Race and intelligence

The first practical intelligence test was developed between 1905 and 1908 by Alfred Binet in France for school placement of children. Binet warned that results from his test should not be assumed to measure innate intelligence or used to label individuals permanently.[2] Binet's test was translated into English and revised in 1916 by Lewis Terman (who introduced IQ scoring for the test results) and published under the name Stanford–Binet Intelligence Scales. In 1916 Terman wrote that Mexican-Americans, African-Americans, and Native Americans have a mental "dullness [that] seems to be racial, or at least inherent in the family stocks from which they come."[3].

The US Army used a different set of tests developed by Robert Yerkes to evaluate draftees for World War I. Based on the Army's data, prominent psychologists and eugenicists such as Henry H. Goddard, Harry H. Laughlin, and Princeton professor Carl Brigham wrote that people from southern and eastern Europe were less intelligent than native-born Americans or immigrants from the Nordic countries, and that black Americans were less intelligent than white Americans.[4] The results were widely publicized by a lobby of anti-immigration activists, including the conservationist and theorist of scientific racism Madison Grant, who considered the so-called Nordic race to be superior, but under threat because of immigration by "inferior breeds." In his influential work, A Study of American Intelligence, psychologist Carl Brigham used the results of the Army tests to argue for a stricter immigration policy, limiting immigration to countries considered to belong to the "Nordic race".[5]

In the 1920s, some US states enacted eugenic laws, such as Virginia's 1924 Racial Integrity Act, which established the one-drop rule (of 'racial purity') as law. Many scientists reacted negatively to eugenicist claims linking abilities and moral character to racial or genetic ancestry. They pointed to the contribution of environment (such as speaking English as a second language) to test results.[6] By the mid-1930s, many psychologists in the US had adopted the view that environmental and cultural factors played a dominant role in IQ test results. The psychologist Carl Brigham repudiated his own earlier arguments, explaining that he had come to realize that the tests were not a measure of innate intelligence.

Discussions of the issue in the United States, especially in the writings of Madison Grant, influenced German Nazi claims that the "Nordics" were a "master race."[7] As American public sentiment shifted against the Germans, claims of racial differences in intelligence increasingly came to be regarded as problematic.[8] Anthropologists such as Franz Boas, Ruth Benedict, and Gene Weltfish did much to demonstrate that claims about racial hierarchies of intelligence were unscientific.[9] Nonetheless, a powerful eugenics and segregation lobby funded largely by textile-magnate Wickliffe Draper continued to use intelligence studies as an argument for eugenics, segregation, and anti-immigration legislation.[10]

As the desegregation of the American South gained traction in the 1950s, debate about black intelligence resurfaced. Audrey Shuey, funded by Draper's Pioneer Fund, published a new analysis of Yerkes' tests, concluding that black people really were of inferior intellect to white people. This study was used by segregationists to argue that it was to the advantage of black children to be educated separately from the superior white children.[11] In the 1960s, the debate was revived when William Shockley publicly defended the view that black children were innately unable to learn as well as white children.[12] Arthur Jensen expressed similar opinions in his Harvard Educational Review article, "How Much Can We Boost IQ and Scholastic Achievement?," which questioned the value of compensatory education for African-American children.[13] He suggested that poor educational performance in such cases reflected an underlying genetic cause rather than lack of stimulation at home or other environmental factors.[14][15]

Another revival of public debate followed the appearance of The Bell Curve (1994), a book by Richard Herrnstein and Charles Murray that supported the general viewpoint of Jensen.[16] A statement in support of Herrnstein and Murray titled "Mainstream Science on Intelligence," was published in The Wall Street Journal with 52 signatures. The Bell Curve also led to critical responses in a statement titled "Intelligence: Knowns and Unknowns" of the American Psychological Association and in several books, including The Bell Curve Debate (1995), Inequality by Design (1996) and a second edition of The Mismeasure of Man (1996) by Stephen Jay Gould.[17][18][17]

