Houlihan|2009}} Recent findings of gene associations with normally varying intellectual differences in adults and children continue to show weak effects for any one gene.<ref name="Davies2011">{{cite journal |vauthors=Davies G, Tenesa A, Payton A, Yang J, Harris SE, Liewald D, Ke X, Le Hellard S |year=2011 |title=Genome-wide association studies establish that human intelligence is highly heritable and polygenic |journal=Mol Psychiatry |volume=16 |issue=10 |pages=996–1005 |doi=10.1038/mp.2011.85 |pmid=21826061 |pmc=3182557 |display-authors=0}}</ref><ref name="Benyamin2013">{{cite journal |display-authors=8 |vauthors= Benyamin B, Pourcain B, Davis OS, Davies G, Hansell NK, Brion MJ, Kirkpatrick RM, Cents RA, Franic S, Miller MB, Haworth CM, Meaburn E, Price TS, Evans DM, Timpson N, Kemp J, Ring S, McArdle W, Medland SE, Yang J, Harris SE, Liewald DC, Scheet P, Xiao X, Hudziak JJ, de Geus C, ((Wellcome Trust Case Control Consortium 2)), Jaddoe VW, Starr JM, Verhulst FC, Pennell C, Tiemeier H, Iacono WG, Palmer LJ, Montgomery GW, Martin NG, Boomsma DI, Posthuma D, McGue M, Wright MJ, Smith GD, Deary IJ, Plomin R, Visscher PM |year=2013 |title= Childhood intelligence is heritable, highly polygenic and associated with FNBP1L |journal=Mol Psychiatry |volume=19| issue=2| pages=253–258| doi=10.1038/mp.2012.184 |pmid=23358156 |pmc=3935975}}</ref>

A 2017 [[Meta-analysis|meta-analysis]] conducted on approximately 78,000 subjects identified 52 genes associated with intelligence. <ref>Sniekers S, Stringer S, Watanabe K, Jansen PR, Coleman JRI, Krapohl E, Taskesen E, Hammerschlag AR, Okbay A, Zabaneh D, Amin N, Breen G, Cesarini D, Chabris CF, Iacono WG, Ikram MA, Johannesson M, Koellinger P, Lee JJ, Magnusson PKE, McGue M, Miller MB, Ollier WER, Payton A, Pendleton N, Plomin R, Rietveld CA, Tiemeier H, van Duijn CM, Posthuma D. Genome-wide association meta-analysis of 78,308 individuals identifies new loci and genes influencing human intelligence. Nat Genet. 2017 Jul;49(7):1107-1112. doi: 10.1038/ng.3869. Epub 2017 May 22. Erratum in: Nat Genet. 2017 Sep 27;49(10 ):1558. PMID: 28530673; PMCID: PMC5665562.</ref> [[Biology:FNBP1L|FNBP1L]] is reported to be the single gene most associated with both adult and child intelligence.<ref>Benyamin B, Pourcain B, Davis OS, Davies G, Hansell NK, Brion MJ, Kirkpatrick RM, Cents RA, Franić S, Miller MB, Haworth CM, Meaburn E, Price TS, Evans DM, Timpson N, Kemp J, Ring S, McArdle W, Medland SE, Yang J, Harris SE, Liewald DC, Scheet P, Xiao X, Hudziak JJ, de Geus EJ; Wellcome Trust Case Control Consortium 2 (WTCCC2); Jaddoe VW, Starr JM, Verhulst FC, Pennell C, Tiemeier H, Iacono WG, Palmer LJ, Montgomery GW, Martin NG, Boomsma DI, Posthuma D, McGue M, Wright MJ, Davey Smith G, Deary IJ, Plomin R, Visscher PM. Childhood intelligence is heritable, highly polygenic and associated with FNBP1L. Mol Psychiatry. 2014 Feb;19(2):253-8. doi: 10.1038/mp.2012.184. Epub 2013 Jan 29. PMID: 23358156; PMCID: PMC3935975.</ref>

===Gene-environment interaction===
[[Biography:David C. Rowe|David Rowe]] reported an interaction of genetic effects with [[Social:Socioeconomic status|socioeconomic status]], such that the heritability was high in high-SES families, but much lower in low-SES families.<ref name="Rowe">{{cite journal |last1=Rowe |first1=D. C. |last2=Jacobson |first2=K. C. |year=1999 |title=Genetic and environmental influences on vocabulary IQ: parental education level as moderator |journal=Child Development |volume=70 |issue=5 |pages=1151–62 |doi=10.1111/1467-8624.00084|pmid=10546338 |s2cid=10959764}}</ref> In the US, this has been replicated in infants,<ref name="Tucker-Drob2011">{{cite journal |last1=Tucker-Drob |first1=E. M. |last2=Rhemtulla |first2=M. |last3= Harden |first3=K. P. |last4=Turkheimer |first4=E. |last5=Fask |first5=D. |year=2011 |title=Emergence of a Gene x Socioeconomic Status Interaction on Infant Mental Ability Between 10 Months and 2 Years |journal=Psychological Science |volume=22 |issue=1| pages=125–33 |doi=10.1177/0956797610392926 |pmid=21169524 |pmc=3532898}}</ref> children,<ref>{{cite journal |last1=Turkheimer |first1=E. |last2=Haley |first2=A. |last3=Waldron |first3=M. |last4=D'Onofrio |first4=B. |last5=Gottesman |first5=I. I. |year=2003 |title=Socioeconomic status modifies heritability of IQ in young children |journal=Psychological Science |volume=14 |issue=6 |pages=623–628 |doi=10.1046/j.0956-7976.2003.psci_1475.x |pmid=14629696 |s2cid=11265284}}</ref> adolescents,<ref>{{cite journal |last1=Harden |first1=K. P. |last2=Turkheimer |first2=E. |last3=Loehlin |first3=J. C. |year=2005 |title=Genotype environment interaction in adolescents' cognitive ability |doi=10.1007/s10519-005-7287-9 |journal=Behavior Genetics |volume=35 |issue=6| page=804 |s2cid=189842802}}</ref> and adults.<ref>{{cite journal |last1=Bates |first1=Timothy C. |last2=Lewis |first2=Gary J. |last3=Weiss |first3=Alexander |date=3 September 2013 |title=Childhood Socioeconomic Status Amplifies Genetic Effects on Adult Intelligence |journal=Psychological Science |volume=24 |issue=10 |pages=2111–2116 |doi=10.1177/0956797613488394 |pmid=24002887 |s2cid=1873699 |url=https://www.pure.ed.ac.uk/ws/files/10602446/GxSES_Bates_Lewis_Weiss_2013_Psychological_Science.pdf|hdl=20.500.11820/52797d10-f0d4-49de-83e2-a9cc3493703d |hdl-access=free }}</ref> Outside the US, studies show no link between heritability and SES.<ref name=":0">{{cite journal |last1=Tucker-Drob |first1=Elliot M. |last2=Bates |first2=Timothy C. |title=Large Cross-National Differences in Gene × Socioeconomic Status Interaction on Intelligence |journal=Psychological Science |date=15 December 2015 |volume=27 |issue=2 |pages=138–149 |doi=10.1177/0956797615612727 |pmid=26671911 |pmc=4749462}}</ref> Some effects may even reverse sign outside the US.<ref name=":0" /><ref name="Hanscombe2012">{{cite journal |last1=Hanscombe |first1=K. B. |last2=Trzaskowski |first2=M. |last3=Haworth |first3=C. M. |last4=Davis |first4=O. S. |last5=Dale |first5=P. S. |last6=Plomin |first6=R. |year=2012 |title=Socioeconomic Status (SES) and Children's Intelligence (IQ): In a UK-Representative Sample SES Moderates the Environmental, Not Genetic, Effect on IQ. |journal=PLOS ONE |volume=7 |issue=2 |page=e30320 |doi=10.1371/journal.pone.0030320 |pmid=22312423 |pmc=3270016 |bibcode=2012PLoSO...730320H|doi-access=free }}</ref>

Dickens and Flynn (2001) have argued that genes for high IQ initiate an environment-shaping [[Feedback|feedback cycle]], with genetic effects causing bright children to seek out more stimulating environments that then further increase their IQ. In Dickens' model, environment effects are modeled as decaying over time. In this model, the Flynn effect can be explained by an increase in environmental stimulation independent of it being sought out by individuals. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they enduringly raised children's drive to seek out cognitively demanding experiences.<ref name="DickensFlynn2001">{{cite journal |last1=Dickens |first1=William T. |last2=Flynn |first2=James R. |title=Heritability estimates versus large environmental effects: The IQ paradox resolved. |journal=Psychological Review |volume=108 |issue=2 |pages=346–69 |year=2001 |pmid=11381833 |doi=10.1037/0033-295X.108.2.346 |url=http://www.apa.org/journals/features/rev1082346.pdf |citeseerx=10.1.1.139.2436}}</ref><ref>{{cite journal|first1=William T. |last1=Dickens |first2=James R. |last2=Flynn |title=The IQ Paradox: Still Resolved |journal=Psychological Review |volume=109 |issue=4 |year=2002 |doi=10.1037/0033-295x.109.4.764 |pages=764–771 |url=http://www.brookings.edu/views/papers/dickens/20020205.pdf |url-status=dead |archive-url= https://web.archive.org/web/20070319031706/http://www.brookings.edu/views/papers/dickens/20020205.pdf |archive-date=19 March 2007}}</ref>

==Interventions==
In general, educational interventions, as those described below, have shown short-term effects on IQ, but long-term follow-up is often missing. For example, in the US, very large intervention programs such as the Head Start Program have not produced lasting gains in IQ scores. Even when students improve their scores on standardized tests, they do not always improve their cognitive abilities, such as memory, attention and speed.<ref>{{Cite news |last=Bidwell|first=Allie|date=13 December 2013|title=Study: High Standardized Test Scores Don't Translate to Better Cognition|work=U.S. News & World Report |url=https://www.usnews.com/news/articles/2013/12/13/study-high-standardized-test-scores-dont-translate-to-better-cognition |url-status=dead |access-date=1 July 2017 |archive-url=https://web.archive.org/web/20131214082855/https://www.usnews.com/news/articles/2013/12/13/study%2Dhigh%2Dstandardized%2Dtest%2Dscores%2Ddont%2Dtranslate%2Dto%2Dbetter%2Dcognition |archive-date=14 December 2013}}</ref> More intensive, but much smaller projects, such as the Abecedarian Project, have reported lasting effects, often on socioeconomic status variables, rather than IQ.{{sfn|Neisser et al.|1995}}

