Race and intelligence: The genetics or not debate

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This article on Race and intelligence: The genetics or not debate is a sub-article of the main Race and intelligence article. See the main article for necessary background.

The scientific debate regarding whether the measured racial IQ differences (in particular the well-studied US Black-White gap) are partially genetic involves numerous different scientific studies using many different kinds of evidence. It sometimes uses non-trivial mathematics. These arguments have been summarized in several long review articles which are linked to in the "External links" section. This article will only review some more important arguments and misconceptions. The terms Blacks and Whites in this article refer to these groups in the United States unless otherwise stated.


Opinion of experts

Opinion of experts in 1987

The 1987 "Survey of Expert Opinion on Intelligence and Aptitude Testing" was published in American Psychologist (the official journal of the American Psychological Association). It surveyed 1020 IQ experts, 661 of which completed the questionnaire. One question was "Which of the following best characterizes your opinion of the heritability of black-white differences in IQ?"[1]

  • 14% declined to answer the question.
  • 24% voted that there was insufficient evidence to give an answer.
  • 1% voted that the gap was "due entirely to genetic variation".
  • 15% voted that it "due entirely to environmental variation".
  • 45% voted that it was a "product of genetic and environmental variation".[1]

The survey formed one part of the 1988 book The IQ Controversy, the Media and Public Policy, which argued that the public was misled regarding the expert opinion on the issue.[2]

Opinion of experts in 2013

In 2013, a similar survey was presented. Researchers were invited to participate only if they had recent intelligence-related publications in peer-reviewed journals. Invitations were emailed to 1237 persons. 228 (18 %) answered the survey (70 fully and 158 partially). As far as the authors could make it out, “lefties” and “righties” turned down the offer in equal numbers.[3]

Asked: "What is the influence of average cognitive ability level and highly cognitive competent persons on positive development of society, the economy, technology, democracy and culture?"

  • All of the results were above the mid point, suggesting agreement about a positive relationship between high intelligence and social progress.[3]

Asked: "What are the sources of U.S. black-white differences in IQ?"

  • 0% of differences due to genes: 17%
  • 0-40% of differences due to genes: 42%
  • 50% of differences due to genes: 18%
  • 60-100% of differences due to genes: 39%
  • 100% of differences due to genes: 5%
  • On average for all experts, 47% of the difference was due to genes (SD=31%).[3]

Out of 26 media sources on intelligence, only 3 were rated better than 5 out of 9. Experts rated public debates on intelligence as twice as likely to be ideological than scientific.[3]

Opinion of experts in 2016

A survey published in 2016 surveyed experts who had published articles on or after 2010 in journals on intelligence, cognitive abilities, and student achievement. A total of 1345 people were invited. 71 responded and answered the questions on cross-national cognitive ability differences. Those surveyed were asked to state a percentage rating indicating the importance of 15 possible causes of measured cognitive ability differences.[4]

Many of the 15 possible factors included in the survey may be influenced by genetics and genetic groups differences. The survey did not include a single binary question on which overall factor, genetics or environment, is the more important explanation for the differences.[4] Thus, it cannot be compared in that respect with the 1987 and 2013 surveys.

Also, cross-national differences are not the same as the US Black-White differences asked about in the 1987 and 2013 surveys. Also supporters of a partially genetic explanation generally argue that environmental factors are more important in developing countries than in developed countries.

"Seventy-one experts rated possible causes of cross-national differences in cognitive ability based on psychometric IQs and student assessment studies (e.g., PISA, PIRLS, TIMSS). Genes were rated as the most important cause (17%), followed by educational quality (11.44%), health (10.88%), and educational quantity (10.20%) [...] Only 5 of 71 experts (7%) who responded to the genetic item thought that genes had no influence. [...] Items with lower percentages (< 10%) included wealth, culture, and modernization (7–9%). Methodological bias factors (sampling error, test knowledge, test bias) were rated as less important (3–6%, together 11.78%). [...] The low ratings for methodological factors suggest that international assessments were perceived to be valid indicators of cognitive ability and cross-country patterns. [...] Because environmental theories are rarely questioned in research, their corroboration by an expert survey is not astonishing. More important is the support for the frequently hotly disputed genetic explanations. Assuming that the survey is representative of expert opinions, genetic factors should receive more attention in future research and public debates."[4]

There were also similar questions on some more specific groups such as "Immigrants from the Middle East (Arabian and Muslim countries) [...] In the current study, experts attributed the low test results primarily to genetic factors (23.01%) and culture (17.26%). Discrimination was rated as relatively unimportant for Middle Eastern immigrants (2.27%) and for other groups and countries (1.25%)."[4]

IQ test reliability/validity

Criticisms against the existence of races and IQ tests

Regarding the existence of races, see Race, Arguments regarding the existence of races, and Race and intelligence: The existence of races.

Regarding IQ, see Intelligence quotient and Race and intelligence: Significance of IQ differences.

Test bias

The American Psychological Association's report Intelligence: Knowns and Unknowns stated that the US Black-White IQ gap "does not result from any obvious biases in test construction and administration, nor does it simply reflect differences in socioeconomic status." It also stated that "Considered as predictors of future performance, the tests do not seem to be biased against African Americans" and "It has been suggested that various aspects of the way tests are formulated and administered may put African Americans at a disadvantage. [...] Many of these suggestions are plausible, and such mechanisms may play a role in particular cases. Controlled studies have shown, however, that none of them contributes substantially to the Black/White differential [...] Moreover, efforts to devise reliable and valid tests that would minimize disadvantages of this kind have been unsuccessful."[5] Also many later studies are argued to support this conclusion.[6] The US Black-White gap is of particular interest, since the genetics or not debate is primarily about this very well studied gap, with the results regarding the other gaps usually only used as supplementary evidence.

There may be problems with testing US immigrants or other persons who are not native English speakers, if using English verbal IQ tests. On the other hand, such groups may be tested with non-verbal tests, such as Raven's Progressive Matrices. Hereditarians have argued that studies have shown that IQ test scores predict school grades and job performance equally well for Africans and non-Africans.[7][8][9]

If two groups are tested on the same test and if this test accurately measures the same trait in both groups, then even if the groups have different mean trait levels, the order of difficulty of the items or subtests should be similar. This has been found to generally be the case for Raven's Progressive Matrices for a wide variety of tested groups from different parts of the world.[10]

There are also some measurements described in this article that are completely different from traditional IQ tests, such as measurements of brain size, reaction time, and DNA.

Stereotype threat and stereotype expectation

Stereotype threat is an argued fear that a person's behavior will confirm to an existing stereotype of a group to which the person belongs. This may in turn lead to an impairment of the person's performance. This has been seen as one explanation for various not politically correct gaps.

Early laboratory experiments finding a stereotype threat effect have been greatly misreported (in both popular and academic literature) and falsely described as having shown that stereotype threat explains the whole gap(s).[11] A 2015 meta-analysis of gender stereotype threat for schoolgirls on math, science, and spatial tests found only a small effect, that likely was inflated due to publication bias (studies finding no effect for various reasons not being published).[12]

A 2014 book on topics such as stereotypes and self-fulfilling prophecies stated that "Although the scientific evidence may be equivocal regarding whether teacher expectation effects on IQ are nonexistent or reliably very small, it is completely unequivocal that such effects, if they occur at all, are not very large by any standard."[13]

Stability and similarity of IQ test results

Stability of IQ in adulthood

In adulthood, IQ is very stable. This is problematic for many kinds of environmental explanations which assume that IQ is greatly malleable by environmental factors.[14][6]

More variable IQ during childhood must not necessarily be due to (or only due to) environmental factors. For example, it is possible that different sets of genes affect the developing brain at different time periods during childhood, which could cause fluctuations in IQ scores and IQ differences during childhood.