Some of the authors proposing genetic explanations for group differences have received funding from the Pioneer Fund, which was headed by J. Philippe Rushton until his death in 2012.[10][17][19][20][21] Arthur Jensen, who jointly with Rushton published a 2005 review article arguing that the difference in mean IQs between blacks and whites is partly due to genetics,[22] received $1.1 million from the Pioneer Fund.[23][24] According to Ashley Montagu, "The University of California's Arthur Jensen, cited twenty-three times in The Bell Curve's bibliography, is the book's principal authority on the intellectual inferiority of blacks."[25]

The Southern Poverty Law Center lists the Pioneer Fund as a hate group, citing the fund's history, its funding of race and intelligence research, and its connections with racist individuals.[26] Other researchers have criticized the Pioneer Fund for promoting scientific racism, eugenics and white supremacy.[10][27][28][29]

The concept of intelligence and the degree to which intelligence is measurable are matters of debate. There is no consensus about how to define intelligence; nor is it universally accepted that it is something that can be meaningfully measured by a single figure.[30] A recurring criticism is that different societies value and promote different kinds of skills and that the concept of intelligence is therefore culturally variable and cannot be measured by the same criteria in different societies.[30] Consequently, some critics argue that it makes no sense to propose relationships between intelligence and other variables.[31]

Correlations between scores on various types of IQ tests led English psychologist Charles Spearman to propose in 1904 the existence of an underlying factor, which he referred to as "g" or "general intelligence". With regard to this "g factor", Spearman claimed that "a person can no more be trained to have it in higher degree than he can be trained to be taller."[32] More recent proponents of this view include Arthur Jensen, who has argued that test score differences, especially in tasks considered to be particularly "g-loaded," reflect the test taker's innate capability.[33] This view, however, has been contradicted by a number of studies showing that even when accounting for "g-loading", education and changes in environment can significantly improve IQ test results.[34][35][36]

Other psychometricians have argued that, whether or not there is such a thing as a general intelligence factor, performance on tests relies crucially on knowledge acquired through prior exposure to the types of tasks that such tests contain. This means that comparisons of test scores between persons with widely different life experiences and cognitive habits do not reveal their relative innate potentials.[37]

The majority of anthropologists today consider race to be a sociopolitical phenomenon rather than a biological one,[38][39] a view supported by considerable genetics research.[40][41] The current mainstream view in the social sciences and biology is that race is a social construction based on folk ideologies that construct groups based on social disparities and superficial physical characteristics.[42] Sternberg, Grigorenko & Kidd (2005) state, "Race is a socially constructed concept, not a biological one. It derives from people's desire to classify."[31] The concept of human "races" as natural and separate divisions within the human species has also been rejected by the American Anthropological Association. The official position of the AAA, adopted in 1998, is that advances in scientific knowledge have made it "clear that human populations are not unambiguous, clearly demarcated, biologically distinct groups" and that "any attempt to establish lines of division among biological populations [is] both arbitrary and subjective."[43] A more recent statement from the American Association of Physical Anthropologists (2019) declares that "Race does not provide an accurate representation of human biological variation. It was never accurate in the past, and it remains inaccurate when referencing contemporary human populations. Humans are not divided biologically into distinct continental types or racial genetic clusters."[44]

In studies of human intelligence, race is almost always determined using self-reports rather than analyses of genetic characteristics. According to psychologist David Rowe, self-report is the preferred method for racial classification in studies of racial differences because classification based on genetic markers alone ignore the "cultural, behavioral, sociological, psychological, and epidemiological variables" that distinguish racial groups.[45] Hunt and Carlson write that "Nevertheless, self-identification is a surprisingly reliable guide to genetic composition. Tang et al. (2005) applied mathematical clustering techniques to sort genomic markers for over 3,600 people in the United States and Taiwan into four groups. There was almost perfect agreement between cluster assignment and individuals' self-reports of racial/ethnic identification as white, black, East Asian, or Latino."[46] Sternberg and Grigorenko disagree with Hunt and Carlson's interpretation of Tang's results as supporting the view that racial divisions are biological; rather, "Tang et al.'s point was that ancient geographic ancestry rather than current residence is associated with self-identification and not that such self-identification provides evidence for the existence of biological race."[47]

