Editing Race and intelligence (section)

==Research into possible genetic factors==
{{see also|Heritability of IQ}}
Although IQ differences between individuals have been shown to have a large hereditary component, it does not follow that mean group-level disparities (between-group differences) in IQ necessarily have a genetic basis.<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 |date=2012 |title=Intelligence: New findings and theoretical developments. |journal=American Psychologist |language=en |volume=67 |issue=2 |pages=130–159 |doi=10.1037/a0026699 |issn=1935-990X |pmid=22233090}}</ref><ref name="Nisbett-2012" /> Today, the scientific consensus is that [[genetics]] does not explain differences in IQ test performance between groups.<ref>{{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>{{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"}}{{sfn|Hunt|2010|page=447|ps= , "It is worth remembering that no genes related to difference in cognitive skills across the various racial and ethnic groups have ever been discovered. The argument for genetic differences has been carried forward largely by circumstantial evidence. Of course, tomorrow afternoon genetic mechanisms producing racial and ethnic differences in intelligence might be discovered, but there have been a lot of investigations, and tomorrow has not come for quite some time now."}}{{sfn|Mackintosh|2011||pages=334–338, 344}}<ref name="Nisbett-2012">{{cite journal |last1=Nisbett |first1=Richard E. |last2=Aronson |first2=Joshua |last3=Blair |first3=Clancy |last4=Dickens |first4=William |last5=Flynn |first5=James |author-link5=Jim Flynn (academic) |last6=Halpern |first6=Diane F. |author-link6=Diane F. Halpern |last7=Turkheimer |first7=Eric |date=2012 |title=Group differences in IQ are best understood as environmental in origin |url=https://archive.org/details/sim_american-psychologist_2012-09_67_6/page/n80 |journal=American Psychologist |volume=67 |number=6 |pages=503–504 |doi=10.1037/a0029772 |issn=0003-066X |pmid=22963427 |author-link1=Richard E. Nisbett}}</ref><ref name="Kaplan-2015">{{Cite journal |last=Kaplan |first=Jonathan Michael |date=January 2015 |title=Race, IQ, and the search for statistical signals associated with so-called "X"-factors: environments, racism, and the "hereditarian hypothesis" |journal=Biology & Philosophy |language=en |volume=30 |issue=1 |pages=1–17 |doi=10.1007/s10539-014-9428-0 |s2cid=85351431 |issn=0169-3867}}</ref><ref>{{Cite journal |last1=Panofsky |first1=Aaron |last2=Dasgupta |first2=Kushan |last3=Iturriaga |first3=Nicole |title=How White nationalists mobilize genetics: From genetic ancestry and human biodiversity to counterscience and metapolitics |journal=American Journal of Physical Anthropology |year=2021 |volume=175 |issue=2 |pages=387–398 |doi=10.1002/ajpa.24150 |issn=0002-9483 |pmid=32986847 |pmc=9909835 |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>{{sfn|Neisser|Boodoo|Bouchard|Boykin|1996}} Growing evidence indicates that environmental factors, not genetic ones, explain the racial IQ gap.{{sfn|Dickens|Flynn|2006}}<ref name="Nisbett-2012" />{{sfn|Nevid|2014|page=271}}<ref name="Kaplan-2015" />

===Genetics of race and intelligence===
{{main|Race and genetics}}
Geneticist [[Alan R. Templeton]] argued that the question about the possible genetic effects on the test score gap is muddled by the general focus on "race" rather than on populations defined by gene frequency or by geographical proximity, and by the general insistence on phrasing the question in terms of heritability.<ref name="Templeton 2001">{{harvnb|Templeton|2001}}</ref> Templeton pointed out that racial groups neither represent [[sub-species]] nor distinct [[lineage (evolution)|evolutionary lineages]], and that therefore there is no basis for making claims about the general intelligence of races.<ref name="Templeton 2001"/> He argued that, for these reasons, the search for possible genetic influences on the black–white test score gap is ''a priori'' flawed, because there is no genetic material shared by all Africans or by all Europeans. {{harvp|Mackintosh|2011}}, on the other hand, argued that by using genetic cluster analysis to correlate gene frequencies with continental populations it might be possible to show that African populations have a higher frequency of certain genetic variants that contribute to differences in average intelligence. Such a hypothetical situation could hold without all Africans carrying the same genes or belonging to a single evolutionary lineage. According to Mackintosh, a biological basis for the observed gap in IQ test performance thus cannot be ruled out on ''a priori'' grounds.{{Page needed|date=January 2022}}