Similarity of racial IQ gaps worldwide

Hereditarians have argued that similar racial IQ gaps and IQ related racial gaps are found in and between many different examined countries worldwide, including in examined multiracial countries. This regardless of if a race is the majority group (such as East Asians in East Asia) or if the race is a minority group (such as East Asians in many other countries worldwide and always scoring high, despite their ancestors sometimes being very poor laborers when they arrived). When there are mixed race groups present in a country, their average IQ score is between the average IQ scores of the ancestral races of the mixed race group.[15][7][16]

See also Effects of race mixing: Latin America.

Lynn stated in 2010 that "The evolutionary theory does however predict that when different races occupy approximately similar environments, such as for instance in the United States, Britain and the Netherlands, the intelligence differences will remain. This prediction has been examined in twenty three societies worldwide in Lynn (2008) and has been confirmed in every case."[17][16]


As stated in the section "Similarity of the racial IQ gaps worldwide", similar racial IQ gaps are argued to be found worldwide. This applies also to immigrant groups and their descendants.

The book The Nature of Race: the Genealogy of the Concept and the Biological Construct’s Contemporaneous Utility stated regarding immigrant cognitive ability that "The matter, of course, is complicated by migrant selectivity, ethnic identification attrition, differential breeding patterns, non-trivial environmental influence on measures, and so on. Yet, were a racial hereditarian position correct, one would expect to find, when looking across numerous countries, a robust statistical association between region of origin scores and migrant scores" and "the cognitive ability scores of international migrants tend to correlate with the cognitive ability scores of those from the regions of origin. That is, to some extent, contemporaneous migrants carry their region of origin abilities with them and the differences brought persist at least until the second or third generation (Carabaña, 2011; De Philippis, 2013; Fuerst, 2014; Kirkegaard, 2015)." This has been argued to support a partially genetic explanation.[18]

A 2014 meta-analysis of the first, second, and third+ generation US immigrants of different races/ethnicities (Asians, Blacks, Hispanics, and Whites) found relatively stable IQ gaps between the different races. The Hispanic-White IQ gap had narrowed somewhat in the third+ generation of immigrants, as compared to first generation of immigrants, but was still large (0.57 SD). The narrowing could be due to improved language abilities and to increasing share of White ancestry in later generations, which may be due to race mixing with Whites. However, improved language ability was argued to be unlikely as an explanation for the IQ gap for second+ generation Hispanics, for reasons such as English-only speaking 2nd+ generation Hispanics not having better scores.[19]

West Indians of African origin that emigrate to the US are more successful than US Blacks. This has been seen as possible evidence for some environmental factor affecting US Blacks negatively. Another explanation is that these emigrants are not a random sample of West Indians of African origin, but a selected group with unusual characteristics. One 2008 study wrote that "West Indian success can be attributed entirely to the greater talent and ambition of those who choose to move. Similarly, the subset of African Americans who are voluntary internal migrants are better off than their less venturesome counterparts. Once this point is clear, it is easy to see why West Indian success offers no lessons for African American improvement."[20]

African immigrants to the United Kingdom perform well on measures such as the GCSE (General Certificate of Secondary Education). However, this as evidence for general African IQ has been questioned, for reasons such as questioning the GCSE as a proxy for IQ, the results possibly being unrepresentative of African immigrants to the United Kingdom since students may only attempt those exams they can do well on, politically correct bias against White students, and Africans immigrants to the UK possibly having higher IQ than Africans in general (a US study found that African immigrants had 1/3 SD higher IQ than the mean IQ of their home countries). Furthermore, the results may be due to many immigrants being Igbos, a Nigerian group that has for a long time specialized in trading lifestyle and that is well-known for high academic achievement. Immigrants from other African countries perform much worse. More generally, many "Blacks" in the United Kingdom are likely mixed race.[21]

See also Countries and intelligence: Immigrant social outcomes and country of origin average IQ.

Historical differences

See Race and intelligence: Historical differences.

Regression to the mean

Regression to the mean refers to the tendency for an exceptional result, such as getting all 6s or all 1s when rolling several dice, to be followed by a less exceptional result (regression), that is closer to the average (mean) result. Regression to the mean regarding IQ does not mean complete but partial regression. Thus, it does not imply that the children of exceptional parents will have an IQ equal to the mean group IQ, but instead that they (on average) will have somewhat less exceptional IQs that still differ from the mean group IQ.

Hereditarians argue that the relatives of Blacks and Whites with exceptional IQs, low or high, will show predictable differences in regression to the mean(s), due to the Black and the White populations having different genotypic average (mean) IQs. These predictions are argued to be confirmed. This pattern of regression is fairly constant across generations, across social classes, and across the IQ spectrum. The same effects may possibly happen also due to environmental factors that behave similarly to IQ genes, but hereditarians have argued that this is unlikely and that no such factors have been presented.[8][7][6]

Other hereditarians have instead argued that regression to the mean is evidence against many but not all environmental explanations, such as being evidence against environmental factors that are not equally depressing all Blacks families across all social classes and across the IQ spectrum. It is also argued to be evidence against unshared environmental factors (see the section "Shared and unshared environmental factors").[6][22][23]

A genetic regression to the mean can explain why Black children born to well-educated, affluent parents have test scores 2 to 4 points lower than do White children born to poorly-educated, impoverished parents.[8]

Argued narrowing of the US Black-White gap and the Flynn effect

Non-hereditarians have frequently argued for a narrowed US Black-White gap and more generally pointed to the Flynn effect as arguments for changing IQ test results and the gaps possibly disappearing in the future. Regarding an argued narrowed US Black-White gap, see Race and intelligence: United States Black-White gap and in this article the section "Shared and unshared environmental factors". Regarding the Flynn effect, see the section "The Flynn effect".

Specific proposed environmental explanations

Socioeconomic factors

See also: The sociologist's fallacy

Controlling for different average socioeconomic status (SES) of Blacks and Whites only reduces the Black-White IQ gap by a third or 5 points. Furthermore, if the Black-White IQ gap is in part caused by genetics, then this number is overstated since the Black-White SES gap is partially caused by the Black-White IQ gap.[7] Not considering such effects is one example of The sociologist's fallacy.

Studies, although not studying racial IQ differences, have found that genetic factors affect SES and its association with IQ. In recent years such studies have used recently available genetic methods. One study stated that "our results emphasize the need to consider genetics in research and policy on family SES and its association with children's IQ."[24][25][26]

Diagram from a 2013 analysis showing that higher parental education is associated with offspring having higher IQ, but not with reduced Black-White IQ gap.[27]
Similar diagram based on the 1994 book The Bell Curve.[28]

An environment-only explanation for the Black-White IQ gap, arguing for poorer Black SES as the explanation for the gap, predicts that the IQ gap should be smaller at higher levels of parental SES, since these children should be less exposed to the environmental factors supposedly lowering IQ. However, the gap is actually equal or larger at higher parental SES levels.[7] In contrast, hereditarians can explain this by regression to different racial genetic averages (see the section "Regression to the mean" below).[27] Another explanation is Black parents having lower average genetic IQ than White parents, despite having similar SES. This may be due to factors such as affirmative action causing discrimination against Whites in education/employment.[16]

Many other factors potentially affecting IQ are likely affected by socioeconomic factors. For example, the income level can affect the quality of health care, nutrition, and exposure to environmental toxins during pregnancy/childhood. Thus, if the gap is not decreased at higher socioeconomic levels, then this problematic also for these factors as explanations for gap.