Anthropologist C. Loring Brace[48] and geneticist Joseph Graves disagree with the idea that cluster analysis and the correlation between self-reported race and genetic ancestry support the notion of biological races.[49] They argue that while it is possible to find biological and genetic variation corresponding roughly to the groupings normally defined as races, this is true for almost all geographically distinct populations. The cluster structure of the genetic data is dependent on the initial hypotheses of the researcher and the populations sampled. When one samples continental groups, the clusters become continental; if one had chosen other sampling patterns, the clusters would be different. Kaplan 2011 concludes that, while differences in particular allele frequencies can be used to identify populations that loosely correspond to the racial categories common in Western social discourse, the differences are of no more biological significance than the differences found between any human populations (e.g., the Spanish and Portuguese).

In the US, generally individuals identifying themselves as Asian tend to score higher on IQ tests than do Caucasians, who score higher than Hispanics, who score higher than African Americans. Nevertheless, greater variation in IQ scores exists within each ethnic group than between them.[52] Roth et al. (2001), in a review of the results of a total of 6,246,729 participants in other tests of cognitive ability or aptitude, found a difference in mean scores between black people and white people of 1.1 SD. Consistent results were found for college and university application tests such as the Scholastic Aptitude Test (N = 2.4 million) and Graduate Record Examination (N = 2.3 million), as well as for tests of job applicants in corporate settings (N = 0.5 million) and in the military (N = 0.4 million).[53]

In response to the controversial 1994 book The Bell Curve, the American Psychological Association (APA) formed a task-force of eleven experts, which issued a report "Intelligence: Knowns and Unknowns" in 1996.[54] Regarding group differences, the report reaffirmed the consensus that differences within groups are much wider than differences between groups, and that claims of ethnic differences in intelligence should be scrutinized carefully, as such claims had been used to justify racial discrimination. The report also acknowledged problems with the racial categories used, as these categories are neither consistently applied, nor homogeneous (see also race and ethnicity in the United States).[55]

In the UK, some African groups have higher educational attainment and standardized test scores than the population on average.[56] In 2010 - 2011, white British pupils were 2.3% less likely to have gained 5 A*–C grades at GCSE than the national average, whereas the likelihood was 21.8% above average for those of Nigerian and 5.5% for those of Ghanaian origin.[57][58] In 2014, Black-African pupils of 11 language groups were more likely to pass Key Stage 2 Maths 4+ in England than the national average. While the White British average was 84.6%, there were four Black African groups (Yoruba, Igbo, Hausa and Amharic speakers) that had a pass rate of over 90%[59]. In 2017/2018, percentage of pupils getting a strong pass (grade 5 or above) in English and maths GCSE (in Key Stage 4) was higher among Black-African pupils (44.3%) than white British pupils (42.67%).[60].

During the 20th century raw scores on IQ tests were rising; this score increase is known as the "Flynn effect," named after James R. Flynn. In the United States, the increase was continuous and approximately linear from the earliest years of testing to about 1998 when the gains stopped and some tests even showed decreasing test scores. For example, the average scores of black people on some IQ tests in 1995 were the same as the scores of white people in 1945.[61] As one pair of academics phrased it, "the typical African American today probably has a slightly higher IQ than the grandparents of today's average white American."[62]

Flynn has argued that, given that these changes took place between one generation and the next, it is highly unlikely that genetic factors could have accounted for the increasing scores, which must then have been caused by environmental factors. The importance of the Flynn effect in the debate over the causes of the black/white IQ gap lies in demonstrating that environmental factors may cause changes in test scores on the scale of 1 SD. This had previously been doubted.

A separate phenomenon from the Flynn effect has been the discovery that the IQ gap was gradually closing over the last decades of the 20th century, as black test-takers increased their average scores relative to white test-takers. For instance, Vincent reported in 1991 that the black–white IQ gap was decreasing among children, but that it was remaining constant among adults.[63] Similarly, a 2006 study by Dickens and Flynn estimated that the difference between mean scores of black people and white people closed by about 5 or 6 IQ points between 1972 and 2002,[64] a reduction of about one-third. In the same period, the educational achievement disparity also diminished.[65] Reviews by Flynn and Dickens,[66] Mackintosh,[67] and Nisbett et al[68] accept the gradual closing of the gap as a fact.