{{harvtxt|Hunt|2010|page=447}} noted that "no genes related to difference in cognitive skills across the various racial and ethnic groups have ever been discovered. The argument for genetic differences has been carried forward largely by circumstantial evidence. Of course, tomorrow afternoon genetic mechanisms producing racial and ethnic differences in intelligence might be discovered, but there have been a lot of investigations, and tomorrow has not come for quite some time now." {{harvtxt|Mackintosh|2011|page=344}} concurred, noting that while several environmental factors have been shown to influence the IQ gap, the evidence for a genetic influence has been negligible. A 2012 review by {{harvp|Nisbett|Aronson|Blair| Dickens|2012a}} concluded that the entire IQ gap can be explained by known environmental factors, and Mackintosh found this view to be plausible.

More recent research attempting to identify genetic loci associated with individual-level differences in IQ has yielded promising results, which led the editorial board of ''[[Nature (journal)|Nature]]'' to issue a statement differentiating this research from the "racist" pseudoscience which it acknowledged has dogged intelligence research since its inception.<ref name="Nature-2017" /> It characterized the idea of genetically determined differences in intelligence between races as definitively false.<ref name="Nature-2017">{{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|doi-access=free }}</ref> Analysis of polygenic scores sampled from the 1000 Genomes Project has likewise found no evidence that intelligence was under diversifying selection in Africans and Europeans, suggesting that genetic differences make up a negligible component of the observed Black-White gap in IQ.<ref>{{Cite journal |last=Bird |first=Kevin A. |date=2 February 2021 |title=No support for the hereditarian hypothesis of the Black–White achievement gap using polygenic scores and tests for divergent selection |url=https://onlinelibrary.wiley.com/doi/10.1002/ajpa.24216 |journal=[[American Journal of Physical Anthropology]] |language=en |volume=175 |issue=2 |pages=465–476 |doi=10.1002/ajpa.24216 |pmid=33529393 |issn=0002-9483 |access-date=1 November 2024 |via=Wiley Online Library}}</ref>

===Heritability within and between groups===
[[File:Heritability plants.jpeg|thumb| An environmental factor that varies between groups but not within groups can cause group differences in a trait that is otherwise 100 percent [[heritable]].|330x330px]]

[[Twin studies]] of intelligence have reported high heritability values. However, these studies have been criticized for being based on questionable assumptions.<ref>{{Cite book |last1=Carson |first1=Michael |title='Race', IQ and Genes |last2=Beckwith |first2=Jon |date=2001 |publisher=John Wiley & Sons, Ltd |isbn=978-0-470-01590-2 |pages=1–5 |language=en |doi=10.1002/9780470015902.a0005689.pub3}}</ref><ref>{{Cite journal |last1=Beckwith |first1=Jon |last2=Morris |first2=Corey A. |date=December 2008 |title=Twin Studies of Political Behavior: Untenable Assumptions? |journal=Perspectives on Politics |language=en |volume=6 |issue=4 |pages=785–791 |doi=10.1017/S1537592708081917 |s2cid=55630117 |issn=1541-0986 }}</ref><ref>{{Cite journal |last1=Kamin |first1=Leon J. |last2=Goldberger |first2=Arthur S. |date=February 2002 |title=Twin Studies in Behavioral Research: A Skeptical View |journal=Theoretical Population Biology |volume=61 |issue=1 |pages=83–95 |doi=10.1006/tpbi.2001.1555 |pmid=11895384 |bibcode=2002TPBio..61...83K |issn=0040-5809}}</ref> When used in the context of human [[behavior genetics]], the term "heritability" can be misleading, as it does not necessarily convey information about the relative importance of genetic or environmental factors on the development of a given trait, nor does it convey the extent to which that trait is genetically determined.{{sfn|Moore|Shenk|2016}} Arguments in support of a genetic explanation of racial differences in IQ are sometimes fallacious. For instance, hereditarians have sometimes cited the failure of known environmental factors to account for such differences, or the high heritability of intelligence within races, as evidence that racial differences in IQ are genetic.<ref>{{harvnb|Mackenzie|1984}}</ref>