There are also studies finding higher average IQ for East Asians, American Indians, and Inuit with similar or worse SES than Blacks. Studies have also found that when comparing Black and White children regarding the geographical areas of their homes, the schools they attend, and other fine grade socioeconomic indicators, then the Black children from the best areas and schools (those producing the highest average scores) still average slightly lower on IQ than the White children with the worst socioeconomic factors.[7]

One early view was that the US Black-White IQ gap was caused by the segregated schools. However, the 1954 Supreme Court decision against segregated schooling and the consequent nationwide program of school busing did not cause the gap to disappear. Furthermore, the Coleman Report found little support for the schools being an important explanation for the Black-White IQ gap or IQ results in general. Negligible, and in some cases, negative correlations were found between IQ and variables such as pupil expenditure, teachers’ salaries, teachers’ qualifications, student/teacher ratios, and the availability of other school professionals. Also, IQ group differences are found also in European countries with desegregated schools.[7][15]

Contrary to popular belief, spending per pupil is actually higher for Black students than it is for White students. Furthermore, a 2015 report found that the achievement gap between Blacks and Whites was the same no matter what proportion of the schools’ population was Black.[29]

More generally and globally, regarding education and scholastic attainment tests such as PISA (which are related to IQ), it has been argued that the poverty in poor countries significantly affects education and the scholastic results, but that diminishing returns occur, and that in wealthy countries the degree of investment in education explains very little of student attainment differences. Stated problematic country examples for those arguing for the importance of education include that Botswana has a low measured average IQ (73), despite being considered a long-term well-run African country that has invested heavily in education. Another is that Saudi Arabia has a low measured average IQ (78), despite having had plenty of money to spend on education for almost 50 years.[30]

Socioeconomic factors are also discussed in several other sections, such as the sections on "Heritability".

Environmental factors specific to developing countries

Certain factors that are common in developing countries, such as iodine deficiency and certain tropical diseases such as malaria, are known to affect IQ negatively (in particular or only during pregnancy/early childhood, but causing lifelong effects). However, these factors are much rarer in developed countries (due to factors such as iodized salt) and thus cannot explain, for example, the US Black-White IQ gap.

That malnutrition would be more common among Blacks than Whites in the US is argued to be excluded by the absence of height differences and nutritional studies.[15]

A 2013 study examined to what degree the average country IQ differences are caused by poor living conditions at or near the test-takers' time of birth and stated that "The paper finds that the impact of living conditions is of much smaller magnitude than is suggested by just looking at correlations between average IQ scores and socioeconomic indicators. [...] As far as IQ and the wealth of nations are concerned, causality thus appears to run mostly from the former to the latter. The test-takers' region of ancestry dominates the regression results. While differences in average scores worldwide can thus be plausibly viewed as being influenced by genetic differences across world regions, it is also possible that score differences are influenced by regional differences in culture that are independent of genetic factors. Differences in average IQ across world regions may change in the years ahead insofar as the strength of Flynn effects may not be uniform, but some regional differences in average g levels seem likely to continue indefinitely."[31]

Environmental factors specific to the pregnancy/early childhood period in developed nations

Also for developed countries, several factors especially important during the pregnancy/early childhood period (when the growing brain is especially sensitive) have been proposed as explanations for racial IQ differences. They include factors such as frequency of low birth weight, breastfeeding, iron deficiency, lead exposure, and some childhood diseases. There are a variety of specific counter-arguments against such specific factors, such as the association between longer breastfeeding and higher child IQ being substantially reduced if controlling for maternal IQ. More generally, differing racial frequencies of, for example, low birth weight have, based on genetic studies, been argued to be partially due to racial genetic differences. Many of these factors should likely be affected by SES but as noted in the section "Socioeconomic factors" the gap is not reduced at higher SES levels. There are also a variety of general arguments against such factors, as explained in the section "Undiscovered environmental factors". Finally, even if using the estimates by non-hereditarians, such factors can only explain a small part of the gap.[6]

There may also be problematic factors during pregnancy/early childhood factors that are more frequent for Whites, such as later age of reproduction, which increases the risk for some complications.[6]

Environmental factors specific to only one (or some) race(s)

As a response to the high measured heritability of IQ (see the section on "Heritability"), non-hereditarians have often pointed to the theoretical possibility of environmental "X-factors", which could theoretically exist despite a measured heritability of 100%. Examples and images, such this one showing a theoretical situation for two groups of plants, are often used to illustrate this theoretical possibility. Hereditarians have criticized this as discussed in the section "Environmental factors specific to only one (or some) race(s)".

"X-factors" are proposed environmental factors that would only affect one group (such as Blacks) and affect all group members equally. This wold cause them to be undetected by some statistical methods such as studies of heritability, which are argued to be problematic for a 100% environmental explanation (see the section "Heritability and the "default hypothesis""). Hereditarians have argued that, while there may be factors that could affect only Blacks (such as racism or a specific Black culture), it is very unlikely that such factors would affect all Blacks equally. That there would be such environmental factors affecting all group members equally have been argued to be especially unlikely considering that the degree of stereotypical "Black" appearance varies for different individuals identifying as Blacks. Another argued problematic group for X-factors is the continuum of genetically mixed-race individuals, who score intermediately between Blacks and Whites and who likely experience a continuum of environmental factors.[6]

Also factors that affect only one race (such as Blacks), but not all race members equally, are argued to be implausible causes of the IQ gap. This since hereditarians have argued that if such important negative environmental factor(s) are depressing the IQ of (in particular some) Blacks but not Whites (such as racism or Black cultural factors), then this would cause the correlations between background variables (such as home environment and peer characteristics) and developmental outcomes (such as achievement and delinquency) to differ between Blacks and Whites. However, several studies have examined this and found no racial differences and have argued that a partially genetic explanation is the best model. Also, the results described in the section "Regression to the mean" are argued to be evidence against environmental factors affecting the members of one group unequally.[8][32][6]

Other argued problems with these explanations include that affirmative action measures have caused discrimination in favor of Blacks and that the impact of such factors would likely vary over time, but adult IQ is stable (see the section "Stability of IQ in adulthood").[6]

Non-hereditarians have sometimes claimed that the existence of the Flynn effect somehow would support the existence of "X-factors", despite the Flynn effect not referring to racial differences, but to time differences, and the Flynn effect having other properties completely different from those of the supposed "X-factors".[32] See also the section "The Flynn effect".


See Other race differences: Self-esteem and narcissism.