Recent studies have shown that training in using one's [[Working memory|working memory]] may increase IQ. A study on young adults published in April 2008 by a team from the Universities of Michigan and Bern supports the possibility of the transfer of fluid intelligence from specifically designed [[Working memory training|working memory training]].<ref name="ReferenceA">{{cite journal |last1=Jaeggi |first1=S. M. |last2=Buschkuehl |first2=M. |last3=Jonides |first3=J. |last4=Perrig |first4=W. J. |title=From the Cover: Improving fluid intelligence with training on working memory |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=19 |pages=6829–33 |year=2008 |doi=10.1073/pnas.0801268105 |pmc=2383929 |pmid=18443283|bibcode=2008PNAS..105.6829J|doi-access=free }}</ref> Further research will be needed to determine nature, extent and duration of the proposed transfer. Among other questions, it remains to be seen whether the results extend to other kinds of fluid intelligence tests than the matrix test used in the study, and if so, whether, after training, fluid intelligence measures retain their correlation with educational and occupational achievement or if the value of fluid intelligence for predicting performance on other tasks changes. It is also unclear whether the training is durable for extended periods of time.<ref>{{cite journal |last1=Sternberg |first1=R. J. |title=Increasing fluid intelligence is possible after all |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=19 |pages=6791–2 |year=2008 |doi=10.1073/pnas.0803396105 |pmc=2383939 |pmid=18474863|bibcode=2008PNAS..105.6791S|doi-access=free }}</ref>

==Music==
{{further|Social:Mozart effect}}
Musical training in childhood correlates with higher than average IQ.<ref name="glenn">{{cite journal |last1=Glenn Schellenberg |first1=E. |title=Music Lessons Enhance IQ |journal=Psychological Science |volume=15 |issue=8 |pages=511–514 |year=2004 |pmid=15270994 |doi=10.1111/j.0956-7976.2004.00711.x|doi-access=free |citeseerx=10.1.1.152.4349}}</ref><ref>{{cite journal |last1=Glenn Schellenberg |first1=E. |title=Long-term positive associations between music lessons and IQ |journal=Journal of Educational Psychology |volume=98 |issue=2 |pages=457–468 |year=2006 |doi=10.1037/0022-0663.98.2.457 |citeseerx= 10.1.1.397.5160}}</ref> However, a study of 10,500 twins found no effects on IQ, suggesting that the correlation was caused by genetic confounders.<ref>{{cite journal |last1=Mosing |first1=Miriam A. |last2=Madison |first2=Guy |last3=Pedersen |first3=Nancy L. |last4=Ullén |first4=Fredrik |title=Investigating cognitive transfer within the framework of music practice: genetic pleiotropy rather than causality |journal=Developmental Science |volume=19 |issue=3 |pages=504–512 |date=1 May 2015 |doi=10.1111/desc.12306 |pmid=25939545}}</ref> A meta-analysis concluded that "Music training does not reliably enhance children and young adolescents' cognitive or academic skills, and that previous positive findings were probably due to confounding variables."<ref>{{Cite journal|last1=Sala|first1=Giovanni |last2= Gobet|first2=Fernand|date=1 February 2017|title=When the music's over. Does music skill transfer to children's and young adolescents' cognitive and academic skills? A meta-analysis|journal=Educational Research Review |volume=20|pages=55–67|doi=10.1016/j.edurev.2016.11.005|issn=1747-938X|doi-access=free}}</ref>

It is popularly thought that listening to classical music raises IQ. However, multiple attempted replications (e.g.<ref>{{cite journal |last1=Stough |first1=C. |last2=Kerkin |first2=B. |last3=Bates |first3=T. C. |last4=Mangan |first4=G. |year=1994 |title=Music and spatial IQ |journal=Personality and Individual Differences |volume=17 |issue=5 |page=695 |doi=10.1016/0191-8869(94)90145-7 |url=https://www.researchgate.net/publication/222240707 |access-date=3 October 2020}}</ref>) have shown that this is at best a short-term effect (lasting no longer than 10 to 15 minutes), and is not related to IQ-increase.<ref>{{cite journal |last1=Chabris |first1=C. F. |year=1999 |title=Prelude or requiem for the 'Mozart effect'? |doi=10.1038/23608 |journal=Nature |volume=400 |issue=6747 |pages=826–827 |pmid=10476958 |bibcode=1999Natur.400..826C |s2cid=898161}}</ref>

==Brain anatomy==
{{Main|Biology:Neuroscience and intelligence}}

Several neurophysiological factors have been correlated with intelligence in humans, including the ratio of brain weight to body weight and the size, shape, and activity level of different parts of the brain. Specific features that may affect IQ include the size and shape of the frontal lobes, the amount of blood and chemical activity in the frontal lobes, the total amount of gray matter in the brain, the overall thickness of the cortex, and the glucose metabolic rate.<ref>{{cite journal | last1 = Deary | first1 = I.J. | last2 = Penke | first2 = L. | last3 = Johnson | first3 = W. | year = 2010 | title = The neuroscience of human intelligence differences | url = http://www.larspenke.eu/pdfs/Deary_Penke_Johnson_2010_-_Neuroscience_of_intelligence_review.pdf | journal = Nature Reviews Neuroscience | volume = 11 | issue = 3| pages = 201–211 | doi=10.1038/nrn2793 | pmid=20145623| s2cid = 5136934| hdl = 20.500.11820/9b11fac3-47d0-424c-9d1c-fe6f9ff2ecac | hdl-access = free }}</ref>

==Health==
Health is important in understanding differences in IQ test scores and other measures of cognitive ability. Several factors can lead to significant cognitive impairment, particularly if they occur during pregnancy and childhood when the brain is growing and the blood–brain barrier is less effective. Such impairment may sometimes be permanent, or sometimes be partially or wholly compensated for by later growth.<ref>{{Cite journal |last1=Buss |first1=Claudia |last2=Entringer |first2=Sonja |last3=Swanson |first3=James M. |last4=D. Wadhwa |first4=Pathik |date=2012 |title=The Role of Stress in Brain Development: The Gestational Environment's Long-Term Effects on the Brain |journal=Cerebrum |volume=4 |page=4 |pmid=23447790 |pmc=3574809 }}</ref>

Since about 2010, researchers such as Eppig, Hassel, and MacKenzie have found a very close and consistent link between IQ scores and infectious diseases, especially in the infant and preschool populations and the mothers of these children.<ref name=":7">Eppig. Christopher. Scientific American."Why is average IQ higher in some places?" 2011.</ref> They have postulated that fighting infectious diseases strains the child's metabolism and prevents full brain development. Hassel postulated that it is by far the most important factor in determining population IQ. However, they also found that subsequent factors such as good nutrition and regular quality schooling can offset early negative effects to some extent.

Developed nations have implemented several health policies regarding nutrients and toxins known to influence cognitive function. These include laws requiring fortification of certain food products<ref>{{Cite journal |last1=Olson |first1=Rebecca |last2=Gavin-Smith |first2=Breda |last3=Ferraboschi |first3=Chiara |last4=Kraemer |first4=Klaus |date=April 2021 |title=Food Fortification: The Advantages, Disadvantages and Lessons from Sight and Life Programs |journal=Nutrients |volume=13 |issue=4 |pages=1118 |doi=10.3390/nu13041118 |pmid=33805305 |pmc=8066912 |doi-access=free }}</ref> and laws establishing safe levels of pollutants (e.g. [[Chemistry:Lead|lead]], [[Chemistry:Mercury (element)|mercury]], and organochlorides). Improvements in nutrition, and in public policy in general, have been implicated in IQ increases.<ref>{{Cite news |last=Nisen |first=Max |date=July 22, 2013 |title=How Adding Iodine To Salt Resulted In A Decade's Worth Of IQ Gains For The United States |work=Business Insider |url=https://www.businessinsider.com/iodization-effect-on-iq-2013-7#ixzz2ZuFjmyY7 |access-date=December 16, 2023}}</ref>

[[Philosophy:Cognitive epidemiology|Cognitive epidemiology]] is a field of research that examines the associations between intelligence test scores and health. Researchers in the field argue that intelligence measured at an early age is an important predictor of later health and mortality differences.{{sfn|Deary|Batty|2007}}

==Social correlations==

===School performance===
The [[Organization:American Psychological Association|American Psychological Association]]'s report ''Intelligence: Knowns and Unknowns'' states that wherever it has been studied, children with high scores on tests of intelligence tend to learn more of what is taught in school than their lower-scoring peers. The correlation between IQ scores and grades is about .50. This means that the explained variance is 25%. Achieving good grades depends on many factors other than IQ, such as "persistence, interest in school, and willingness to study" (p.&nbsp;81).{{sfn|Neisser et al.|1995}}

It has been found that the correlation of IQ scores with school performance depends on the IQ measurement used. For undergraduate students, the Verbal IQ as measured by WAIS-R has been found to correlate significantly (0.53) with the grade point average (GPA) of the last 60 hours (credits). In contrast, Performance IQ correlation with the same GPA was only 0.22 in the same study.<ref>{{cite book |last=Kamphaus |first=Randy W. |title=Clinical assessment of child and adolescent intelligence |publisher=Springer |year=2005 | isbn=978-0-387-26299-4}}</ref>

Some measures of educational aptitude correlate highly with IQ tests{{snd}}for instance, {{harvp|Frey|Detterman|2004}} reported a correlation of 0.82 between ''g'' (general intelligence factor) and SAT scores;{{sfn|Frey|Detterman|2004}} another research found a correlation of 0.81 between ''g'' and GCSE scores, with the explained variance ranging "from 58.6% in Mathematics and 48% in English to 18.1% in Art and Design".{{sfn|Deary|Strand|Smith|Fernandes|2007}}

===Job performance===
According to Schmidt and Hunter, "for hiring employees without previous experience in the job the most valid predictor of future performance is general mental ability."<ref name="Schmidt98">{{cite journal |last1=Schmidt |first1=Frank L. |last2=Hunter |first2=John E. |title=The validity and utility of selection methods in personnel psychology: Practical and theoretical implications of 85 years of research findings |journal=Psychological Bulletin |volume=124 |issue=2 |pages=262–74 |year=1998 |citeseerx=10.1.1.172.1733 |doi=10.1037/0033-2909.124.2.262 |s2cid=16429503 |url=http://www.moityca.com.br/pdfs/SchmidteHunter1998.pdf |access-date=25 October 2017 |archive-url=https://web.archive.org/web/20140602034440/http://www.moityca.com.br/pdfs/SchmidteHunter1998.pdf |archive-date=2 June 2014 |url-status=dead}}</ref> The validity of IQ as a predictor of [[Social:Job performance|job performance]] is above zero for all work studied to date, but varies with the type of job and across different studies, ranging from 0.2 to 0.6.<ref name="Hunter84">{{cite journal |last1=Hunter |first1=John E. |last2=Hunter |first2=Ronda F. |title=Validity and utility of alternative predictors of job performance |journal=Psychological Bulletin |volume=96 |issue=1 |pages=72–98 |year=1984 |doi=10.1037/0033-2909.96.1.72 |s2cid=26858912}}</ref> The correlations were higher when the unreliability of measurement methods was controlled for.{{sfn|Neisser et al.|1995}} While IQ is more strongly correlated with reasoning and less so with motor function,<ref>{{cite journal |last1=Warner |first1=Molly |last2=Ernst |first2=John |last3=Townes |first3=Brenda |last4=Peel |first4=John |last5=Preston |first5=Michael |title=Relationships Between IQ and Neuropsychological Measures in Neuropsychiatric Populations: Within-Laboratory and Cross-Cultural Replications Using WAIS and WAIS-R |journal=Journal of Clinical and Experimental Neuropsychology |volume=9 |issue=5 |pages=545–62 |year=1987 |pmid=3667899 |doi=10.1080/01688638708410768}}</ref> IQ-test scores predict performance ratings in all occupations.<ref name="Schmidt98"/> 