Psychometricians have found that intelligence is substantially heritable within populations, with 30–50% of variance in IQ scores in early childhood being attributable to genetic factors in analyzed US populations, increasing to 75–80% by late adolescence.{{sfn|Neisser|Boodoo|Bouchard|Boykin|1996}}<ref name="Deary, Johnson & Houlihan 2009">{{harvnb|Deary|Johnson|Houlihan|2009}}</ref> In biology heritability is defined as the ratio of variation attributable to genetic differences in an observable [[trait (biology)|trait]] to the trait's total observable variation. The heritability of a trait describes the proportion of variation in the trait that is attributable to genetic factors within a particular population. A heritability of 1 indicates that variation correlates fully with genetic variation and a heritability of 0 indicates that there is no correlation between the trait and genes at all. In psychological testing, heritability tends to be understood as the degree of correlation between the results of a test taker and those of their biological parents. However, since high heritability is simply a correlation between child and parents, it does not describe the causes of heritability which in humans can be either genetic or environmental.

Therefore, a high heritability measure does not imply that a trait is genetic or unchangeable. In addition, environmental factors that affect all group members equally will not be measured by heritability, and the heritability of a trait may also change over time in response to changes in the distribution of genetic and environmental factors.{{sfn|Neisser|Boodoo|Bouchard|Boykin|1996}} High heritability does not imply that all of the heritability is genetically determined; rather, it can also be due to environmental differences that affect only a certain genetically defined group (indirect heritability).<ref name="Block 2002">{{harvnb|Block|2002}}</ref>

The figure to the right demonstrates how heritability works. In each of the two gardens the difference between tall and short cornstalks is 100% heritable, as cornstalks that are genetically disposed for growing tall will become taller than those without this disposition. But the difference in height between the cornstalks to the left and those on the right is 100% environmental, as it is due to different nutrients being supplied to the two gardens. Hence, the causes of differences within a group and between groups may not be the same, even when looking at traits that are highly heritable.<ref name="Block 2002" />

=== Spearman's hypothesis ===
{{Main|Spearman's hypothesis}}
Spearman's hypothesis states that the magnitude of the black–white difference in tests of cognitive ability depends entirely or mainly on the extent to which a test measures general mental ability, or ''g''. The hypothesis was first formalized by [[Arthur Jensen]], who devised the statistical "method of correlated vectors" to test it. If Spearman's hypothesis holds true, then the cognitive tasks that have the highest ''g''-load are the tasks in which the gap between black and white test takers are greatest. Jensen and Rushton took this to show that the cause of ''g'' and the cause of the gap are the same—in their view, genetic differences.{{sfn|Rushton|Jensen|2005}}

{{harvtxt|Mackintosh|2011|pages=338–39}} acknowledges that Jensen and Rushton showed a modest correlation between ''g''-loading, heritability, and the test score gap, but does not agree that this demonstrates a genetic origin of the gap. Mackintosh argues that it is exactly those tests that Rushton and Jensen consider to have the highest ''g''-loading and heritability, such as the Wechsler test, that have seen the greatest increases in black performance due to the Flynn effect. This likely suggests that they are also the most sensitive to environmental changes, which undermines Jensen's argument that the black–white gap is most likely caused by genetic factors. {{harvtxt|Nisbett|Aronson|Blair|Dickens|2012a|page=146}} make the same point, noting also that the increase in the IQ scores of black test takers necessarily indicates an increase in ''g''.