Undiscovered environmental factors

Another argument is that there may be various undiscovered environmental factors that explain the gap. As being undiscovered, they may not be replied to specifically, but hereditarians argue that there are a variety of general arguments against their existence and ability to explain the gap as being 100% environmental, as discussed in many other sections, such as the sections "Heritability and the "default hypothesis"", "Spearman's hypothesis and g factor differences", "Regression to the mean", "Socioeconomic factors", "Stability of IQ in adulthood", and other sections.[6]


Intervention programs

In the middle of the twentieth century, a large number of early childhood intervention programs, such as the Head Start program, were tried with one expectation being that these would eliminate or substantially reduce various IQ gaps, including the racial IQ gaps. Large initial IQ gains were also found, but the initial enthusiasm declined, as it became apparent that the IQ or achievement tests gains soon faded away, as the children grew older. For example, a 1995 review of 36 such early intervention programs found no consistent pattern of lasting effects on IQ or achievement tests.[33]

A few of the programs have found longer lasting effects, contrary to this general pattern. The most well-known may be the Abecedarian Early Intervention Project, which found limited IQ gains lasting to adulthood. However, there have been various criticisms. One is that there is evidence for the intervention and control groups being dissimilar due to pure chance (e.g., sampling error) or non-random attrition of participants. The other few claimed exceptions have been criticized due to poor methodology, "teaching the test", and even a conviction of misuse of federal funds. A 2014 article stated lack of good evidence for anything except a null (or small) long-term effect from intervention programs (as well as adoptions) on the g factor.[33][34][35]

A 2010 meta-analysis of 177 interventions programs stated that "By and large effect sizes tended to be modestly (but insignificantly) larger if the children were under the age of 3 when the programs began. Effect sizes varied little by program duration." Studies with follow-ups more than 2 years after the intervention found only very small effects (effect sizes of only 0.01-0.05 SD).[36][6]

A 2014 meta-analysis of the initial IQ gains during the Head Start program found that they were not on the g factor and similar to gains from greater test experience (restest gains).[37]

A 2015 meta-analysis again found that interventions may temporarily raise IQ, but that the effect fade away with time.[38]


The Minnesota Trans-Racial Adoption Study studied 265 children (Black, White, and mixed race) adopted by White upper-middle-class parents with an average IQ of 120. Despite this similar environment, consistent racial differences were found on IQ, school grades, class ranks, and aptitude tests. At age 17, Whites on average scored 106, mixed race 99, and Blacks 89. 89 was also the average score for Blacks in general in Minnesota. The same difference between mixed race and Black children occurred also in some cases in which the adopting parents wrongly thought that mixed race children had two Black parents. These results caused considerable debate. Non-hereditarians have raised objections such as the Black and/or mixed race children having psychological problems due to identity issues, possibly being placed in somewhat relatively poorer homes, being adopted somewhat later (on average a 13 months difference between the Black and the White adoptees), and having more prior foster care placements. Hereditarians have argued that none of these are convincing explanations. For example, the children were in foster care prior to adoption, but there is no indication that the foster homes did not provide a good environment. A large-scale study that specifically addressed the effect of early versus late age of adoption on children’s later IQ found no effect at school age. Furthermore, later adoption time for Blacks cannot explain the intermediate result of the mixed race adoptees, since they were on average adopted before the White adoptees (on average 10 months earlier). Another argued problem is possible non-random drop-out from the study by adopted Whites causing misleadingly high average IQ, but the average IQ of the non-adopted White children of the parents in the study was even higher (which hereditarians argue is a result to be expected, due to a high average genotypic IQ of the parents).[8][7][39][6]

Non-hereditarians have argued that two other and smaller adoption studies (a 1972 study by Tizard and colleagues and a 1986 study by Moore) found opposing results. Hereditarians have argued that these results are dubious since the children were only tested when young. This since it is well-established that the effect of genes on IQ becomes stronger and the effect of the family environment weakens as children age. This effect was seen in the Minnesota study, where the Black children scored much higher (97) at age 7 than at age 17 (89).[8] See also the section "Shared and unshared environmental factors" regarding increasing influence of genetic factors with increasing age.

Hereditarians have also argued that there are also three studies of adopted East Asian children, who in some cases were malnourished and adopted late. Despite this, and presumably also identity issues, they scored highly on IQ tests.[8]

Furthermore, adoption results from the High School Longitudinal Study of 2009 are also argued to support the hereditarian position.[40][6]

A 2008 study in Sweden examined the results from cognitive tests at age 18 at military conscription during the years 1968-1976 (mandatory for all men at this age in Sweden during these years). Adoptees from South Korea scored much higher than persons adopted from other non-Western countries (the most common being India, Thailand, Chile, Ethiopia, Colombia and Sri Lanka). A large difference was found also when only looking at the group aged 0-6 months at adoption. The difference was so large that it could not be explained solely by differences regarding average country IQ. The Korean children may have had some advantages compared to the other adopted children (better care before adoption, better adoption agencies, and being adopted away for cultural reasons rather than reasons such as poverty), but they also had disadvantages (by being adopted later and having foster parents with shorter education than the other adoptees). The study also found only a weak relationship between length of the education of the foster parent and the IQ of their adopted children. However, it found a much stronger such relationship between parents and their biological children. This was interpreted as evidence for genetic parental IQ affecting the length of their education and the IQ of their biological children, but that a presumably more intellectually stimulating environment created by the parents with the longer education did not affect the IQ of their children.[41]

Non-hereditarians have sometimes cited studies showing relatively large IQ gains from adopting into high socioeconomic status (SES) homes as compared to adopting into low SES homes. Criticisms of this include that these adoption studies did not study racial differences, that the above mentioned Minnesota study studied Black and White children adopted by high SES homes but still found large racial differences, and that studies have found that the relatively large gains from being adopted into high SES vs. low SES homes occurred only on non-g factors, while the adopted children's g factor scores instead mainly reflected their biological parents SES, which implies that g is less amenable to such environmental manipulations.[8]

A 2015 meta-analysis of adoption IQ gains found that they were not on the g factor.[42]

Racial admixture

A higher degree of European ancestry in American countries is associated with higher achievement test scores according to a 2014 analysis using genetic data.[43]
A higher degree of Amerindian ancestry in Mexican districts is associated with lower achievement test scores according to a 2014 analysis using genetic data.[44]

Some non-hereditarians have argued that racial admixture and adoption studies provide the best evidence against hereditarians. Hereditarians have in return replied that the cited studies are very old (median publishing date 1960), not replicated, non-decisive, and exclude many studies on the issues supporting a partially genetic position.[8][6]

For example Wikipedia selectively cites only a few old adoption/admixture studies and ignores numerous other studies on these issues, including recent studies using new methods such as genetic analyses.

Some of the adoption studies discussed in the "Adoptions" section included mixed race adopted children and are therefore also racial admixture studies. For example, the Minnesota Trans-Racial Adoption Study found results for the mixed race children which are argued to support the hereditarians. See the "Adoptions" section regarding these studies.

A 1961 study by Eyferth, often cited by non-hereditarians, compared 83 German White children with 98 mixed race children born to post-WWII German mothers and "Black" soldiers. The study found only very small IQ differences. Similarly to adoption studies on young children, this study has been criticized for not having any follow-up when the children were older (see the "Adoptions" section). 20-25% of the "Black" soldiers were from French North Africa. The Black soldiers from the US almost certainly had higher average IQ than the average US Black IQ, due to Army General Classification Test excluding 30% of Blacks.[8] Furthermore, the results for the White children differed greatly for the boys and the girls. The expectation would be similar results and the large difference may be an indication of methodological/sampling problems with the study.