That said, for highly qualified activities (research, management) low IQ scores are more likely to be a barrier to adequate performance, whereas for minimally-skilled activities, athletic strength (manual strength, speed, stamina, and coordination) is more likely to influence performance.<ref name="Schmidt98"/> The prevailing view among academics is that it is largely through the quicker acquisition of job-relevant knowledge that higher IQ mediates job performance. This view has been challenged by Byington & Felps (2010), who argued that "the current applications of IQ-reflective tests allow individuals with high IQ scores to receive greater access to developmental resources, enabling them to acquire additional capabilities over time, and ultimately perform their jobs better."<ref>{{Cite journal |last1=Byington |first1=Eliza |last2=Felps |first2=Will |title=Why do IQ scores predict job performance? |journal=Research in Organizational Behavior |volume=30 |pages=175–202 |doi=10.1016/j.riob.2010.08.003 |year=2010}}</ref>

Newer studies find that the effects of IQ on job performance have been greatly overestimated. The current estimates of the correlation between job performance and IQ are about 0.23 correcting for unreliability and range restriction.<ref>{{cite journal |url=https://ink.library.smu.edu.sg/lkcsb_research/7188/ |title=Revisiting the design of selection systems in light of new findings regarding the validity of widely used predictors |last1=Sackett |first1=Paul R. |last2=Zhang |first2=Charlene |last3=Berry |first3=Christopher M. |last4=Lievens |first4=Filip |year=2023 |journal=Industrial and Organizational Psychology: Perspectives on Science and Practice |volume=16 |issue=3 |access-date=24 April 2023}}</ref><ref>{{cite journal|doi=10.1037/apl0000994 |title=Revisiting meta-analytic estimates of validity in personnel selection: Addressing systematic overcorrection for restriction of range |year=2022 |last1=Sackett |first1=Paul R. |last2=Zhang |first2=Charlene |last3=Berry |first3=Christopher M. |last4=Lievens |first4=Filip |journal=Journal of Applied Psychology |volume=107 |issue=11 |pages=2040–2068 |pmid=34968080 |s2cid=245594032 |url=https://ink.library.smu.edu.sg/lkcsb_research/6894 }}</ref>

In establishing a causal direction to the link between IQ and work performance, longitudinal studies by Watkins and others suggest that IQ exerts a causal influence on future academic achievement, whereas academic achievement does not substantially influence future IQ scores.<ref>{{cite journal |last1=Watkins |first1=M |last2=Lei |first2=P |last3=Canivez |first3=G |title=Psychometric intelligence and achievement: A cross-lagged panel analysis |journal=Intelligence |volume=35 |issue=1 |pages=59–68 |year=2007 |doi=10.1016/j.intell.2006.04.005|citeseerx=10.1.1.397.3155}}</ref> Treena Eileen Rohde and Lee Anne Thompson write that general cognitive ability, but not specific ability scores, predict academic achievement, with the exception that processing speed and spatial ability predict performance on the SAT math beyond the effect of general cognitive ability.<ref>{{cite journal |last1=Rohde |first1=T |last2=Thompson |first2=L |title=Predicting academic achievement with cognitive ability |journal=Intelligence |volume=35 |issue=1 |pages=83–92 |year=2007 |doi=10.1016/j.intell.2006.05.004}}</ref>

However, large-scale longitudinal studies indicate an increase in IQ translates into an increase in performance at all levels of IQ: i.e. ability and job performance are [[Monotonic function|monotonically]] linked at all IQ levels.<ref>{{cite journal |last1=Coward |first1=W. Mark |last2=Sackett |first2=Paul R. |title=Linearity of ability-performance relationships: A reconfirmation |journal=Journal of Applied Psychology |volume=75 |issue=3 |pages=297–300 |year=1990 |doi=10.1037/0021-9010.75.3.297}}</ref><ref>{{cite journal |last1=Robertson |first1=Kimberley Ferriman |last2=Smeets |first2=Stijn |last3=Lubinski |first3=David |last4=Benbow |first4=Camilla P. |title=Beyond the Threshold Hypothesis |journal=Current Directions in Psychological Science |date=14 December 2010 |volume=19 |issue=6 |pages=346–351 |doi=10.1177/0963721410391442 |s2cid=46218795 |url=https://my.vanderbilt.edu/smpy/files/2013/02/Ferriman_20101.pdf}}</ref>

===Income===
<!-- This whole section needs to be checked against the sources for missing information and undue weight -->
<!--
{| class="wikitable"
|-
|+ '''Relation between IQ and earnings in the U.S.'''
! IQ || <75 || 75–90 || 90–110 || 110–125 || >125
|-
| Age 18 || 2,000 || 5,000 || 8,000 || 8,000 || 21,000
|-
| Age 26 || 3,000 || 10,000 || 16,000 || 20,000 || 42,000
|-
| Age 32 || 5,000 || 12,400 || 20,000 || 27,000 || 48,000
|-
| colspan="6" | Values are the average earnings (1993 US Dollars) of each IQ sub-population.<ref> {{cite journal | last1 = Murray | first1 = C | year = 1997 | title = IQ and economic success | journal = Public Interest | volume = 128 | pages = 21–35}}</ref>
|}
-->

It has been suggested that "in economic terms it appears that the IQ score measures something with decreasing marginal value" and it "is important to have enough of it, but having lots and lots does not buy you that much".{{sfn |Detterman|Daniel|1989}}<ref>
{{cite web
 |url=http://www.americanscientist.org/template/AssetDetail/assetid/24538/page/4
 |title=The Role of Intelligence in Modern Society (July–Aug, 1995)
 |pages=4 (Nonlinearities in Intelligence)
 |date=July 1995
 |first=Earl B. |last=Hunt
 |publisher=American Scientist
|archive-url=https://web.archive.org/web/20060521185816/http://www.americanscientist.org/template/AssetDetail/assetid/24538/page/4 |archive-date=21 May 2006}}</ref>

The link from IQ to wealth is much less strong than that from IQ to job performance. Some studies indicate that IQ is unrelated to net worth.<ref>{{Cite news |work=The Times |location=London |title=Brains don't make you rich IQ study finds |first=Mark |last=Henderson |date=25 April 2007 |url=http://www.timesonline.co.uk/tol/news/uk/science/article1701377.ece |access-date=5 May 2010}}</ref><ref>{{cite web |title=You Don't Have To Be Smart To Be Rich, Study Finds |website=ScienceDaily |url=https://www.sciencedaily.com/releases/2007/04/070424204519.htm |access-date=26 August 2014}}</ref> The American Psychological Association's 1995 report ''[[Philosophy:Intelligence: Knowns and Unknowns|Intelligence]]'' stated that IQ scores accounted for about a quarter of the social status variance and one-sixth of the income variance. Statistical controls for parental SES eliminate about a quarter of this predictive power. Psychometric intelligence appears as only one of a great many factors that influence social outcomes.{{sfn|Neisser et al.|1995}} Charles Murray (1998) showed a more substantial effect of IQ on income independent of family background.{{sfn|Murray|1998|p={{Page needed|date=January 2011}}}} In a meta-analysis, Strenze (2006) reviewed much of the literature and estimated the correlation between IQ and income to be about 0.23.<ref name="Strenze2007">{{cite journal |last1=Strenze |first1=Tarmo |date=September 2007 |title=Intelligence and socioeconomic success: A meta-analytic review of longitudinal research |journal=Intelligence |volume=35 |issue=5 |pages=401–426 |doi=10.1016/j.intell.2006.09.004 |quote=The correlation with income is considerably lower, perhaps even disappointingly low, being about the average of the previous meta-analytic estimates (.15 by Bowles et al., 2001; and .27 by Ng et al., 2005). But...other predictors, studied in this paper, are not doing any better in predicting income, which demonstrates that financial success is difficult to predict by any variable. This assertion is further corroborated by the meta-analysis of Ng et al. (2005) where the best predictor of salary was educational level with a correlation of only .29. It should also be noted that the correlation of .23 is about the size of the average meta-analytic result in psychology(Hemphill, 2003) and cannot, therefore, be treated as insignificant.}}</ref>

Some studies assert that IQ only accounts for (explains) a sixth of the variation in income because many studies are based on young adults, many of whom have not yet reached their peak earning capacity, or even their education. On pg 568 of ''The g Factor'', [[Biography:Arthur Jensen|Arthur Jensen]] says that although the correlation between IQ and income averages a moderate 0.4 (one-sixth or 16% of the variance), the relationship increases with age, and peaks at middle age when people have reached their maximum career potential. In the book, ''A Question of Intelligence'', Daniel Seligman cites an IQ income correlation of 0.5 (25% of the variance).

A 2002 study<ref>{{cite journal |last1=Bowles |first1=Samuel |last2=Gintis |first2=Herbert |title=The Inheritance of Inequality |journal=Journal of Economic Perspectives |volume=16 |issue=3 |pages=3–30 |year=2002 |doi=10.1257/089533002760278686|doi-access=free}}</ref> further examined the impact of non-IQ factors on income and concluded that an individual's location, inherited wealth, race, and schooling are more important as factors in determining income than IQ.

===Crime===
The American Psychological Association's 1995 report ''Intelligence: Knowns and Unknowns'' stated that the [[Correlates of crime#Psychological traits|correlation between IQ and crime]] was −0.2. This association is generally regarded as small and prone to disappearance or a substantial reduction after controlling for the proper covariates, being much smaller than typical sociological correlates.<ref>{{cite journal |last1=Cullen |first1=Francis T. |last2=Gendreau |first2=Paul |last3=Jarjoura |first3=G. Roger |last4=Wright |first4=John Paul |title=Crime and the Bell Curve: Lessons from Intelligent Criminology |journal=Crime & Delinquency |date=October 1997 |volume=43 |issue=4 |pages=387–411 |doi=10.1177/0011128797043004001 |s2cid=145418972}}</ref> It was −0.19 between IQ scores and the number of juvenile offenses in a large Danish sample; with social class controlled for, the correlation dropped to −0.17. A correlation of 0.20 means that the explained variance accounts for 4% of the total variance. The causal links between psychometric ability and social outcomes may be indirect. Children with poor scholastic performance may feel alienated. Consequently, they may be more likely to engage in delinquent behavior, compared to other children who do well.{{sfn|Neisser et al.|1995}}

In his book ''The g Factor'' (1998), [[Biography:Arthur Jensen|Arthur Jensen]] cited data which showed that, regardless of race, people with IQs between 70 and 90 have higher crime rates than people with IQs below or above this range, with the peak range being between 80 and 90.