Percentage of children aged 1–5 with blood lead levels at least 10 µg/dL. Black and Hispanic children have much higher levels than white children. A 10 µg/dL increase in blood lead at 24 months is associated with a 5.8-point decline in IQ.[69] Although the Geometric Mean Blood Lead Levels (GM BLL) are declining, a CDC report (2002) states that: "However, the GM BLL for non-Hispanic black children remains higher than that for Mexican-American and non-Hispanic white children, indicating that differences in risk for exposure still persist."[70]

Environmental factors including childhood lead exposure,[69] low rates of breast feeding,[71] and poor nutrition[72][73] can significantly affect cognitive development and functioning. For example, childhood exposure to lead, associated with homes in poorer areas[74] causes an average IQ drop of 7 points,[75] and iodine deficiency causes a fall, on average, of 12 IQ points.[76][77] Such impairments may sometimes be permanent, sometimes be partially or wholly compensated for by later growth. The first two years of life is the critical time for malnutrition, the consequences of which are often irreversible and include poor cognitive development, educability, and future economic productivity.[78] The African American population of the United States is statistically more likely to be exposed to many detrimental environmental factors such as poorer neighborhoods, schools, nutrition, and prenatal and postnatal health care.[79][80] Mackintosh points out that for American black people infant mortality is about twice as high as for white people, and low birthweight is twice as prevalent. At the same time white mothers are twice as likely to breastfeed their infants, and breastfeeding is highly correlated with IQ for low birthweight infants. In this way a wide number of health related factors that influence IQ are unequally distributed between the two groups.[81]

The Copenhagen consensus in 2004 stated that lack of both iodine and iron has been implicated in impaired brain development, and this can affect enormous numbers of people: it is estimated that one-third of the total global population are affected by iodine deficiency. In developing countries, it is estimated that 40% of children aged four and under suffer from anaemia because of insufficient iron in their diets.[82]

Other scholars have found that simply the standard of nutrition has a significant effect on population intelligence, and that the Flynn effect may be caused by increasing nutrition standards across the world.[83] James Flynn has himself argued against this view.[84]

Some recent research has argued that the retardation caused in brain development by infectious diseases, many of which are more prevalent in non-white populations, may be an important factor in explaining the differences in IQ between different regions of the world.[85] The findings of this research, showing the correlation between IQ, race and infectious diseases was also shown to apply to the IQ gap in the US, suggesting that this may be an important environmental factor.[86]

A 2013 meta-analysis by the World Health Organization found that, after controlling for maternal IQ, breastfeeding was associated with IQ gains of 2.19 points. The authors suggest that this relationship is causal but state that the practical significance of this gain is debatable; however, they highlight one study suggesting an association between breastfeeding and academic performance in Brazil, where "breastfeeding duration does not present marked variability by socioeconomic position."[87] Colen and Ramey (2014) similarly find that controlling for sibling comparisons within families, rather than between families, reduces the correlation between breastfeeding status and WISC IQ scores by nearly a third, but further find the relationship between breastfeeding duration and WISC IQ scores to be insignificant. They suggest that "much of the beneficial long-term effects typically attributed to breastfeeding, per se, may primarily be due to selection pressures into infant feeding practices along key demographic characteristics such as race and socioeconomic status."[88] Reichman estimates that no more than 3 to 4% of the black–white IQ gap can be explained by black–white disparities in low birth weight.[89]

Several studies have proposed that a large part of the gap can be attributed to differences in quality of education.[90] Racial discrimination in education has been proposed as one possible cause of differences in educational quality between races.[91] According to a paper by Hala Elhoweris, Kagendo Mutua, Negmeldin Alsheikh and Pauline Holloway, teachers' referral decisions for students to participate in gifted and talented educational programs were influenced in part by the students' ethnicity.[92]