There is also a 1974 study by Willerman which is cited by non-hereditarians. It found a significant IQ difference between mixed race children born to White mothers versus Black mothers. This is argued to support an environmental explanation. Again it has been criticized for only testing the children when young. The White mothers had longer education and thus likely a higher IQ. Also, the two groups of mixed race children on average scored intermediately between the average IQs of the Black and White children in the study.[8]

US Blacks have on average a low degree of European ancestry. If the partially genetic explanation is correct, then one would expect that those with more European genetics would have higher IQ and brain weight. Studies using skin color as an indirect measure of the degree of European ancestry of Blacks have found weak such correlations, argued to be explained by skin color of African Americans being only a weak indicator of the degree of European ancestry. The results are argued to support that 50-75% of the IQ gap is explained by genetic factors. Non-hereditarians have argued that not all studies have found this correlation but non-hereditarians have argued that this was due to these studies being too small. Another argument is that the correlation may be due to societal advantages causing higher IQ for Blacks with lighter skin color. Hereditarians have argued that this is unlikely, East Asians have been discriminated against, but still do not score low on IQ tests, and a study regarding if Blacks with darker skin color were more discriminated found no or contradictory results.[45][46][7][6]

There are also several other kinds of empirical evidence against "colorism" (skin color differences as the cause of group differences due to factors such as racism). For example, darker skin color not being associated with more negative outcomes after controlling for IQ differences.[47]

A few other studies have used physical appearance indexes other than skin color. They have overall found weak correlations, which as for skin color, have been argued to support a partially genetic explanation.[6]

Another method is by examining the relationship between the degree of European blood groups and IQ. Two studies from the 1970s argued that there were no such relationship. These have been criticized for using genetic makers that would have been unable to detect a relationship.[8][48][6]

A study from the 1930s using self-reported degree of European ancestry found a small negative correlation with IQ. However, self-report has been criticized as being very uncertain and in particular regarding possible race mixing during the slavery period.[8] Another criticism is that closer examination is argued to show that the results support a partially genetic explanation or are inconclusive. Furthermore, two other genealogically based studies are argued to support a partially genetic explanation.[6][49]

Hereditarians have pointed to more recent studies in the United States, Brazil, and South Africa finding that the average IQ of populations of mixed Black and White origin is intermediate between that of Blacks and Whites. In the case of the United States, study explanations based on social class and "discrimination based on skin tone" were argued to be ruled out or controlled for. Furthermore, hereditarians have argued that in certain United States areas where Blacks have a low degree European ancestry, their average IQ is unusually low.[8][7][15]

Several other studies and data sets, ranging from very old ones (such as a 1914 study examining the relationship between degree of Africa ancestry and cognitive test results) to recent ones (such as data from the General Social Survey, the Programme for International Student Assessment (PISA), and the National Assessment of Educational Progress (NAEP)), have also been argued to support the hereditarian position.[6]

A 2002 study stated that "Mean levels of three characteristics—verbal IQ, number of sexual partners, and birth weight—were examined in African American, White (European-descent) Americans, and Black/White mixed race American adolescents. The sample came from Wave 1 of the National Longitudinal Study of Adolescent Health. The mean age was 16 years. According to their interviewers, the mixed race children had an African American physical appearance. The African American adolescents had a lower birth weight, a lower verbal IQ, and a higher number of sexual partners than did White adolescents. For each characteristic, the mixed race mean fell between the means of the two parental populations."[50]

Hereditarians have furthermore argued that studies have found that other mixed race groups, such as groups of mixed European/Amerindian origins as well as groups of mixed European/Australian aboriginal origins, score on average on IQ tests between the average IQ scores of their ancestral groups. One of the studies examined mixed European/Amerindian groups where the degree of European ancestry varied and found higher average IQ scores for groups with a higher degree of European ancestry.[15]

A 2014 article found 31 genetic admixture studies which reported, for individuals residing in the Americas, associations between continental ancestry (e.g., European, Amerindian, Sub-Saharan African, East Asian, and Pacific Islander) and some index of educational attainment or socioeconomic status. None of the associations went in a direction opposite to that predicted by the average IQ scores of the ancestral populations. The results were argued to "support a racial hereditarian hypothesis along with others that predict a fairly internationally consistent association between continental ancestry and cognitively correlated indices of socioeconomic status such as education, income, and job prestige".[51]

There have also been similar results for degree of African ancestry and socioeconomic status variables among African American.[52][6]

A 2016 study "conducted novel analyses regarding the association between continental racial ancestry, cognitive ability and socioeconomic outcomes across 6 datasets: states of Mexico, states of the United States, states of Brazil, departments of Colombia, sovereign nations and all units together. We find that European ancestry is consistently and usually strongly positively correlated with cognitive ability and socioeconomic outcomes. [...] It was found that the effect of European ancestry on socioeconomic outcomes was mostly mediated by cognitive ability (Section 13). We failed to find evidence of international colorism or culturalism (i.e., neither skin reflectance nor self-reported race/ethnicity showed incremental predictive ability once genomic ancestry had been taken into account) (Section 14). The association between European ancestry and cognitive outcomes was robust across a number of alternative measures of cognitive ability (Section 15)."[53]

Another 2016 study extended the previous analysis to the United States.[54]

Various mixed population IQ studies are discussed in the articles linked to in these references and are argued to generally support a hereditarian standpoint.[55][56]

Race differences on different tests and subtests

Sibling similarity/dissimilarity

The degree to which siblings differ on different tests and subtests can be measured. This is one measure of which tests or subtests are relatively more dependent on IQ genes as compared to environmental factors. Black-White IQ differences are largest on those tests and subtests where siblings show the smallest differences. This has been argued to support a genetic influence.[7]

In contrast, most non-hereditarian explanations for the gap are argued to predict the opposite pattern.[6]

Inbreeding depression

Inbreeding depression is a concept applicable to many genetically influenced characteristics, but as applied to IQ it refers to the lowered IQ in the children of very closely related parents. Different IQ tests, and the different subtests in a test, are affected to different degrees by inbreeding depression. This is another measure of which tests or subtests are relatively more dependent on IQ genes as compared to environmental factors. Black-White IQ differences are largest on those tests and subtests most affected by inbreeding depression. This has been argued to support a genetic influence.[8]

In contrast, most non-hereditarian explanations for the gap are argued to predict the opposite pattern.[6]

Spearman's hypothesis and g factor differences

The g factor (general factor) is the argued underlying general mental ability that is measured more or less well by different cognitive tests. The g factor has been argued, based on evidence such as twin studies, to have a high heritability. A test's g loading, or a subtest's g loading, refers to how well it correlates with the g factor.[8]

Regarding general criticisms of g see Race and intelligence: Significance of IQ and g

Spearman's hypothesis states that differences between groups on the subtests of an IQ battery are a function of the cognitive complexity of these subtests: large differences between groups on high-cognitively complex/high g-factor loaded subtests and small differences between groups on low-cognitively complex/low g loaded subtests. It has been argued to have been tested on different racial groups and confirmed (a Spearman/Jensen effect) in the large majority of studies.[57][58][10][59]

A proven Spearman effect is argued to make several possible environmental explanations much more difficult. It is indicative of a difference in a broad ability, as opposed to numerous deficits in specific abilities. One argued implication is that motivational explanations can not easily account of the gap. Another is that ability specific factors, at least individually, can not either.[6]