James Flynn argued that his findings undermine Spearman's hypothesis.<ref>{{Cite journal |last1=Flynn |first1=J.R. |year=1999 |title=Searching for justice: the discovery of IQ gains over time |url=http://www.stat.columbia.edu/~gelman/stuff_for_blog/flynn.pdf |url-status=live |journal=American Psychologist |volume=54 |pages=5–9 |doi=10.1037/0003-066X.54.1.5 |archive-url=https://web.archive.org/web/20100625085640/http://www.stat.columbia.edu/~gelman/stuff_for_blog/flynn.pdf |archive-date=25 June 2010 |access-date=26 October 2017}}</ref> In a 2006 study, he and William Dickens found that between 1972 and 2002 "The standard measure of the ''g'' gap between Blacks and Whites declined virtually in tandem with the IQ gap."{{sfn|Dickens|Flynn|2006}} Flynn also criticized Jensen's basic assumption that a correlation between ''g''-loading and test score gap implies a genetic cause for the gap.<ref>{{cite journal |last=Flynn |first=James R. |year=2010 |title=The spectacles through which I see the race and IQ debate |url=http://www.iapsych.com/iqmr/fe/LinkedDocuments/flynn2010a.pdf |journal=Intelligence |volume=38 |issue=4 |pages=363–366 |doi=10.1016/j.intell.2010.05.001 |access-date=2011-02-18 |archive-date=2020-12-07 |archive-url=https://web.archive.org/web/20201207224050/http://www.iapsych.com/iqmr/fe/LinkedDocuments/flynn2010a.pdf |url-status=live }}</ref> In a 2014 suite of meta-analyses, along with co-authors Jan te Nijenhuis and Daniel Metzen, he showed that the same negative correlation between IQ gains and ''g''-loading obtains for cognitive deficits of known environmental cause: [[iodine deficiency]], [[prenatal cocaine exposure]], [[Fetal alcohol spectrum disorder|fetal alcohol syndrome]], and [[traumatic brain injury]].<ref>{{Cite journal |last1=Flynn |first1=James R. |last2=te Nijenhuis |first2=Jan |last3=Metzen |first3=Daniel |date=2014 |title=The g beyond Spearman's g: Flynn's paradoxes resolved using four exploratory meta-analyses |url=https://james-flynn.net/wp-content/uploads/2020/12/flynn2014-The-g-beyond-Spearmans-g-Flynns-paradoxes-resolved-using-four-exploratory-meta-analyses.pdf |journal=Intelligence |volume=44 |pages=1–10|doi=10.1016/j.intell.2014.01.009 }}</ref>
===Adoption studies===
A number of IQ studies have been done on the effect of similar rearing conditions on children from different races. The hypothesis is that this can be determined by investigating whether black children adopted into white families demonstrated gains in IQ test scores relative to black children reared in black families. Depending on whether their test scores are more similar to their biological or adoptive families, that could be interpreted as supporting either a genetic or an environmental hypothesis. Critiques of such studies question whether the environment of black children—even when raised in white families—is truly comparable to the environment of white children. Several reviews of the [[adoption study]] literature have suggested that it is probably impossible to avoid confounding biological and environmental factors in this type of study.{{sfn|Mackintosh|2011|p=337}} Another criticism by {{harvtxt|Nisbett|Aronson|Blair|Dickens|2012a|pages=134}} is that adoption studies on the whole tend to be carried out in a restricted set of environments, mostly in the medium-high SES range, where heritability is higher than in the low-SES range.