The 2009 ''Handbook of Crime Correlates'' stated that reviews have found that around eight IQ points, or 0.5 SD, separate criminals from the general population, especially for persistent serious offenders. It has been suggested that this simply reflects that "only dumb ones get caught" but there is similarly a negative relation between IQ and self-reported offending. That children with [[Medicine:Conduct disorder|conduct disorder]] have lower IQ than their peers "strongly argues" for the theory.<ref>''Handbook of Crime Correlates''; Lee Ellis, Kevin M. Beaver, John Wright; 2009; Academic Press</ref>

A study of the relationship between US county-level IQ and US county-level crime rates found that higher average IQs were very weakly associated with lower levels of property crime, burglary, larceny rate, motor vehicle theft, violent crime, robbery, and aggravated assault. These results were "not confounded by a measure of concentrated disadvantage that captures the effects of race, poverty, and other social disadvantages of the county."<ref>{{cite journal |last1=Beaver |first1=Kevin M. |last2=Schwartz |first2=Joseph A. |last3=Nedelec |first3=Joseph L. |last4=Connolly |first4=Eric J. |last5=Boutwell |first5=Brian B. |last6=Barnes |first6=J.C. |title=Intelligence is associated with criminal justice processing: Arrest through incarceration |journal=Intelligence |date=September 2013 |volume=41 |issue=5 |pages=277–288 |doi=10.1016/j.intell.2013.05.001}}</ref> However, this study is limited in that it extrapolated Add Health estimates to the respondent's counties, and as the dataset was not designed to be representative on the state or county level, it may not be generalizable.<ref>{{cite journal |last1=Beaver |first1=Kevin M. |last2=Wright |first2=John Paul |title=The association between county-level IQ and county-level crime rates |journal=Intelligence |date=January 2011 |volume=39 |issue=1 |pages=22–26 |doi=10.1016/j.intell.2010.12.002}}</ref>

It has also been shown that the effect of IQ is heavily dependent on socioeconomic status and that it cannot be easily controlled away, with many methodological considerations being at play.<ref>{{cite journal |last1=Mears |first1=Daniel P. |last2=Cochran |first2=Joshua C. |title=What is the effect of IQ on offending? |journal=Criminal Justice and Behavior |date=November 2013 |volume=40 |issue=11 |pages=1280–1300 |doi=10.1177/0093854813485736 |s2cid=147219554}}</ref> Indeed, there is evidence that the small relationship is mediated by well-being, substance abuse, and other confounding factors that prohibit simple causal interpretation.<ref>{{cite journal |last1=Freeman |first1=James |title=The relationship between lower intelligence, crime and custodial outcomes: a brief literary review of a vulnerable group |journal=Vulnerable Groups & Inclusion |date=January 2012 |volume=3 |issue=1 |pages=14834 |doi=10.3402/vgi.v3i0.14834 |s2cid=145305072|doi-access=free }}</ref> A recent meta-analysis has shown that the relationship is only observed in higher risk populations such as those in poverty without direct effect, but without any causal interpretation.<ref>{{cite journal |last1=Ttofi |first1=Maria M. |last2=Farrington |first2=David P. |last3=Piquero |first3=Alex R. |last4=Lösel |first4=Friedrich |last5=DeLisi |first5=Matthew |last6=Murray |first6=Joseph |title=Intelligence as a protective factor against offending: A meta-analytic review of prospective longitudinal studies |journal=Journal of Criminal Justice |date=1 June 2016 |volume=45 |pages=4–18 |doi=10.1016/j.jcrimjus.2016.02.003}}</ref> A nationally representative longitudinal study has shown that this relationship is entirely mediated by school performance.<ref>{{cite journal |last1=McGloin |first1=Jean Marie |last2=Pratt |first2=Travis C. |last3=Maahs |first3=Jeff |title=Rethinking the IQ-delinquency relationship: A longitudinal analysis of multiple theoretical models |journal=Justice Quarterly |date=1 September 2004 |volume=21 |issue=3 |pages=603–635 |doi=10.1080/07418820400095921 |s2cid=143305924}}</ref>

===Health and mortality===
Multiple studies conducted in Scotland have found that higher IQs in early life are associated with lower mortality and morbidity rates later in life.<ref>{{Cite journal |last1=Gottfredson |first1=Linda S. |last2=Deary |first2=Ian J. |date=22 June 2016 |title=Intelligence Predicts Health and Longevity, but Why? |journal=Current Directions in Psychological Science |language=en |volume=13 |issue=1 |pages=1–4 |doi=10.1111/j.0963-7214.2004.01301001.x |s2cid=15176389}}</ref><ref>{{Cite journal |last1=Batty |first1=G. David |last2=Deary |first2=Ian J. |last3=Gottfredson |first3=Linda S. |title=Premorbid (early life) IQ and Later Mortality Risk: Systematic Review |journal=Annals of Epidemiology |volume=17 |issue=4 |pages=278–288 |doi=10.1016/j.annepidem.2006.07.010 |pmid=17174570 |year=2007|citeseerx=10.1.1.693.9671}}</ref>

===Other accomplishments===
<!-- This whole section, repetitive of a section above, needs to be checked against reliable sources for missing information and due weight -->
{| class=wikitable
|+ Average adult combined IQs associated with real-life accomplishments by various tests:{{sfn|Kaufman|2009|p=126}}{{sfn|Kaufman|Lichtenberger|2006}}
! Accomplishment !! IQ !! Test/study !! Year
|-
| MDs, JDs, and PhDs || 125 || WAIS-R || 1987
|-
| rowspan=3 | College graduates || rowspan=2 | 112 || KAIT || 2000
|-
| K-BIT || 1992
|-
| 115 || WAIS-R ||
|-
| rowspan=3 | 1–3 years of college || rowspan=2 | 104 || KAIT ||
|-
| K-BIT ||
|-
| 105–110 || WAIS-R ||
|-
| Clerical and sales workers || 100–105 || ||
|-
| rowspan=3 | High school graduates, skilled workers (e.g., electricians, cabinetmakers) || rowspan=2 | 100 || KAIT ||
|-
| WAIS-R ||
|-
| 97 || K-BIT ||
|-
| rowspan=3 | 1–3 years of high school (completed 9–11 years of school) || 94 || KAIT ||
|-
| 90 || K-BIT ||
|-
| 95 || WAIS-R ||
|-
| Semi-skilled workers (e.g. truck drivers, factory workers) || 90–95 || ||
|-
| Elementary school graduates (completed eighth grade) || 90 || ||
|-
| Elementary school dropouts (completed 0–7 years of school) || 80–85 || ||
|-
| Have 50/50 chance of reaching high school || 75 || ||
|}

{| class=wikitable
|+ Average IQ of various occupational groups:{{sfn|Kaufman|2009|p=132}}
! Accomplishment !! IQ !! Test/study !! Year
|-
| Professional and technical || 112 || ||
|-
| Managers and administrators || 104 || ||
|-
| Clerical workers, sales workers, skilled workers, craftsmen, and foremen || 101 || ||
|-
| Semi-skilled workers (operatives, service workers, including private household) || 92 || ||
|-
| Unskilled workers || 87 || ||
|}

{| class=wikitable
|+ Type of work that can be accomplished:{{sfn|Kaufman|2009|p=126}}
! Accomplishment !! IQ !! Test/study !! Year
|-
| Adults can harvest vegetables, repair furniture || 60 || ||
|-
| Adults can do domestic work || 50|| ||
|}

There is considerable variation within and overlap among these categories. People with high IQs are found at all levels of education and occupational categories. The biggest difference occurs for low IQs with only an occasional college graduate or professional scoring below 90.<ref name=Kaufman2009/>

==Group differences==
Among the most controversial issues related to the study of intelligence is the observation that IQ scores vary on average between ethnic and racial groups, though these differences have fluctuated and in many cases steadily decreased over time.<ref name=":8">{{Cite journal |last1=Nisbett |first1=Richard E. |last2=Aronson |first2=Joshua |last3=Blair |first3=Clancy |last4=Dickens |first4=William |last5=Flynn |first5=James |last6=Halpern |first6=Diane F. |last7=Turkheimer |first7=Eric |date=2012 |title=Intelligence: New findings and theoretical developments |journal=American Psychologist |volume=67 |issue=2 |pages=130–159 |doi=10.1037/a0026699 |issn=1935-990X |pmid=22233090}}</ref> While there is little scholarly debate about the continued existence of some of these differences, the current scientific consensus is that they stem from environmental rather than genetic causes.{{sfn|Ceci|Williams|2009|pages=788–789, "There is an emerging consensus about racial and gender equality in genetic determinants of intelligence; most researchers, including ourselves, agree that genes do not explain between-group differences"}}<ref>{{Cite journal |date=25 May 2017 |title=Intelligence research should not be held back by its past |journal=Nature |volume=545 |issue=7655 |pages=385–386 |doi=10.1038/nature.2017.22021 |pmid=28541341 |bibcode=2017Natur.545R.385. |s2cid=4449918 |quote=Intelligence science has undoubtedly been dogged by ugly prejudice. Historical measurements of skull volume and brain weight were done to advance claims of the racial superiority of white people. More recently, the (genuine but closing) gap between the average IQ scores of groups of black and white people in the United States has been falsely attributed to genetic differences between the races.|doi-access=free }}</ref><ref name=":03" /> The existence of differences in IQ between the sexes has been debated, and largely depends on which tests are performed.<ref name="Mackintosh2">{{harvnb|Mackintosh|2011|pp=[https://books.google.com/books?id=BcKcAQAAQBAJ&pg=PA362 362–363]}}</ref><ref name="Hunt 389">{{harvnb|Hunt|2011|p=[https://books.google.com/books?id=DwO4TtKAiCoC&pg=PA389 389]}}</ref>

===Race===

A 1996 task force investigation on intelligence sponsored by the [[Organization:American Psychological Association|American Psychological Association]] concluded that there were significant variations in IQ across races.{{sfn|Neisser et al.|1995}} However, a systematic analysis by William Dickens and James Flynn (2006) showed the gap between black and white Americans to have closed dramatically during the period between 1972 and 2002, suggesting that, in their words, the "constancy of the Black–White IQ gap is a myth".<ref name=":32">{{Cite journal|last1=Dickens|first1=William T.|last2=Flynn|first2=James R.|date=2006|title=Black Americans Reduce the Racial IQ Gap: Evidence from Standardization Samples |url=http://www.iapsych.com/iqmr/fe/LinkedDocuments/dickens2006a.pdf|journal=Psychological Science |volume=17|issue=10|pages=913–920|doi=10.1111/j.1467-9280.2006.01802.x|pmid=17100793|s2cid=6593169}}</ref>