More generally, a Spearman effect is argued to be strong support for that racial IQ differences are racial g differences. This is also argued to be supported by statistical analysis of g and its relationship to cognitive abilities and other variables, which show similar results for both Blacks and Whites. Racial g differences are in turn is argued to support a hereditarian explanation for reasons such as g being highly heritable, g being the argued biologically rooted backbone of IQ tests ability to explain individual and groups differences and their ability to predict future outcomes, and g having many neurophysiological correlates such as brain neural conduction velocity, cerebral glucose metabolic rate, the latency and amplitude of evoked electrical brain potentials, the volume of white and grey matter, the mass of the prefrontal lobe, etc.[6]

Studies on cognitive ability differences between different primate species have found that they are concentrated on a highly heritable factor corresponding to the human g factor and have found support for this factor having evolved rapidly and being the principal focus of selection in the evolution of primate intelligence.[60][61]

No other factor except the g factor is argued to be able to predict the magnitude of the Black-White difference on diverse tests.[6]

Non-hereditarians have argued that a Spearman effect could in theory be caused by environmental and non-g factors. Thus, there could be a general cognitive ability test gap between groups due to factors such as a general "education gap", and the cognitive ability test gaps could be larger on more complex tests due to such tests being more affected by training and there also being a general "training gap" between groups, and assuming that such environmental and non-g "gaps" are equal for all different kinds of tests and subtests, then this could in theory produce an environmental and non-g Spearman effect. Hereditarians argue that this is highly unlikely.[6]

A 2015 study, comparing the IQ test results of ethnic minority immigrants from non-Western countries and of native Dutch, found a Spearman effect, only small effect of language bias, and very little place left for cultural bias, and no clear indications of publication bias.[62]

Group differences must not necessarily show a Spearman effect. For example, IQ group differences between deaf, blind, and non-handicapped children, IQ group differences due to adoption of low SES children by high SES parents, and IQ group differences clearly due to tests practice and educational differences, do not show a Spearman effect. This is argued to be because these IQ group differences are not due to genetic factors affecting g. Also, the Flynn effect does not show a Spearman effect (see the section "The Flynn effect"). In contrast, a Spearman effect exists between Blacks and Whites both as adults and at younger ages, between those having reduced IQ due to brain damage due to exposure to fetal alcohol and those not exposed, between those having reduced IQ due to inbreeding and those not having such reduced IQ, and between groups differentiated by differences regarding volume of brain structures related to IQ. A Spearman effect between Blacks and Whites also exists on tests of reaction time (see the section "Reaction time").[6]

A 2017 meta-analysis stated the existence of a Spearman effect between Whites and minority groups also on alternative tests of intelligence.[63]

Furthermore, there are no differences between Blacks and Whites regarding rote memory or psychomotor ability (e.g., reactivity to sensory stimulation and coordination). This is problematic for a number of potential environmental explanation (such as lead poisoning, mercury poisoning, malnutrition, etc.) which are empirically shown to affect rote memory and psychomotor ability.[6]

The Flynn effect

The Flynn effect refers to the (earlier) observed worldwide increase in average IQ test scores. Several explanations have been proposed, such as increased familiarity with taking tests, increased education, a more complex world, reduced infectious disease burden (such as from malaria), and improved nutrition, with the developing brain being particularly sensitive. The cause(s) may possibly differ during different time periods and in different places (for example, improved nutrition and reduced infections diseases may be important in developing countries, but less so in developed countries). Another issue is if the gains during a particular time period in a particular place reflect g factor gains, gains on more narrow cognitive abilities, and/or simply increased test taking skill(s).

The Flynn effect has been seen as evidence for that IQ can be changed significantly by environmental factors and that the racial IQ gaps may eventually disappear. However, at least in the US and some other developed countries, the gains from the Flynn effect correlate negatively with g loadings and inbreeding depression (see the earlier sections on g and inbreeding depression regarding these subjects). This is argued to show that, at least in those countries, the environmentally caused Flynn effect mostly does not affect g and will not significantly narrow the largely genetically influenced Black-White IQ (g) gap.[64] A 2013 meta-analysis concluded that "It appears that the Flynn effect and group differences have different causes."[65] A 2014 study found that some of the Flynn effect is due to enhanced guessing on harder (more g loaded) test items, rather than solving them. Controlling for this greatly increased the above mentioned negative correlations "indicating that the authentic Flynn effect is substantially more independent from g than previously thought."[66]

Furthermore, in order to establish that the same trait is being measured in different tested groups, there must be an analysis of measurement invariance. Several studies have investigated this and found that the Black-White IQ gap reflects genuine group differences, while the Flynn effect (group differences between different age groups) does not reflect genuine group differences (that is, the measured group differences may be due to factors such as measurement biases and other measurement errors). This indicates that intelligence is not genuinely increasing over time due to the Flynn effect, at least in developed countries.[67]

A trend that has accompanied the Flynn effect is increasing education and learning of various narrow cognitive abilities and skills. This has also increased the ability to understand and follow relatively simple rule systems, such as those used in basic mathematics. Some IQ tests that have seen particularly large Flynn effect gains are argued to use similar rule systems for solving the problems, implying that such test gains may be due to increased learning of such rule systems, rather than being genuine g gains. It should be noted that even if g has not increased, the learning such more narrow abilities and skills may still be practically important for individuals and groups, and are associated not just with the Flynn effect, but also with effects such as increasing cognitive specialization and economic growth.[68][69]

See Dysgenics: Intelligence on studies finding that the Flynn effect has now reversed and that the average measured IQ is now declining in many developed nations, as well as on studies arguing that results on biological measures, such as reaction time, indicate that genotypic g has been declining for a long time period, also while the Flynn effect previously occurred.

This "anti-Flynn effect" have shown the opposite pattern to Flynn effect regarding g loadings, which has been argued to indicate that this "anti-Flynn effect" is due to reduced genetic g, unlike the Flynn effect.[70][71][72]

See Dysgenics: Declining innovation rates on studies arguing that this has been associated with negative effects, such as reduced per person innovation rates, also while the Flynn effect previously occurred.


Heritability and the "default hypothesis"

"Heritability" refers to the proportion of the variation of a trait that is due to genetic factors. Heritability of IQ, as determined from twin studies, is 50-80% in adults in the United States. This applies both to the population as a whole and to Blacks and to Whites studied separately. Non-hereditarians have argued that these numbers are too high, in particular for Blacks. Hereditarians have countered that the critical studies were done on children, who are known to have lower heritability than adults, or contradicted by many more studies supporting the high heritability numbers. If the heritability figures are equal for both races, then this is evidence against environmental factors that are negatively affecting IQ more often affecting Blacks. Furthermore, while a 50-80% heritability is not definitive evidence for that the measured IQ gap between Blacks and Whites is 50-80% genetically caused, hereditarians have argued that this makes the 50-80% genetic explanation the most probable "default hypothesis".[7][8][73][74][6]

One criticism of heritability estimates from twin studies is that estimates from some earlier DNA studies have sometimes been lower (20-50%). However, this has been argued to reflect incomplete understanding of what genetic mechanisms influence IQ. A 2017 DNA study, using a large sample and a methodology enabling detection of more kinds of genetic influences, found a heritability of at least 54%.[75]