The [[Minnesota Transracial Adoption Study]] (1976) examined the [[intelligence quotient|IQ]] test scores of 122 [[adopted]] children and 143 nonadopted children reared by advantaged white families. The children were restudied ten years later.<ref name="Weinberg 1992">{{harvnb|Weinberg|Scarr|Waldman|1992}}</ref>{{sfn|Scarr|Weinberg|1976}}{{sfn|Loehlin|2000|p=185}} The study found higher IQ for white people compared to black people, both at age 7 and age 17.<ref name="Weinberg 1992"/> Acknowledging the existence of confounding factors, Scarr and Weinberg, the authors of the original study, did not consider that it provided support for either the hereditarian or environmentalist view.{{sfn|Scarr|Weinberg|1990}}

Three other studies lend support to environmental explanations of group IQ differences: 
*{{harvp|Eyferth|1961}} studied the out-of-wedlock children of black and white soldiers stationed in Germany after World War II who were then raised by white German mothers in what has become known as the [[Eyferth study]]. He found no significant differences in average IQ between groups. 
*{{harvp|Tizard et al.|1972}} studied black (West Indian), white, and mixed-race children raised in British long-stay residential nurseries. Two out of three tests found no significant differences. One test found higher scores for non-white people.
*{{harvp|Moore|1986}} compared black and mixed-race children adopted by either black or white middle-class families in the United States. Moore observed that 23 black and interracial children raised by white parents had a significantly higher mean score than 23 age-matched children raised by black parents (117 vs 104), and argued that differences in early socialization explained these differences.

Frydman and Lynn (1989) showed a mean IQ of 119 for Korean infants adopted by Belgian families. After correcting for the [[Flynn effect]], the IQ of the adopted Korean children was still 10 points higher than that of the Belgian children.{{sfn|Loehlin|2000|p=187}}<ref name="Frydman and Lynn">{{cite journal |author=Frydman and Lynn |journal=Personality and Individual Differences |volume=10 |issue=12 |pages=1323–1325 |year=1989 |title=The intelligence of Korean children adopted in Belgium |url=https://archive.org/details/sim_personality-and-individual-differences_1989_10_12/page/1323 |doi=10.1016/0191-8869(89)90246-8}}</ref>

Reviewing the evidence from adoption studies, Mackintosh finds that environmental and genetic variables remain confounded and considers evidence from adoption studies inconclusive, and fully compatible with a 100% environmental explanation.{{sfn|Mackintosh|2011|page=337}} Similarly, Drew Thomas argues that race differences in IQ that appear in adoption studies are in fact an artifact of methodology, and that East Asian IQ advantages and black IQ disadvantages disappear when this is controlled for.<ref>{{cite journal |last1=Thomas |first1=Drew |year=2017 |title=Racial IQ Differences among Transracial Adoptees: Fact or Artifact? |journal=Journal of Intelligence |volume=5 |issue=1 |page=1 |doi=10.3390/jintelligence5010001 |pmid=31162392 |pmc=6526420 |doi-access=free}}</ref>

===Racial admixture studies===
Most people have ancestry from different geographical regions. In particular, African Americans typically have ancestors from both Africa and Europe, with, on average, 20% of their genome inherited from European ancestors.<ref>{{harvnb|Bryc et al.|2009}}</ref> If racial IQ gaps have a partially genetic basis, one might expect black people with a higher degree of European ancestry to score higher on IQ tests than black people with less European ancestry, because the genes inherited from European ancestors would likely include some genes with a positive effect on IQ.{{sfn|Loehlin|2000}} Geneticist [[Alan Templeton]] has argued that an experiment based on the Mendelian "common garden" design, where specimens with different hybrid compositions are subjected to the same environmental influences, are the only way to definitively show a causal relation between genes and group differences in IQ. Summarizing the findings of admixture studies, he concludes that they have shown no significant correlation between any cognitive ability and the degree of African or European ancestry.{{sfn|Templeton|2001}}