The problem of determining the causes underlying racial variation has been discussed at length as a classic question of "nature versus nurture", for instance by Alan S. Kaufman{{sfn|Kaufman|2009|page=[https://archive.org/details/iqtestingpsych00phdd/page/n187 173]}} and Nathan Brody.{{sfn|Brody|2005|p={{page needed|date=October 2020}}}} Researchers such as statistician Bernie Devlin have argued that there are insufficient data to conclude that the black–white gap is due to genetic influences.<ref>{{Cite book |url=https://archive.org/details/intelligencegene00bern |title=Intelligence, Genes, and Success: Scientists Respond to the Bell Curve|publisher=[[Company:Springer Science+Business Media|Springer Verlag]]|year=1997|isbn=978-0-387-98234-2 |editor1=Bernie Devlin|location=New York |editor2=Stephen E. Fienberg|editor3=Daniel P. Resnick|editor4=Kathryn Roeder}}{{Page needed|date=January 2011}}</ref> Dickens and Flynn argued more positively that their results refute the possibility of a genetic origin, concluding that "the environment has been responsible" for observed differences.<ref name=":32" /> A review article published in 2012 by leading scholars on human intelligence reached a similar conclusion, after reviewing the prior research literature, that group differences in IQ are best understood as environmental in origin.<ref>{{cite journal |last1=Nisbett |first1=Richard E. |last2=Aronson |first2=Joshua |last3=Blair |first3=Clancy |last4=Dickens |first4=William |last5=Flynn |first5=James |last6=Halpern |first6=Diane F. |last7=Turkheimer |first7=Eric |author-link5=Jim Flynn (academic) |author-link6=Diane F. Halpern |title=Group differences in IQ are best understood as environmental in origin |journal=American Psychologist |doi=10.1037/a0029772 |pmid=22963427 |issn=0003-066X |volume=67 |number=6 |date=2012 |pages=503–504 |url=http://people.virginia.edu/~ent3c/papers2/Articles%20for%20Online%20CV/Nisbett%20(2012)%20Group.pdf |access-date=22 July 2013}}</ref> More recently, geneticist and neuroscientist Kevin Mitchell has argued, on the basis of basic principles of population genetics, that "systematic genetic differences in intelligence between large, ancient populations" are "inherently and deeply implausible".<ref name=":02">{{Cite news|last=Mitchell|first=Kevin |date=2 May 2018|title=Why genetic IQ differences between 'races' are unlikely: The idea that intelligence can differ between populations has made headlines again, but the rules of evolution make it implausible |work=The Guardian|url=https://www.theguardian.com/science/blog/2018/may/02/why-genetic-iq-differences-between-races-are-unlikely|access-date=13 June 2020}}</ref>

The effects of [[Philosophy:Stereotype threat|stereotype threat]] have been proposed as an explanation for differences in IQ test performance between racial groups,{{sfn|Mackintosh|2011|p=348}}<ref>{{cite book |last1=Inzlicht |first1=Michael |date=2011 |title=Stereotype Threat: Theory, Process, and Application |publisher=Oxford University Press |pages=5, 141–143 |isbn= 978-0199732449}}</ref> as have issues related to cultural difference and access to education.<ref>{{Cite book |last1=Shuttleworth-Edwards|first1=Ann B.|title=Minority and cross-cultural aspects of neuropsychological assessment|last2=Van der Merwe|first2=Adele S. |publisher=Swets & Zeitlinger|year=2002|isbn=9026518307|editor-last=Ferraro |editor-first=F. Richard |location=Exton, PA |pages=72–75|chapter=WAIS-III and WISC-IV South African Cross-Cultural Normative Data Stratified for Quality of Education}}</ref><ref name=":6">Barbara P. Uzzell, Marcel Ponton, Alfredo Ardila International Handbook of Cross-Cultural Neuropsychology book {{ISBN|978-0805835854}} (2013)</ref>

===Sex===
{{Main|Philosophy:Sex differences in intelligence}}

With the advent of the concept of ''g'' or general intelligence, many researchers have found that there are no significant sex differences in general intelligence,<ref name="Hunt 389"/><ref name="Plotnik2">{{cite book |url=https://books.google.com/books?id=6jLTCQAAQBAJ&pg=PT324|title=Introduction to Psychology |vauthors=Plotnik R, Kouyoumdjian H|publisher=Cengage Learning|year=2013|isbn=978-1133939535|pages=282–283}}</ref><ref name="Hunt 378-379">{{harvnb|Hunt|2011|pp=[https://books.google.com/books?id=DwO4TtKAiCoC&pg=PA378 378–379]}}</ref> though ability in particular types of intelligence does vary.<ref name="Mackintosh2"/><ref name="Hunt 378-379"/> Thus, while some test batteries show slightly greater intelligence in males, others show greater intelligence in females.<ref name="Mackintosh2"/><ref name="Hunt 378-379"/> In particular, studies have shown female subjects performing better on tasks related to [[Social:Linguistic intelligence|verbal ability]],<ref name="Hunt 389"/> and males performing better on tasks related to rotation of objects in space, often categorized as [[Spatial ability|spatial ability]].<ref name="Terry2">{{cite book |url=https://books.google.com/books?id=4s5WCgAAQBAJ&pg=PA356|title=Learning and Memory: Basic Principles, Processes, and Procedures, Fourth Edition|vauthors=Terry WS|publisher=Psychology Press |year=2015|isbn=978-1317350873 |page=356}}</ref> These differences remain, as {{harvp|Hunt|2011}} observes, "even though men and women are essentially equal in general intelligence".

Some research indicates that male advantages on some [[Philosophy:Cognitive test|cognitive test]]s are minimized when controlling for socioeconomic factors.<ref name="Mackintosh2"/><ref name="Plotnik2"/> Other research has concluded that there is slightly [[Philosophy:Variability hypothesis|larger variability]] in male scores in certain areas compared to female scores, which results in slightly more males than females in the [[Social:Intellectual giftedness|top]] and [[Medicine:Intellectual disability|bottom]] of the IQ distribution.<ref name="Chrisler JC">{{cite book|url= https://books.google.com/books?id=Xtq0M1f_aIMC&pg=PA302 |title=Handbook of Gender Research in Psychology: Volume 1: Gender Research in General and Experimental Psychology |vauthors=Chrisler JC, McCreary DR |publisher=Springer Science & Business Media |year=2010 |isbn=978-1441914651|page=302}}</ref>

The existence of differences between male and female performance on math-related tests is contested,<ref name=":5">{{cite journal|last1=Hyde|first1=J. S.|last2=Linn|first2=M. C.|date=27 October 2006 |title=DIVERSITY: Enhanced: Gender Similarities in Mathematics and Science|journal=Science|volume=314 |issue=5799 |pages=599–600|doi=10.1126/science.1132154|pmid=17068246|s2cid=34045261}}</ref> and a meta-analysis focusing on average gender differences in math performance found nearly identical performance for boys and girls.<ref>{{cite journal|last1=Hyde|first1=Janet S.|last2=Fennema|first2=Elizabeth|last3=Lamon|first3=Susan J.|date=1990|title=Gender differences in mathematics performance: A meta-analysis.|journal=Psychological Bulletin|volume=107|issue=2|pages=139–155|doi=10.1037/0033-2909.107.2.139|pmid=2138794}}</ref> Currently, most IQ tests, including popular batteries such as the WAIS and the WISC-R, are constructed so that there are no overall score differences between females and males.{{sfn|Neisser et al.|1995}}<ref name="apa">{{cite journal |last1=Nisbett |first1=Richard E. |last2=Aronson |first2=Joshua |last3=Blair |first3=Clancy |last4=Dickens |first4=William |last5=Flynn |first5=James |last6=Halpern |first6=Diane F. |last7=Turkheimer |first7=Eric |title=Intelligence: New findings and theoretical developments. |journal=American Psychologist |date=2012 |volume=67 |issue=2 |pages=130–159 |doi=10.1037/a0026699 |pmid=22233090 |s2cid=7001642}}</ref>{{sfn|Jensen|1998|p=531}}

==Public policy==

In the United States, certain public policies and laws regarding military service,<ref>{{cite report|url=https://www.rand.org/content/dam/rand/pubs/technical_reports/2005/RAND_TR193.pdf |title=Determinants of Productivity for Military Personnel|last=Kavanagh|first=Jennifer|date=2005 |publisher=RAND Corporation|location=Santa Monica, CA |isbn=0-8330-3754-4|access-date=1 July 2017}}</ref><ref>{{cite report |url=https://www.rand.org/content/dam/rand/pubs/monograph_reports/2009/MR818.pdf |title=Estimating AFQT Scores for National Educational Longitudinal Study (NELS) Respondents|last1=Kilburn |first1=M. Rebecca|last2=Hanser |first2=Lawrence M.|date=2009 |publisher=RAND Corporation|location=Santa Monica, CA |last3=Klerman|first3=Jacob A.|access-date=1 July 2017}}</ref> education, public benefits,<ref>{{Cite web |url=https://www.ssa.gov/disability/professionals/bluebook/12.00-MentalDisorders-Adult.htm |title=12.00-Mental Disorders-Adult |website=www.ssa.gov|publisher=U. S. Social Security Administration |access-date=1 July 2017}}</ref> capital punishment,{{sfn|Flynn|2009|p={{Page needed|date=January 2011}}}} and employment incorporate an individual's IQ into their decisions. However, in the case of ''Griggs v. Duke Power Co.'' in 1971, for the purpose of minimizing employment practices that disparately impacted racial minorities, the U.S. Supreme Court banned the use of IQ tests in employment, except when linked to job performance via a [[Social:Job analysis|job analysis]]. Internationally, certain public policies, such as improving nutrition and prohibiting neurotoxins, have as one of their goals raising, or preventing a decline in, intelligence.

A diagnosis of [[Medicine:Intellectual disability|intellectual disability]] is in part based on the results of IQ testing. [[Medicine:Borderline intellectual functioning|Borderline intellectual functioning]] is the categorization of individuals of below-average cognitive ability (an IQ of 71–85), although not as low as those with an intellectual disability (70 or below).

In the United Kingdom, the eleven plus exam which incorporated an intelligence test has been used from 1945 to decide, at eleven years of age, which type of school a child should go to. They have been much less used since the widespread introduction of comprehensive schools.