Hereditarians also argue that a high heritability implies that, if there are environmental factors causing the entire US Black-White IQ gap, then the environment must very markedly differ for US Blacks and US Whites, implying that the environment for US Blacks supposedly must be equivalent to the environment for the few percent (or even under 1%) of US Whites having the worst environment.[6][76]

There are in principle several ways for a 100% environmental explanation to avoid some of the above stated problems for the explanation if heritability is high: test bias (see the section on this), unique environmental factors only affecting one race (see the section "Environmental factors specific to only one (or some) race(s)"), and gene-environment interactions (a possibility which is argued by hereditarians to be unlikely and having various problems and limitations).[6]

A famous study on heritability was a 2003 study by the Jewish researcher Turkheimer and colleagues, which found that in impoverished families, heritability was very low. However, it has later been revealed that "Despite the strong SES-heritability interaction, there is no race-heritability interaction in this sample [...] The upshot is that while environmental deprivation may render genetic differences less important in the determination of children’s IQ, the typical black child in this large and downscale sample had apparently not been raised in deprived circumstances any more frequently than the typical white child in the sample. The lower IQs of blacks in this sample cannot therefore be put down to them having been exposed to environments less conducive to the expression of genetic variance in IQ than the environments experienced by whites. Had the race-specific results been published by Turkheimer et al. back in 2003, we would have been spared much misleading speculation."[77]

Also, there are other studies that have studied SES-heritability interaction, but hereditarians argue that most have found no interaction effects or interactions that vary unpredictably.[8][78]

Even if there is a large SES-heritability interaction, then the gap is despite this equally large at high SES, see the section "Socioeconomic factors".[6]

Shared and unshared environmental factors

Another aspect of heritability is that the environmental part of the IQ variation can be divided into two parts.

Shared environmental factors

One part of the he environmental part of the IQ variation is due to factors affecting the siblings (such as the twins in twin studies) in a family similarly, including often popular and politically correct factors such as social class, religion, culture, absence or not of a father, and educational method.

Studies have found that the shared environmental factors account for about 30% of the IQ variation before adulthood, but that these factors are unimportant in adulthood. Instead, the genetic part of the IQ variation increases in importance in adulthood. This may be explained by individuals in childhood being forced to experience a similar environment, but after reaching adulthood they are more free to choose an environment they prefer - a preference partly dependent on genetic factors. This has been argued to be evidence against such often popular and politically correct shared environmental factors being important for racial IQ differences in adulthood.[7]

The US Black-White gap has been argued to have become narrower for younger age groups, but, according to longitudinal studies, this narrowing to decrease as the children grow up, with the gap when the individuals reach their mid-twenties argued to be almost unchanged. This phenomenon has been argued to be difficult to explain using 100% environmental explanations, but can by hereditarians be explained by mechanisms such as greatly improved shared environmental factors for Blacks, but genetic factors becoming increasingly more important and shared environmental factors decreasingly less important with increasing age.[6]

That the gap has actually narrowed at younger ages is disputed. See Race and intelligence: United States Black White gap.

Unshared environmental factors

Another part of the he environmental part of the IQ variation is due to factors only affecting one sibling (such as only one twin), such as an accident or a disease.

At least for Whites, most of the non-genetic adult IQ variation has been argued to result from random adverse effects, such as prenatal problems, complications in the birth process, maternal health, and childhood disease and trauma.[7] Such factors may often not be easily predicted and prevented and may cause "hard-wiring" effects that cannot easily be reversed later in life. As such, these factors can often be considered to be politically incorrect, despite being environmental.

Many unshared environmental factors are argued to have their greatest or only effect during pregnancy/(early) childhood, when the rapidly growing brain is particularly sensitive. However, an argued substantially smaller Black-White IQ gap at a very young age, that increases as the children age, is problematic for the theory that unshared environmental factors during pregnancy and early childhood are the main explanations for the gap.[6]

Also, for example "regression to the mean" studies (see separate section) are argued to be evidence against unshared environmental factors explaining the Black-White gap.[6]

Mental retardation rates, brain size, and reaction time

Mental retardation rates

Another argument for genetic causes is, at least in the United States, an increased Black rate of mental retardation even after controlling for socioeconomic and other factors.[79][6]

Brain size

Hereditarians have argued that brain size is highly heritable. A 2009 review stated that indirectly measured brain size (such as from skull measurements) have a 0.2 correlation with IQ and MRI measured brain size have a 0.4 correlation with IQ. If using the method of correlated vectors to distill g from the subtests of an IQ test, then the correlation was on average 0.63. One study found a correlation of 0.89 between g loadings and number of gray matter clusters.[80] A larger 2014 meta-analysis of studies using modern brain imaging methods found a correlation of 0.24 between brain volume and IQ.[81][82]

A large number of studies have found racial differences in brain size in both the US and worldwide. Non-hereditarians have argued that there are a few studies finding no racial differences or no relationship between brain size and IQ. Furthermore, brain size differences may be due to poor environmental factors during and after pregnancy. Hereditarians have in return criticized the argued exceptions. They argue that racial differences in brain size can be observed as early as the 9th week of pregnancy. Nutrition as an explanation is argued to be unlikely, at least in the US, since Blacks in one US study with 40,000 participants were found to have smaller cranial volumes at birth, despite being taller and heavier at birth.[8][83][84]

Differences in brain size between different species are associated with differences in various musculo-skeletal traits. This can partly be explained as adaptations to an increasingly larger brain. The same musculo-skeletal differences are seen between different human races, which has been argued to further support the existence of brain size differences and also make an environmental explanation much more difficult.[85][86]

Critics may refer to megalencephaly, a genetic growth development disorder which cause abnormal brain growth and various problems.[87] However, this is a medical disorder disrupting normal brain development and thus not relevant for discussions regarding normal brains.

Another criticism is that some early studies produced somewhat different results than modern ones. A 2001 article thus stated that early studies placed "Caucasoids" as having a higher measured average cranial capacity than "Mongoloids".[88] However, possible errors and problems (such as sampling problems) with such often very old (nineteenth century) studies is not evidence for that modern studies are incorrect. Also, it is only Northeast Asians that are argued to have an average IQ above Europeans, not "Mongoloids" in general. Furthermore, the argued average IQ difference between Europeans and Northeast Asians is not large, which may make it more difficult to correctly detect differences between these groups. In addition, if looking at more groups rather than just the above two, then there is arguably more agreement between old and modern studies, with the 2001 article stating that none of the examined studies (old or modern) found that "Negroids" had a higher measured average than "Caucasoids" or "Mongoloids".[88]

Yet another argument by critics is that certain animals, such as elephants and whales, have large brains. However, these animals have large body sizes which require large brains for body maintenance and control. See the article on Neanderthals regarding their brain size, which has been argued to be somewhat larger than that of moderns humans.