Studies have employed different ways of measuring or approximating relative degrees of ancestry from Africa and Europe. Some studies have used skin color as a measure, and others have used blood groups. {{harvp|Loehlin|2000}} surveys the literature and argues that the blood groups studies may be seen as providing some support to the genetic hypothesis, even though the correlation between ancestry and IQ was quite low. He finds that studies by {{harvp|Eyferth|1961}}, Willerman, Naylor & Myrianthopoulos (1970) did not find a correlation between degree of African/European ancestry and IQ. The latter study did find a difference based on the race of the mother, with children of white mothers with black fathers scoring higher than children of black mothers and white fathers. Loehlin considers that such a finding is compatible with either a genetic or an environmental cause. All in all Loehlin finds admixture studies inconclusive and recommends more research.

Reviewing the evidence from admixture studies {{harvp|Hunt|2010}} considers it to be inconclusive because of too many uncontrolled variables. {{harvtxt|Mackintosh|2011|p=338}} quotes a statement by {{harvp|Nisbett|2009}} to the effect that admixture studies have not provided a shred of evidence in favor of a genetic basis for the IQ gap.

===Mental chronometry===
{{Main|Mental chronometry}}
[[Mental chronometry]] measures the elapsed time between the presentation of a sensory stimulus and the subsequent behavioral response by the participant. These studies have shown inconsistent results when comparing black and white populations groups, with some studies showing whites outperforming blacks, and others showing blacks outperforming whites.{{sfn|Sheppard|Vernon|2008}} 

Arthur Jensen argued that this reaction time (RT) is a measure of the speed and efficiency with which the brain processes information,<ref name="Jensen 2006">{{harvnb|Jensen|2006}}</ref> and that scores on most types of RT tasks tend to correlate with scores on standard IQ tests as well as with ''g''.<ref name="Jensen 2006" /> Nisbett argues that some studies have found correlations closer to 0.2, and that a correlation is not always found.<ref name="Nisbett 2009">{{harvnb|Nisbett|2009}}</ref> Nisbett points to the {{harvp|Jensen|Whang|1993}} study in which a group of Chinese Americans had longer reaction times than a group of European Americans, despite having higher IQs. Nisbett also mentions findings in {{harvp|Flynn|1991}} and {{harvp|Deary|2001}} suggesting that movement time (the measure of how long it takes a person to move a finger after making the decision to do so) correlates with IQ just as strongly as reaction time, and that average movement time is faster for black people than for white people.{{sfn|Nisbett|2009|pp=221–2}} {{harvtxt|Mackintosh|2011|page=339}} considers reaction time evidence unconvincing and comments that other cognitive tests that also correlate well with IQ show no disparity at all, for example the habituation/[[dishabituation]] test. He further comments that studies show that rhesus monkeys have shorter reaction times than American college students, suggesting that different reaction times may not tell us anything useful about intelligence.

===Brain size===
{{main|Brain size}}
A number of studies have reported a moderate statistical correlation between differences in IQ and brain size between individuals in the same group.{{sfn|Deary|Penke|Johnson|2010}}{{sfn|McDaniel|2005}} Some scholars have reported differences in average brain sizes between racial groups,{{sfn|Ho et al.|1980}} although this is unlikely to be a good measure of IQ as brain size also differs between men and women, but without significant differences in IQ.{{sfn|Nisbett|Aronson|Blair|Dickens|2012a}} At the same time newborn black children have the same average brain size as white children, suggesting that the difference in average size could be accounted for by differences in environment.{{sfn|Nisbett|Aronson|Blair|Dickens|2012a}} Several environmental factors that reduce brain size have been demonstrated to disproportionately affect black children.{{sfn|Nisbett|Aronson|Blair|Dickens|2012a}}

===Archaeological data===
Archaeological evidence does not support claims by Rushton and others that black people's cognitive ability was inferior to white people's during prehistoric times.{{sfn|MacEachern|2006}}