==Classification==
[[File:Stephen Hawking.StarChild.jpg|thumb|upright=0.7|Physicist [[Biography:Stephen Hawking|Stephen Hawking]]. When asked his IQ, he replied: "I have no idea. People who boast about their IQ are losers."<ref>{{Cite news|last=Solomon |first=Deborah |date=12 December 2004|title=The Science of Second-Guessing|work=The New York Times |url=https://www.nytimes.com/2004/12/12/magazine/the-science-of-secondguessing.html}}</ref>]]

IQ classification is the practice used by IQ test publishers for designating IQ score ranges into various categories with labels such as "superior" or "average".{{sfn|Kaufman|Lichtenberger|2006}} IQ classification was preceded historically by attempts to classify human beings by general ability based on other forms of behavioral observation. Those other forms of behavioral observation are still important for validating classifications based on IQ tests.

==High-IQ societies==

There are social organizations, some international, which limit membership to people who have scores as high as or higher than the 98th percentile (two standard deviations above the mean) on some IQ test or equivalent. Mensa International is perhaps the best known of these. The largest 99.9th percentile (three standard deviations above the mean) society is the Triple Nine Society.
{{Clear}}

==See also==
* Emotional competence
* Emotional intelligence (EI), also known as emotional quotient (EQ) and emotional intelligence quotient (EIQ)
* Raven's Progressive Matrices
* [[Social:Stanford–Binet Intelligence Scales|Stanford–Binet Intelligence Scales]]
* Wechsler Adult Intelligence Scale
* [[Social:Woodcock–Johnson Tests of Cognitive Abilities|Woodcock–Johnson Tests of Cognitive Abilities]]

==Citations==
<!-- This article uses the citation templates described in User:RexxS/Cite multiple pages. Thank you to RexxS for the detailed tips. -->

{{Reflist|30em|refs=
<ref name="Gottfredson2009pp31–32">{{Harvnb |Gottfredson|2009|pages=31–32}}</ref>
<ref name="Hunt2011p5">{{Harvnb |Hunt|2011|page=5}} "As mental testing expanded to the evaluation of adolescents and adults, however, there was a need for a measure of intelligence that did not depend upon mental age. Accordingly the intelligence quotient (IQ) was developed.&nbsp;... The narrow definition of IQ is a score on an intelligence test&nbsp;... where 'average' intelligence, that is the median level of performance on an intelligence test, receives a score of 100, and other scores are assigned so that the scores are distributed normally about 100, with a standard deviation of 15. Some of the implications are that: 1. Approximately two-thirds of all scores lie between 85 and 115. 2. Five percent (1/20) of all scores are above 125, and one percent (1/100) are above 135. Similarly, five percent are below 75 and one percent below 65."</ref>
<ref name="TermanOldClasses">{{Harvnb |Terman|1916|page=[https://archive.org/details/measurementofint008006mbp/page/n106 79]}} "What do the above IQ's imply in such terms as feeble-mindedness, border-line intelligence, dullness, normality, superior intelligence, genius, etc.? When we use these terms two facts must be born in mind: (1) That the boundary lines between such groups are absolutely arbitrary, a matter of definition only; and (2) that the individuals comprising one of the groups do not make up a homogeneous type."</ref>
<ref name="WechslerOldClasses">{{Harvnb |Wechsler|1939|page=37}} "The earliest classifications of intelligence were very rough ones. To a large extent they were practical attempts to define various patterns of behavior in medical-legal terms."</ref>
<ref name="Kaufman2009p21">{{Harvnb |Kaufman|2009|page=21}} "Galton's so-called intelligence test was misnamed."</ref>
<ref name="Kaufman2009">{{Harvnb |Kaufman|2009}}</ref>
<ref name="Urbina2011Table2.1">{{Harvnb |Urbina|2011|loc=Table 2.1 Major Examples of Current Intelligence Tests}}</ref>
<ref name="FlanaganHarrison2012chs8-16">{{Harvnb |Flanagan|Harrison|2012|loc=chapters 8–13, 15–16}} (discussing Wechsler, Stanford–Binet, Kaufman, Woodcock–Johnson, DAS, CAS, and RIAS tests)</ref>
<ref name="Kaufman2009Fig5.1">{{Harvnb |Kaufman|2009|loc=Figure 5.1 IQs earned by preadolescents (ages 12–13) who were given three different IQ tests in the early 2000s}}</ref>
<ref name="KaufmanSB2013Fig3.1">{{Harvnb |Kaufman|2013|loc=Figure 3.1 "Source: {{harvp|Kaufman|2009}}. Adapted with permission."}}</ref>
}}