The Abecedarian Early Intervention Project, an intensive early childhood education project, was also able to bring about an average IQ gain of 4.4 points at age 21 in the black children who participated in it compared to controls.[71] Arthur Jensen agreed that the Abecedarian project demonstrated that education can have a significant effect on IQ, but also declared his view that no educational program thus far had been able to reduce the black–white IQ gap by more than a third, and that differences in education are thus unlikely to be its only cause.[93]

A series of studies by Joseph Fagan and Cynthia Holland measured the effect of prior exposure to the kind of cognitive tasks posed in IQ tests on test performance. Assuming that the IQ gap was the result of lower exposure to tasks using the cognitive functions usually found in IQ tests among African American test takes, they prepared a group of African Americans in this type of tasks before taking an IQ test. The researchers found that there was no subsequent difference in performance between the African-Americans and white test takers.[94][95] Daley and Onwuegbuzie conclude that Fagan and Holland demonstrate that "differences in knowledge between black people and white people for intelligence test items can be erased when equal opportunity is provided for exposure to the information to be tested".[96] A similar argument is made by David Marks who argues that IQ differences correlate well with differences in literacy suggesting that developing literacy skills through education causes an increase in IQ test performance.[97][98]

A 2003 study found that two variables—stereotype threat and the degree of educational attainment of children's fathers—partially explained the black–white gap in cognitive ability test scores, undermining the hereditarian view that they stemmed from immutable genetic factors.[99]

Different aspects of the socioeconomic environment in which children are raised have been shown to correlate with part of the IQ gap, but they do not account for the entire gap.[100] According to a 2006 review, these factors account for slightly less than half of one standard deviation.[101]

Other research has focused on different causes of variation within low socioeconomic status (SES) and high SES groups.[102][103][104] In the US, among low SES groups, genetic differences account for a smaller proportion of the variance in IQ than among high SES populations.[105] Such effects are predicted by the bioecological hypothesis—that genotypes are transformed into phenotypes through nonadditive synergistic effects of the environment.[106] Nisbett et al. (2012a) harvp error: multiple targets (2×): CITEREFNisbettAronsonBlairDickens2012a (help) suggest that high SES individuals are more likely to be able to develop their full biological potential, whereas low SES individuals are likely to be hindered in their development by adverse environmental conditions. The same review also points out that adoption studies generally are biased towards including only high and high middle SES adoptive families, meaning that they will tend to overestimate average genetic effects. They also note that studies of adoption from lower-class homes to middle-class homes have shown that such children experience a 12 to 18 point gain in IQ relative to children who remain in low SES homes.[107] A 2015 study found that environmental factors (namely, family income, maternal education, maternal verbal ability/knowledge, learning materials in the home, parenting factors, child birth order, and child birth weight) accounted for the black–white gap in cognitive ability test scores.[108]

A number of studies have reached the conclusion that IQ tests may be biased against certain groups.[109][110][111][112] The validity and reliability of IQ scores obtained from outside the United States and Europe have been questioned, in part because of the inherent difficulty of comparing IQ scores between cultures.[113][114] Several researchers have argued that cultural differences limit the appropriateness of standard IQ tests in non-industrialized communities.[115][116]

A 1996 report by the American Psychological Association states that intelligence can be difficult to compare across cultures, and notes that differing familiarity with test materials can produce substantial differences in test results; it also says that tests are accurate predictors of future achievement for black and white Americans, and are in that sense unbiased.[55] The view that tests accurately predict future educational attainment is reinforced by Nicholas Mackintosh in his 1998 book IQ and Human Intelligence,[117] and by a 1999 literature review by Brown, Reynolds & Whitaker (1999).