Adjusting for body size differences in human brain studies have been argued to not eliminate racial differences regarding average brain size. On the contrary, they may be increased, such as when adjusting for shorter stature of East Asians.[80]

Critics have pointed to the average brain size differences between men and women (even after adjusting for body size differences) and argued that there is no IQ difference, which would indicate that brain size is an unreliable measure. Hereditarians have argued that some recent studies do have shown small average IQ differences between the sexes and that regardless there are clearly proven large differences between the sexes regarding narrower abilities. For example, men have on average greater spatial abilities, which may be very computationally demanding and require the on average larger male brain areas. Thus, the average brain size differences between the sexes are argued to be significant regarding cognitive differences.[8][80][89]

Another criticism is that the racial brain size differences only can explain a small part of racial IQ differences, since the correlation between brain size and IQ in individuals is argued to be relatively low and that calculating the "explained variance" gives an even lower value. One response is that the genes affecting IQ must not necessarily affect whole brain size, but may likely have various effects including non-whole brain size effects, such as on neuronal connections, neuronal metabolism, neuronal insulation, the biochemical environment surrounding neurons, the size of smaller brain structure than the whole brain (see later section on this), and so on. But even if racial whole brain size differences only explain a small part of the racial IQ differences, then this may still be very problematic for a 100% environmental theory.

Furthermore, IQ differences between groups and evolutionary explanations for these IQ differences (see the main Race and intelligence article) provide explanations for why there are measured brain size differences between groups. In contrast, if denying that the brain size differences are related to IQ, then the causes and functions of such brain size differences are unclear.

That brain size differences explain only a small part of racial IQ differences can be questioned. Thus, a common method of calculating "explained variance" have been criticized as usually having little substantive value.[90][78] Also, brain size differences may possible be more important for IQ differences between races than they are for IQ differences between individuals.

In addition, as noted above, the correlation with intelligence was very high in a study looking at more specific brain structures (gray matter clusters) instead of at whole brain size. A 2015 study found that when using several other brain variables in addition to overall brain size, then they accounted for around 20% of the variance in g.[91] Regarding more specific brain structure differences between races, see Other race differences: Brain anatomy.

Brain/cranial cavity size was one of the first studied variables and have been involved in some notable controversies involving Franz Boas and Stephen Jay Gould. See Race and intelligence: Ad hominem, straw men, and guilt by association and The Mismeasure of Man: Samuel George Morton.

A 2014 genetic study found that human brain anatomy differences are to a large degree caused by genetic factors, that these genetic factors are relatively independent of the genetic factors influencing general body size (height), and that the genetic factors influencing brain anatomy differences also influence IQ differences.[92] Another 2014 study found that gene markers associated with intracranial volume differences differed between different populations in a way that similar to gene markers associated with intelligence differences.[93]

A 2016 study found recent human evolutionary developments for a brain size regulating gene and that the frequency of gene variants differed in different human populations.[94]

Reaction time

The reaction time in response to a stimulus can be measured and tested on a variety of different tasks. Differences in reaction time are argued to be due to neurophysiological differences in the brain's ability to process information, which is also what IQ tests measure. Due to the unusual nature of the testing it is unlikely to be influenced by practice or education. Non-hereditarians have dismissed it as having a low and uncertain correlation with IQ. The related measure decision time has been similarly dismissed. Hereditarians have argued that this only applies when only one task is used. When the results from different tasks are combined, as is also done in IQ tests, the correlation between IQ and reaction time is 0.6-0.7. Although not all studies completely agree, overall racial differences are found (both in the US and elsewhere) consistent with those from IQ testing. Just as for IQ, these racial differences are largest on the tasks that best measure the g factor.[8][95]

Direct genetic evidence

Predicted genotypic IQs of several populations in developing countries based on genetic population data according to a 2015 study.[96]
See also: Arguments regarding the existence of races: The existence of races and the existence of genetic differences between human populations.

Direct genetic evidence basically requires two things. One is knowledge about how genes are distributed in different races. This is being rapidly achieved by continued technological developments, which have dramatically lowered the costs for analyzing a person's DNA. Several science projects have already completed analyzing or are in the process of analyzing the DNA of persons from different populations worldwide. One example is the 1000 Genomes Project, which in 2015 completed its final phase, after analyzing the genomes of 2,504 individuals from 26 populations across five continents and 18 countries.[97]

The other thing required is knowledge about which genes influence intelligence. One suggestion, made in 2011, at the start of a research project regarding this, is that "within the next 5-10 years we will identify genes which account for a significant fraction of total IQ variation."[98] As data regarding racial gene differences and data regarding which genes affect IQ increasingly accumulate in the future, so will the evidence for or against a genetic explanation for the racial IQ differences.

Several of the most recent studies mentioned in the "Racial admixture" section above used genetic data and found support for a partially genetic explanation for the racial IQ differences.

Studies in 2010 and 2012 examined genetic haplogroups associated with intelligence in different countries and regions. The haplogroups were argued to probably not themselves affect intelligence, but to indicate different evolutionary developments likely related to intelligence. The results were argued to support that national differences in cognitive ability are caused by both genetic and environmental factors.[99][100]

A 2013 study examined the frequency in different populations of two genetic markers that are associated with intelligence on the individual level. On the population level, high frequencies in a population of these genetic markers were associated with a high average population IQ. There was also evidence regarding one of the genetic markers for an association with climate differences and adoption of an agricultural vs. a hunter-gatherer lifestyle.[101]

Another 2013 study examined the frequency in different populations of ten genetic markers that are associated with educational attainment on the individual level. On the individual level, such educational attainment is highly correlated with IQ and in a subsample, for which cognitive tests scores where available, these genetic markers were associated with higher test scores for individuals. On the population level, a high frequency in a population of genetic markers associated with high individual results was strongly associated with high population educational attainment and high average population IQ. There were similar population differences for two individual genetic markers that have been associated with higher IQ and which are located within genes involved in brain functions. The study stated that "this is the first study to provide systematic (albeit preliminary) evidence that differences between countries and races in IQ and educational attainment are related to genetic factors."[102]

Genetic studies in 2014 found more support for genetic factors as causes of the racial IQ gaps.[103][93][104][105]

In 2015, genetic population data were used to estimate the predicted genotypic IQs of several populations in developing countries. East Asians had the highest predicted genotypic IQs (103-106) and Sub-Saharan Africans had the lowest predicted genotypic IQs (81-84).[106][96]

Also other genetic studies in 2015 found support for genetics as contributing to the racial IQ gaps.[107][108][109][110]

Also genetic studies in 2016 found more support for genetic factors as causes of the racial IQ gaps.[53][111][112][113][114]

Hereditarians have argued that it is already today possible to construct genetic studies that could definitely decide if genetics contribute to the measured average IQ differences, but that such studies require effort, money, and cooperation for which the necessary will simply does not exist, for political reasons. Also James Flynn, well-known for his arguments against hereditarianism, have stated that: "[I]f universities have their way, the necessary research will never be done. They fund the most mundane research projects, but never seem to have funds to test for genetic differences between races. I tell US academics I can only assume that they believe that racial IQ differences have a genetic component, and fear what they might find. They never admit that the politics of races affects their research priorities. It is always just far more important to establish whether squirrel enjoy The Magic Flute."[115][116][18]

External links


The Alternative Hypothesis


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  2. Mark Snyderman and Stanley Rothman. The IQ Controversy, the Media and Public Policy. 1988. Transaction Books.
  3. 3.0 3.1 3.2 3.3 Rindermann, Coyle, and Becker, ISIR 13, 14-XII 13, Expert Survey. Discussed in: J Thompson. (2013, December 14). ISIR What do intelligence researchers really think about intelligence? http://drjamesthompson.blogspot.com/2013/12/isir-what-do-intelligence-researchers.html
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