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* {{cite book |title=Culture and Children's Intelligence: Cross-Cultural Analysis of the WISC-III |first1=James |last1=Georgas |first2=Lawrence |last2=Weiss |first3=Fons |last3=van de Vijver |first4=Donald |last4=Saklofske |editor1-first=James |editor1-last=Georgas |editor2-first=Lawrence |editor2-last=Weiss |editor3-first=Fons |editor3-last=van de Vijver |editor4-first=Donald |editor4-last=Saklofske |editor3-link=Fons van de Vijver |chapter=Preface |pages=xvx–xxxii |date=2003 |publisher=Academic Press |location=San Diego, CA |isbn=978-0-12-280055-9}}
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* {{cite journal |last=Gottfredson |first=Linda S. |date=1997 |title=Mainstream Science on Intelligence (editorial) |journal=Intelligence |issn=0160-2896 |volume=24 |pages=13–23 |url=http://www.udel.edu/educ/gottfredson/reprints/1997mainstream.pdf |doi=10.1016/s0160-2896(97)90011-8 }}
<!-- Gottfredson1997b -->
* {{cite journal |last=Gottfredson |first=Linda S. |title=Why g matters: The complexity of everyday life |journal=Intelligence |volume=24 |issue=1 |date=1997 |pages=79–132 |issn=0160-2896 |doi=10.1016/S0160-2896(97)90014-3 |url=http://www.udel.edu/educ/gottfredson/reprints/1997whygmatters.pdf |access-date=7 July 2014 |citeseerx=10.1.1.535.4596 }}
<!-- Gottfredson1998 -->
* {{cite journal |first1=Linda S. |last1=Gottfredson |title=The general intelligence factor |journal=Scientific American Presents |volume=9 |issue=4 |pages=24–29 |date=1998 |url=http://www.udel.edu/educ/gottfredson/reprints/1998generalintelligencefactor.pdf }}
<!-- Gottfredson2005 -->
* {{cite book |last=Gottfredson |first=Linda S. |title=Destructive Trends In Mental Health: The Well-Intentioned Path to Harm |editor1-last=Wright |editor1-first=Rogers H. |editor2-last=Cummings |editor2-first=Nicholas A. |date=11 March 2005 |publisher=Taylor & Francis |isbn=978-0-203-95622-9 |chapter=Chapter 9: Suppressing Intelligence Research: Hurting Those We Intend to Help |pages=155–186 |chapter-url=http://www.udel.edu/educ/gottfredson/reprints/2005suppressingintelligence.pdf }}
<!-- Gottfredson2006 -->
* {{cite book |last=Gottfredson |first=Linda S. |trans-chapter=Social consequences of group differences in cognitive ability |chapter=Conseqüências sociais das diferenças de grupo na capacidade cognitiva |title=Introdução à Psicologia das Diferenças Individuais |trans-title=Introduction to the psychology of individual differences |editor1-last=Flores-Mendoza |editor1-first=Carmen E. |editor2-last=Colom |editor2-first=Roberto |location=Porto Alegre, Brazil |publisher=ArtMed Publishers |date=2006 |pages=433–456 |isbn=978-85-363-1418-1 |chapter-url=http://www.udel.edu/educ/gottfredson/reprints/2004socialconsequences.pdf }}
<!--Gottfredson2009 -->
* {{cite book |last=Gottfredson |first=Linda S. |chapter=Chapter 1: Logical Fallacies Used to Dismiss the Evidence on Intelligence Testing |title=Correcting Fallacies about Educational and Psychological Testing |editor-last=Phelps |editor-first=Richard F. |date=2009 |publisher=American Psychological Association |location=Washington, DC |isbn=978-1-4338-0392-5}}
<!-- Gould1981 -->
* {{cite book|last=Gould |first=Stephen Jay |title=The Mismeasure of Man |location=New York |publisher=W. W. Norton |isbn=978-0-393-30056-7 |date=1981}}
**{{cite news |author=Christopher Lehmann-Haupt |date=21 October 1981 |title=Books Of The Times: The Mismeasure of Man |type=Review |url=https://www.nytimes.com/books/97/11/09/home/gould-mismeasure.html |newspaper=The New York Times }}
<!-- Gould1996 -->
* {{cite book |last=Gould |first=Stephen Jay |title=The Mismeasure of Man |location=New York |publisher=W. W. Norton |isbn=978-0-393-31425-0 |date=1996 |edition=Rev. and expanded |url=https://archive.org/details/mismeasureofman00goul_1 }}
<!-- Gregory1995 -->
* {{cite encyclopedia |last=Gregory |first=Robert J. |title=Classification of Intelligence |editor-last=Sternberg |editor-first=Robert J. |encyclopedia=Encyclopedia of human intelligence |date=1995 |publisher=Macmillan |isbn=978-0-02-897407-1 |volume=1 |pages=260–266 |url=https://archive.org/details/encyclopediaofhu0000unse/page/260 }}
<!-- Groth-Marnat2009 -->
* {{cite book |title=Handbook of Psychological Assessment |last=Groth-Marnat |first=Gary |date=2009 |publisher=Wiley |location=Hoboken, NJ |isbn=978-0-470-08358-1 |edition=5th}}
<!-- Harris2009 -->
* {{cite book |last=Harris |first=Judith Rich |title=The Nurture Assumption: Why Children Turn Out the Way They Do |date=2009 |edition=2nd |publisher=Free Press |isbn=978-1-4391-0165-0}}
**{{cite interview |subject=Judith Rich Harris |interviewer=Jonah Lehrer |date=9 April 2009 |title=Do Parents Matter? |magazine=Scientific American |url=http://www.scientificamerican.com/article/parents-peers-children/ }}
<!-- HopkinsStanley1981 -->
* {{cite book |last1=Hopkins |first1=Kenneth D. |last2=Stanley |first2=Julian C. |title=Educational and Psychological Measurement and Evaluation |edition=6th |location=Engelwood Cliffs, NJ |publisher=Prentice Hall |isbn=978-0-13-236273-3 |date=1981}}
<!-- Hunt2001 -->
* {{cite journal |last1=Hunt |first1=Earl |title=Multiple Views of Multiple Intelligence |journal=PsycCRITIQUES |volume=46 |issue=1 |pages=5–7 |date=2001 |doi=10.1037/002513}}
<!-- Hunt2011 -->
* {{cite book |last1=Hunt |first1=Earl B. |date=2011 |title=Human Intelligence |publisher=Cambridge University Press |location=Cambridge |isbn=978-0-521-70781-7 |url=https://books.google.com/books?id=DwO4TtKAiCoC }}
<!-- Jensen1969 this is a citation to the one-volume reprint -->
* {{cite book |last=Jensen |first=Arthur |title=Environment, Heredity, and Intelligence |series=Harvard Educational Review Reprint Series |volume=2 |date=1969 |publisher=Harvard Educational Review |location=Cambridge, MA |pages=1–123 |chapter=How Much Can We Boost IQ and Scholastic Achievement? |lccn=71087869 |isbn=978-0916690021}}
**{{cite periodical |date=9 October 1978 |title=Jensen A R. How much can we boost IQ and scholastic achievement? |type=Review |periodical=Citation Classics |number=41 |url=http://www.garfield.library.upenn.edu/classics1978/A1978FQ53600002.pdf }}
<!-- Jensen1980 -->
* {{cite book |last=Jensen |first=Arthur R. |title=Bias in mental testing |location=New York |publisher=Free Press |isbn=978-0-02-916430-3 |date=1980}}<br/>{{*}}{{cite journal|ref=none |last=Scarr |first=Sandra |title=Implicit Messages: A Review of ''Bias in Mental Testing'' |journal=American Journal of Education |volume=89 |issue=3 |date=1981 |pages=330–338 |doi=10.1086/443584 |jstor=1084961|s2cid=147214993 }}
<!-- Jensen1998 -->
* {{cite book |title=The g Factor: The Science of Mental Ability |last=Jensen |first=Arthur R. |date=1998 |publisher=Praeger |location=Westport, CT |isbn=978-0-275-96103-9 |issn=1063-2158 |series=Human Evolution, Behavior, and Intelligence |url=https://archive.org/details/gfactorscienc00jens }}<br/>{{*}}{{cite journal |ref=none |last=Locurto |first=Charles |date=1999 |title=A Balance Sheet on Persistence: Book Review of Jensen on Intelligence-g-Factor |journal=Psycoloquy |volume=10 |issue=59 |at=9 |url=http://www.cogsci.ecs.soton.ac.uk/cgi/psyc/newpsy?10.059 }}
<!-- Jensen2006 -->
* {{cite book |last=Jensen |first=Arthur R. |title=Clocking the Mind: Mental Chronometry and Individual Differences |url=http://www.sciencedirect.com/science/book/9780080449395 |access-date=7 July 2014 |date=10 July 2006 |publisher=Elsevier |isbn=978-0-08-044939-5 }}<br/>{{*}}{{cite journal |ref=none |last=Wai |first=Jonathan |title=Book Review: Jensen, A. R. (2006). Clocking the mind: Mental chronometry and individual differences. Amsterdam: Elsevier. (ISBN 978-0-08-044939-5) |journal=Gifted Child Quarterly |date=2008 |volume=52 |page=99 |doi=10.1177/0016986207310434 |s2cid=143666885 |url=http://www.psychologytoday.com/files/attachments/56143/bookreviewclockingthemind.pdf }}
<!-- Jensen2011 -->
* {{cite journal |title=The Theory of Intelligence and Its Measurement |last=Jensen |first=Arthur R. |date=2011 |journal=Intelligence |volume=39 |issue=4 |pages=171–177 |issn=0160-2896 |doi=10.1016/j.intell.2011.03.004}}
<!-- Johnson2012 -->
* {{cite book |last=Johnson |first=Wendy |title=Developmental Psychology: Revisiting the Classic Studies |editor1-last=Slater |editor1-first=Alan M. |editor2-last=Quinn |editor2-first=Paul C. |chapter=How Much Can We Boost IQ? An Updated Look at Jensen's (1969) Question and Answer |location=Thousand Oaks, CA |publisher=SAGE |isbn=978-0-85702-757-3 |date=2012}}<br/>{{*}}{{cite journal |ref=none |last=Gamboa |first=Camille |date=May 2013 |title=(Review) Developmental Psychology: Revisiting the Classic Studies, [ed.] by Alan M. Slater and Paul C. Quinn |journal=Choice |volume=50 |issue=9 |url=http://www.cne.psychol.cam.ac.uk/pdfs/choice |url-status=dead |archive-url=https://web.archive.org/web/20141010173731/http://www.cne.psychol.cam.ac.uk/pdfs/choice |archive-date=10 October 2014 }}
<!-- JohnsonTurkheimerGottesmanBouchard2009 -->
* {{cite journal |last1=Johnson |first1=Wendy |last2=Turkheimer |first2=E. |last3=Gottesman |first3=Irving |last4=Bouchard |first4=Thomas |date=2009 |title=Beyond Heritability: Twin Studies in Behavioral Research |journal=Current Directions in Psychological Science |volume=18 |issue=4 |pages=217–220 |url=http://people.virginia.edu/~ent3c/papers2/Articles%20for%20Online%20CV/Johnson%20%282009%29.pdf |access-date=29 June 2010 |doi=10.1111/j.1467-8721.2009.01639.x |pmc=2899491 |pmid=20625474 }}
<!-- Kaufman2009 -->
* {{cite book |title=IQ Testing 101 |last=Kaufman |first=Alan S. |date=2009 |publisher=Springer Publishing |location=New York |isbn=978-0-8261-0629-2 |url=https://archive.org/details/iqtestingpsych00phdd |url-access=limited }}
<!-- KaufmanLichtenberger2006 -->
* {{cite book |title=Assessing Adolescent and Adult Intelligence |last1=Kaufman |first1=Alan S. |last2=Lichtenberger |first2=Elizabeth O. |edition=3rd |date=2006 |publisher={{wipe|John Wiley & Sons}} |location=Hoboken, NJ |isbn=978-0-471-73553-3 }}
<!-- KaufmanSB2013 -->
* {{cite book |last=Kaufman |first=Scott Barry |title=Ungifted: Intelligence Redefined |url=https://archive.org/details/ungiftedintellig0000kauf |url-access=registration |access-date=1 October 2013 |date=1 June 2013 |publisher=Basic Books |isbn=978-0-465-02554-1 }}
**{{cite magazine |title=Ungifted: Intelligence Redefined: The Truth about Talent, Practice, Creativity, and the Many Paths to Greatness |magazine=Publishers Weekly |type=Review |url=http://www.publishersweekly.com/978-0-465-02554-1 }}
<!-- KranzlerFloyd2013 -->
* {{cite book |last1=Kranzler |first1=John H. |last2=Floyd |first2=Randy G. |title=Assessing Intelligence in Children and Adolescents: A Practical Guide |date=1 August 2013 |publisher=Guilford Press |isbn=978-1-4625-1121-1 |url=http://www.guilfordpress.co.uk/books/details/9781462511211/ |access-date=9 June 2014 |archive-url=https://web.archive.org/web/20141016201213/http://www.guilfordpress.co.uk/books/details/9781462511211/ |archive-date=16 October 2014 |url-status=dead }}
<!-- LahnEbenstein2009 -->
* {{cite journal |last1=Lahn |first1=Bruce T. |last2=Ebenstein |first2=Lanny |  title=Let's celebrate human genetic diversity |journal=Nature |volume=461 |issue=7265 |pages=726–728 |date=2009 |issn=0028-0836 |pmid=19812654 |doi=10.1038/461726a |bibcode=2009Natur.461..726L |s2cid=205050141}}
<!-- LohmanFoley Nicpon2012 -->