James R. Flynn, surveying studies on the topic, notes that the weight and presence of many test questions depends on what sorts of information and modes of thinking are culturally valued.[118]

Stereotype threat is the fear that one's behavior will confirm an existing stereotype of a group with which one identifies or by which one is defined; this fear may in turn lead to an impairment of performance.[119] Testing situations that highlight the fact that intelligence is being measured tend to lower the scores of individuals from racial-ethnic groups who already score lower on average or are expected to score lower. Stereotype threat conditions cause larger than expected IQ differences among groups.[120] Psychometrician Nicholas Mackintosh considers that there is little doubt that the effects of stereotype threat contribute to the IQ gap between black people and white people.[121]

A large number of studies have shown that systemically disadvantaged minorities, such as the African American minority of the United States, generally perform worse in the educational system and in intelligence tests than the majority groups or less disadvantaged minorities such as immigrant or "voluntary" minorities.[55] The explanation of these findings may be that children of caste-like minorities, due to the systemic limitations of their prospects of social advancement, do not have "effort optimism", i.e. they do not have the confidence that acquiring the skills valued by majority society, such as those skills measured by IQ tests, is worthwhile. They may even deliberately reject certain behaviors that are seen as "acting white."[55][122][123]

Research published in 1997 indicates that part of the black–white gap in cognitive ability test scores is due to racial differences in test motivation.[124]

However, attempts to replicate studies evincing significant effects of stereotype threat have not yielded the same results. In 2004 Sackett et al. found that eliminating stereotype threat does not eliminate the racial test performance gap, and in 2005 Tyson et al. found that African Americans have motivation similar to or even better than that of white Americans.[125][126] Self-affirmation exercises promoted by research scientists such as Geoffrey L. Cohen have not been shown to be effective in attempts to replicate his studies.[127]

Geneticist Alan R. Templeton argues 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.[131] Templeton points out that racial groups neither represent sub-species nor distinct evolutionary lineages, and that therefore there is no basis for making claims about the general intelligence of races.[131] From this point of view 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. Mackintosh (2011) harvp error: multiple targets (2×): CITEREFMackintosh2011 (help), however, argues 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 an average lower 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 gap thus cannot be ruled out on a priori grounds.

Intelligence is a polygenic trait. This means that intelligence is under the influence of several genes, possibly several thousand. The effect of most individual genetic variants on intelligence is thought to be very small, well below 1% of the variance in g. Current studies using quantitative trait loci have yielded little success in the search for genes influencing intelligence. Robert Plomin is confident that QTLs responsible for the variation in IQ scores exist, but due to their small effect sizes, more powerful tools of analysis will be required to detect them.[132] Others assert that no useful answers can be reasonably expected from such research before an understanding of the relation between DNA and human phenotypes emerges.[80] Several candidate genes have been proposed to have a relationship with intelligence.[133][134] However, a review of candidate genes for intelligence published in Deary, Johnson & Houlihan (2009) failed to find evidence of an association between these genes and general intelligence, stating "there is still almost no replicated evidence concerning the individual genes, which have variants that contribute to intelligence differences".[135] In 2001, a review in the Journal of Black Psychology refuted eight major premises on which the hereditarian view regarding race and intelligence is based.[136]

A 2005 literature review article by Sternberg, Grigorenko and Kidd stated that no gene has been shown to be linked to intelligence, "so attempts to provide a compelling genetic link of race to intelligence are not feasible at this time."[137] Hunt (2010, p. 447) concurs with this critique, noting that "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." Mackintosh (2011, p. 344) harvtxt error: multiple targets (2×): CITEREFMackintosh2011 (help) concurs as well, noting that while several environmental factors have been shown to influence the IQ gap, the evidence for a genetic influence has been negligible. The 2012 review by Nisbett et al. (2012a) harvp error: multiple targets (2×): CITEREFNisbettAronsonBlairDickens2012a (help) concluded that "Almost no genetic polymorphisms have been discovered that are consistently associated with variation in IQ in the normal range." They consider the entire IQ gap to be explained by the environmental factors that have thus far been demonstrated to influence it, and Mackintosh finds this view to be reasonable.

An environmental factor that varies between groups but not within groups can cause group differences in a trait that is otherwise 100 percent heritable.

