Editing Intelligence quotient (section)

{{Short description|Score from a test designed to assess intelligence}}
{{about|the history and applications of IQ|the classifications of IQ|IQ classification}}
{{Redirect|IQ}}
{{Use British English|date=February 2023}}
{{Use dmy dates|date=August 2022}}
{{Infobox diagnostic
| name    = Intelligence quotient
| image   = [[File:Raven Matrix.svg|class=skin-invert-image|290px]]
| 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 [[standardized test]]s or subtests designed to assess [[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> Originally, IQ was a score obtained by dividing a person's [[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]]) 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 [[#Current tests|IQ tests]], the [[test score|raw score]] is [[Data transformation (statistics)|transformed]] to a [[normal distribution]] with mean 100 and [[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 [[Intellectual giftedness|above 130]] and [[Intellectual disability|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 [[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 | issn = 0022-3476}}</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 [[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 status]],{{sfn|Neisser et al.|1995}} and [[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 [[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}}<ref>{{Cite journal |last1=Oxley |first1=Florence A. R. |last2=Wilding |first2=Kirsty |last3=von Stumm |first3=Sophie |date=2024-11-01 |title=DNA and IQ: Big deal or much ado about nothing? – A meta-analysis |url=https://www.sciencedirect.com/science/article/abs/pii/S0160289624000655 |journal=Intelligence |volume=107 |pages=101871 |doi=10.1016/j.intell.2024.101871 |issn=0160-2896}}</ref> 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 |author3-link= Kathryn Roeder |title=The heritability of IQ |bibcode=1997Natur.388..468D |s2cid=4313884|doi-access=free }}</ref><ref>{{Cite journal |last1=Plomin |first1=Robert |last2=von Stumm |first2=Sophie |date=March 2018 |title=The new genetics of intelligence |journal=Nature Reviews Genetics |language=en |volume=19 |issue=3 |pages=148–159 |doi=10.1038/nrg.2017.104 |pmid=29335645 |issn=1471-0064|pmc=5985927 }}</ref> Current best estimates for heritability range from 40 to 60% of the variance between individuals in IQ being explained by genetics.<ref>{{Cite journal |last1=de la Fuente |first1=Javier |last2=Davies |first2=Gail |last3=Grotzinger |first3=Andrew D. |last4=Tucker-Drob |first4=Elliot M. |last5=Deary |first5=Ian J. |date=January 2021 |title=A general dimension of genetic sharing across diverse cognitive traits inferred from molecular data |journal=Nature Human Behaviour |language=en |volume=5 |issue=1 |pages=49–58 |doi=10.1038/s41562-020-00936-2 |pmid=32895543 |pmc=9346507 |issn=2397-3374}}</ref><ref>{{Cite journal |last1=Davies |first1=G. |last2=Tenesa |first2=A. |last3=Payton |first3=A. |last4=Yang |first4=J. |last5=Harris |first5=S. E. |last6=Liewald |first6=D. |last7=Ke |first7=X. |last8=Le Hellard |first8=S. |last9=Christoforou |first9=A. |last10=Luciano |first10=M. |last11=McGhee |first11=K. |last12=Lopez |first12=L. |last13=Gow |first13=A. J. |last14=Corley |first14=J. |last15=Redmond |first15=P. |date=October 2011 |title=Genome-wide association studies establish that human intelligence is highly heritable and polygenic |journal=Molecular Psychiatry |language=en |volume=16 |issue=10 |pages=996–1005 |doi=10.1038/mp.2011.85 |pmid=21826061 |issn=1476-5578|pmc=3182557 }}</ref>

IQ scores were used for educational placement, [[IQ classification|assessment of intellectual ability]], and evaluating job applicants. In research contexts, they have been studied as predictors of [[performance rating (work measurement)|job performance]]<ref name="Schmidt98"/> and [[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 [[Flynn effect]]. Investigation of different patterns of increases in subtest scores can also inform current research on human intelligence.

Historically, many proponents of IQ testing have been [[Eugenics|eugenicists]] who used [[pseudoscience]] to push now-debunked views of [[Scientific racism|racial hierarchy]] in order to justify [[Racial segregation|segregation]] and oppose [[immigration]].<ref name=":11">{{Cite web |last=Winston |first=Andrew S. |date=29 May 2020 |title=Scientific Racism and North American Psychology |url=https://oxfordre.com/psychology/display/10.1093/acrefore/9780190236557.001.0001/acrefore-9780190236557-e-516 |website=Oxford Research Encyclopedias: Psychology |doi=10.1093/acrefore/9780190236557.013.516 |isbn=978-0-19-023655-7 |quote=The use of psychological concepts and data to promote ideas of an enduring racial hierarchy dates from the late 1800s and has continued to the present. The history of scientific racism in psychology is intertwined with broader debates, anxieties, and political issues in American society. With the rise of intelligence testing, joined with ideas of eugenic progress and dysgenic reproduction, psychological concepts and data came to play an important role in naturalizing racial inequality. Although racial comparisons were not the primary concern of most early mental testing, results were employed to justify beliefs regarding Black “educability” and the dangers of Southern and Eastern European immigration.}}</ref><ref>{{Cite book |last=Newitz |first=Annalee |author-link=Annalee Newitz |title=Stories Are Weapons: Psychological Warfare and the American Mind |date=June 4, 2024 |publisher=W. W. Norton & Company |chapter=Chapter 4}}</ref> Such views are now rejected by a strong consensus of mainstream science, though fringe figures continue to promote them in [[pseudo-scholarship]] and popular culture.<ref name=":12">{{Cite journal |last1=Bird |first1=Kevin |last2=Jackson |first2=John P. |last3=Winston |first3=Andrew S. |date=2024 |title=Confronting Scientific Racism in Psychology: Lessons from Evolutionary Biology and Genetics |url=https://psycnet.apa.org/doiLanding?doi=10.1037%2Famp0001228 |journal=American Psychologist |volume=79 |issue=4 |pages=497–508 |doi=10.1037/amp0001228 |pmid=39037836 |quote=Recent articles claim that the folk categories of race are genetically meaningful divisions, and that evolved genetic differences among races and nations are important for explaining immutable differences in cognitive ability, educational attainment, crime, sexual behavior, and wealth; all claims that are opposed by a strong scientific consensus to the contrary.}}</ref><ref name=":13">{{Cite journal |last1=Panofsky |first1=Aaron |last2=Dasgupta |first2=Kushan |last3=Iturriaga |first3=Nicole |year=2021 |title=How White nationalists mobilize genetics: From genetic ancestry and human biodiversity to counterscience and metapolitics |journal=American Journal of Physical Anthropology |volume=175 |issue=2 |pages=387–398 |doi=10.1002/ajpa.24150 |issn=0002-9483 |pmc=9909835 |pmid=32986847 |quote=[T]he claims that genetics defines racial groups and makes them different, that IQ and cultural differences among racial groups are caused by genes, and that racial inequalities within and between nations are the inevitable outcome of long evolutionary processes are neither new nor supported by science (either old or new). |doi-access=free}}</ref>