* {{cite book |title=Identification: The Theory and Practice of Identifying Students for Gifted and Talented Education Services |last1=Lohman |first1=David F. |last2=Foley Nicpon |first2=Megan |editor-last=Hunsaker |editor-first=Scott |chapter=Chapter 12: Ability Testing & Talent Identification |pages=287–386 |chapter-url=https://faculty.education.uiowa.edu/docs/default-source/dlohman/ability-testing-and-talent-identification.pdf?sfvrsn=0 |date=2012 |publisher=Prufrock |location=Waco, TX |isbn=978-1-931280-17-4 |access-date=2014-07-07 |archive-date=2016-03-15 |archive-url=https://web.archive.org/web/20160315055952/https://faculty.education.uiowa.edu/docs/default-source/dlohman/ability-testing-and-talent-identification.pdf?sfvrsn=0 |url-status=dead }}
<!-- Mackintosh1998 -->
* {{Cite book |title=IQ and Human Intelligence |last=Mackintosh |first=N. J. |year=1998 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-852367-3 |url=https://archive.org/details/iqhumanintellige00mack }}
<!-- Mackintosh2011 -->
* {{cite book |title=IQ and Human Intelligence |last=Mackintosh |first=N. J. |date=2011 |edition=2nd |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-958559-5 |url=https://books.google.com/books?id=BcKcAQAAQBAJ }}
<!-- Matarazzo1972 -->
* {{cite book |title=Wechsler's Measurement and Appraisal of Adult Intelligence |last=Matarazzo |first=Joseph D. |date=1972 |edition=5th |publisher=Williams & Witkins |location=Baltimore, MD}}
**{{cite journal |author=R. D. Savage |date=April 1974 |title=Wechsler's Measurement and Appraisal of Adult Intelligence, 5th ed |type= Review |journal=British Journal of Industrial Medicine |volume=31 |issue=2 |page=169 |pmc=1009574}}
<!-- McIntoshDixonPierson2012 -->
* {{harvc |last1=McIntosh|first1=David E. |last2=Dixon|first2=Felicia A. |last3=Pierson|first3=Eric E. |in1=Flanagan|in2=Harrison |c=Chapter 25: Use of Intelligence Tests in the Identification of Giftedness |pp=623–642 |year=2012}}
<!-- Murray1998 -->
* {{Cite book |last=Murray |first=Charles |title=Income Inequality and IQ |location=Washington, DC |publisher=AEI Press |isbn=978-0-8447-7094-9 |date=1998 |url=https://www.aei.org/wp-content/uploads/2011/10/20040302_book443.pdf |access-date=7 July 2014 }}<br/>{{*}}{{cite magazine |ref=none |last=Loury |first=Glenn C. |date=18 May 1998 |title=Charles II |department=Hard Questions Column |url=http://www.bu.edu/irsd/articles/charles2.htm |magazine=The New Republic |access-date=7 July 2014 |archive-date=2015-09-24 |archive-url=https://web.archive.org/web/20150924042546/http://www.bu.edu/irsd/articles/charles2.htm |url-status=dead }}
<!-- Naglieri1999 -->
* {{cite book |title=Essentials of CAS Assessment |last=Naglieri |first=Jack A. |date=1999 |publisher=Wiley |location=Hoboken, NJ |series=Essentials of Psychological Assessment |isbn=978-0-471-29015-5}}
<!-- NeisserBoodooBouchard1996 formerly named APA-report -->
* {{cite journal |ref={{sfnref|Neisser et al.|1995}} |display-authors=8 |last1=Neisser |first1=Ulrich |last2=Boodoo |first2=Gwyneth |last3=Bouchard |first3=Thomas J. |last4=Boykin |first4=A. Wade |last5=Brody |first5=Nathan |last6=Ceci |first6=Stephen J. |last7=Halpern |first7=Diane F. |last8=Loehlin |first8=John C. |last9=Perloff |first9=Robert |last10=Sternberg |first10=Robert J. |last11=Urbina |first11=Susana |author-link10=Robert Sternberg |title=Intelligence: Knowns and unknowns |journal=American Psychologist |issn=0003-066X |volume=51 |issue=2 |pages=77–101 |year=1996 |doi=10.1037/0003-066x.51.2.77 |url=http://psych.colorado.edu/~carey/pdfFiles/IQ_Neisser2.pdf |access-date=9 October 2014 |s2cid=20957095 }}
<!-- Noguera2001 -->
* {{Cite journal |last=Noguera |first=Pedro A. |title=Racial politics and the elusive quest for excellence and equity in education |journal=In Motion Magazine |date=30 September 2001 |id=Article # ER010930002 |url=http://www.inmotionmagazine.com/er/pnrp1.html |access-date=7 July 2014 }}
<!-- PerlethSchatzMönks2000 -->
* {{cite book |last1=Perleth |first1=Christoph |last2=Schatz |first2=Tanja |last3=Mönks |first3=Franz J. |title=International Handbook of Giftedness and Talent |url=https://archive.org/details/internationalhan00hell |url-access=limited |editor1-last=Heller |editor1-first=Kurt A. |editor2-last=Mönks |editor2-first=Franz J. |editor3-last=Sternberg |editor3-first=Robert J. |editor4-last=Subotnik |editor4-first=Rena F. |edition=2nd |date=2000 |publisher=Pergamon |location=Amsterdam |isbn=978-0-08-043796-5 |pages=[https://archive.org/details/internationalhan00hell/page/n313 297]–316 |chapter=Early Identification of High Ability |quote=a gifted sample gathered using IQ > 132 using the old SB L-M in 1985 does not contain the top 2% of the population but the best 10%. |editor3-link=Robert Sternberg }}
<!-- PlominDeFriesKnopikNeiderhiser2012 -->
* {{cite book|last1=Plomin |first1=Robert |last2=DeFries |first2=John C. |last3=Knopik |first3=Valerie S. |last4=Neiderhiser |first4=Jenae M. |title=Behavioral Genetics |date=2013 |publisher=Worth Publishers |isbn=978-1-4292-4215-8 |edition=6th}}
<!-- Shurkin1992 -->
* {{cite journal |last=Reddy |first=Ajitha |year=2008 |title=The Eugenic Origins of IQ Testing: Implications for Post-Atkins Litigation |url=https://via.library.depaul.edu/cgi/viewcontent.cgi?article=1270&context=law-review |journal=DePaul Law Review |volume=57 |pages=667–677 }}
* {{cite book |title=Terman's Kids: The Groundbreaking Study of How the Gifted Grow Up |last=Shurkin |first=Joel |date=1992 |publisher=Little, Brown |location=Boston, MA |isbn=978-0-316-78890-8 |url=https://archive.org/details/termanskids00joel }}
** {{cite news |author=Frederic Golden |date=31 May 1992 |title=Tracking the IQ Elite : TERMAN'S KIDS: The Groundbreaking Study of How the Gifted Grow Up, By Joel N. Shurkin |url=http://articles.latimes.com/1992-05-31/books/bk-1247_1_lewis-terman |archive-url=https://web.archive.org/web/20121108031753/http://articles.latimes.com/1992-05-31/books/bk-1247_1_lewis-terman |archive-date=8 November 2012 |newspaper=Los Angeles Times }}
<!-- Stern1912, Stern1914 as harv ref -->
* {{cite book |last=Stern |first=William |location=Baltimore, MD |publisher=Warwick & York |date=1914 |url=https://books.google.com/books?id=D6h9AAAAMAAJ |access-date=15 June 2014 |title=The Psychological Methods of Testing Intelligence |translator=Guy Montrose Whipple |series=Educational psychology monographs |volume=13 |isbn=9781981604999 |oclc=4521857 |lccn=14010447 }}<br/>{{cite book|ref=none |last=Stern |first=William |title=Die psychologischen Methoden der Intelligenzprüfung: und deren Anwendung an Schulkindern |location=Leipzig |publisher=J. A. Barth |date=1912 |language=de |trans-title=The Psychological Methods of Testing Intelligence}}
<!-- Terman et al. 1915 -->
* {{cite journal |ref={{sfnref|Terman et al.|1915}} |last1=Terman |first1=Lewis M. |last2=Lyman |first2=Grace |last3=Ordahl |first3=George |last4=Ordahl |first4=Louise |last5=Galbreath |first5=Neva |last6=Talbert |first6=Wilford |title=The Stanford revision of the Binet–Simon scale and some results from its application to 1000 non-selected children |journal=Journal of Educational Psychology |volume=6 |issue=9 |pages=551–62 |year=1915 |doi=10.1037/h0075455 |url=https://zenodo.org/record/1429169 }}
<!-- Terman1916 -->
* {{cite book |title=The Measurement of Intelligence: An Explanation of and a Complete Guide to the Use of the Stanford Revision and Extension of the Binet–Simon Intelligence Scale |last=Terman |first=Lewis M. |editor=Ellwood P. Cubberley |year=1916 |publisher=Houghton Mifflin |location=Boston |series=Riverside Textbooks in Education |url=https://www.gutenberg.org/ebooks/20662 |access-date=26 June 2010 }}
<!-- TermanMerrill1937 -->
* {{cite book |title=Measuring Intelligence: A Guide to the Administration of the New Revised Stanford–Binet Tests of Intelligence |last1=Terman |first1=Lewis M. |last2=Merrill A. |first2=Maude |year=1937 |publisher=Houghton Mifflin |location=Boston}}
<!-- TermanMerrill1960 -->
* {{cite book |title=Stanford–Binet Intelligence Scale: Manual for the Third Revision Form L-M with Revised IQ Tables by Samuel R. Pinneau |last1=Terman |first1=Lewis M. |last2=Merrill |first2=Maude A. |year=1960 |publisher=Houghton Mifflin |location=Boston, MA}}
<!-- Turkheimer2008 -->
* {{cite journal |last=Turkheimer |first=Eric |date=April 2008 |title=A Better Way to Use Twins for Developmental Research |journal=LIFE Newsletter |volume=2 |issue=1 |pages=2–5 |url=http://people.virginia.edu/~ent3c/papers2/Articles%20for%20Online%20CV/Turkheimer%20%282008%29.pdf |access-date=29 October 2010 }}
<!-- Urbina2011 -->
* {{Cite book |last=Urbina |first=Susana |title=The Cambridge Handbook of Intelligence |url=https://archive.org/details/cambridgehandboo00ster |url-access=limited |editor1-last=Sternberg |editor1-first=Robert J. |editor1-link=Robert Sternberg |editor2-last=Kaufman |editor2-first=Scott Barry |date=2011 |chapter=Chapter 2: Tests of Intelligence |pages=[https://archive.org/details/cambridgehandboo00ster/page/n41 20]–38 |publisher=Cambridge University Press |location=Cambridge |isbn=9780521739115 }}
<!-- Wasserman2012 -->
* {{harvc |last=Wasserman |first=John D. |in1=Flanagan|in2=Harrison |c=Chapter 1: A History of Intelligence Assessment: The Unfinished Tapestry |pp=3–55 |year=2012}}
<!-- Wechsler1939 -->
* {{cite book |last=Wechsler |first=David |title=The Measurement of Adult Intelligence |date=1939 |edition=1st |publisher=Williams & Witkins |location=Baltimore, MD |lccn=39014016}}
<!-- Wechsler1997 -->
* {{Cite book |last=Wechsler |first=David |title=Wechsler Adult Intelligence Scale |edition=3rd |location=San Antonio, TX |publisher=The Psychological Corporation |date=1997}}
<!-- Wechsler2003 -->
* {{Cite book |last=Wechsler |first=David |title=Wechsler Intelligence Scale for Children |edition=4th |location=San Antonio, TX |publisher=The Psychological Corporation |date=2003}}
* {{cite book |editor1-last=Weiner |editor1-first=Irving B. |editor2-last=Graham |editor2-first=John R. |editor3-last=Naglieri |editor3-first=Jack A. |title=Handbook of Psychology, Volume 10: Assessment Psychology |url=http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470891270.html |access-date=25 November 2013 |date=2 October 2012 |publisher=John Wiley & Sons |isbn=978-0-470-89127-8 }}
<!-- WeissSaklofskePrifiteraHoldnack2006 -->
* {{cite book |editor1-last=Weiss |editor1-first=Lawrence G. |editor2-last=Saklofske |editor2-first=Donald H. |editor3-last=Prifitera |editor3-first=Aurelio |editor4-last=Holdnack |editor4-first=James A. |title=WISC-IV Advanced Clinical Interpretation |date=2006 |publisher=Academic Press |location=Burlington, MA |isbn=978-0-12-088763-7 |series=Practical Resources for the Mental Health Professional}} This practitioner's handbook includes chapters by L.G. Weiss, J.G. Harris, A. Prifitera, T. Courville, E. Rolfhus, D.H. Saklofske, J.A. Holdnack, D. Coalson, S.E. Raiford, D.M. Schwartz, P. Entwistle, V. L. Schwean, and T. Oakland.
* {{cite journal |last1=Wicherts |first1=Jelte M. |last2=Dolan |first2=Conor V. |last3=Carlson |first3=Jerry S. |last4=van der Maas |first4=Han L.J. |year=2010|title=Raven's test performance of sub-Saharan Africans: Average performance, psychometric properties, and the Flynn Effect |journal=Learning and Individual Differences |volume=20 |issue=3 |pages=135–151 |doi=10.1016/j.lindif.2009.12.001 }}
* {{cite journal |last1=Wicherts |first1=Jelte M. |last2=Dolan |first2=Conor V. |last3=van der Maas |first3=Han L.J. |year =2010|title=A systematic literature review of the average IQ of sub-Saharan Africans |journal=Intelligence |volume=38 |issue=1 |pages=1–20 |doi=10.1016/j.intell.2009.05.002 }}
{{Refend}}

==External links==
{{Library resources box
|by=no
|onlinebooks=no
|others=no
|about=yes
|label=Intelligence quotient
}}

* [https://psychclassics.yorku.ca/ Classics in the History of Psychology]

{{Human intelligence topics}}

{{DEFAULTSORT:Intelligence Quotient}}
[[Category:Intelligence]]
[[Category:Psychometrics]]

{{Sourceattribution|Intelligence quotient}}