Twin studies of intelligence have reported high heritability values. However, these studies have been criticized for being based on questionable assumptions.[138][139][140] 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.[141] 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.[142]

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.[55][135] In biology heritability is defined as the ratio of variation attributable to genetic differences in an observable 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.[55] 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).[143]

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.[143] In his criticism of the Bell Curve, Noam Chomsky further illustrated this with the example of women wearing earrings:

To borrow an example from Ned Block, "some years ago when only women wore earrings, the heritability of having an earring was high because differences in whether a person had an earring was due to a chromosomal difference, XX vs. XY." No one has yet suggested that wearing earrings, or ties, is "in our genes"...[144]

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 take this to show that the cause of g and the cause of the gap are the same—in their view, genetic differences.[22]

Mackintosh (2011, pp. 338–39) harvtxt error: multiple targets (2×): CITEREFMackintosh2011 (help) 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. Mackintosh also argues that Spearman's hypothesis, which he considers to be likely to be correct, simply shows that the test score gap is based on whatever cognitive faculty is central to intelligence, but does not show what this factor is. Nisbett et al. (2012a, p. 146) harvtxt error: multiple targets (2×): CITEREFNisbettAronsonBlairDickens2012a (help) make the same point, noting also that the increase in the IQ scores of black test takers necessarily indicates an increase in g.

Flynn has criticized Jensen's basic assumption that confirmation of Spearman's hypothesis would support a partially genetic explanation for IQ differences. He argues that, no matter what the causes are of average group IQ differences, one would expect the differences to be greater for more complex tasks. Flynn thus sees the correlation between g-loading and the test score gap to offer no clue to the cause of the gap.[145]

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.[146] Another criticism by Nisbett et al. (2012a, pp. 134) harvtxt error: multiple targets (2×): CITEREFNisbettAronsonBlairDickens2012a (help) 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 IQ test scores of 122 adopted children and 143 nonadopted children reared by advantaged white families. The children were restudied ten years later.[147][148][149] The study found higher IQ for white people compared to black people, both at age 7 and age 17.[147] 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.[150]

Three other studies lend support to environmental explanations of group IQ differences:

• 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.
• 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.
• 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.[151][22][152]

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.[146] 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.[153]

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.[154] 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.[155] 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.[156]

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. 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 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 Hunt (2010) considers it to be inconclusive because of too many uncontrolled variables. Mackintosh (2011, p. 338) harvtxt error: multiple targets (2×): CITEREFMackintosh2011 (help) quotes a statement by 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 measures the elapsed time between the presentation of a sensory stimulus and the subsequent behavioral response by the participant. This reaction time (RT) is considered a measure of the speed and efficiency with which the brain processes information.[157] Scores on most types of RT tasks tend to correlate with scores on standard IQ tests as well as with g, and no relationship has been found between RT and any other psychometric factors independent of g.[157] The strength of the correlation with IQ varies from one RT test to another, but Hans Eysenck gives 0.40 as a typical correlation under favorable conditions.[158] According to Jensen, individual differences in RT have a substantial genetic component, and heritability is higher for performance on tests that correlate more strongly with IQ.[159] Nisbett argues that some studies have found correlations closer to 0.2, and that a correlation is not always found.[160]

Several studies have found differences between races in average reaction times. These studies have generally found that reaction times among black, Asian and white children follow the same pattern as IQ scores.[161][162][163] Black–white differences in reaction time, however, tend to be small (average effect size .18).[164] Rushton & Jensen (2005) have argued that reaction time is independent of culture and that the existence of race differences in average reaction time is evidence that the cause of racial IQ gaps is partially genetic. Responding to this argument in Intelligence and How to Get It, Nisbett points to the 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 Flynn (1991) and 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.[165] Mackintosh (2011, p. 339) harvtxt error: multiple targets (2×): CITEREFMackintosh2011 (help) 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.

A number of studies have reported a moderate statistical correlation between differences in IQ and brain size between individuals in the same group.[166][167] Some scholars have reported differences in average brain sizes between racial groups,[168] 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. 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. Several factors that reduce brain size have been demonstrated to disproportionately affect black children.[107]

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 as a result of evolution.[169]