Race and intelligence

The study of race and intelligence is a controversial field which seeks to determine whether or not human intellectual abilities vary between races.

The modern controversy surrounding intelligence and race focuses primarily on the results of intelligence testing studies conducted since the second half of the 20th century in the United States, Western Europe, and other industrialized nations. There are also controversies over the nature of race, the meaning and importance of intelligence, and whether the intelligence quotient (IQ) is a satisfactory measure thereof.

While the existence of average intelligence test score differences has been shown in many large studies and is therefore not much in dispute, there is a great deal of controversy surrounding causal explanations of the disparities.

Theories about a relationship between race and intelligence have been the subject of speculation and debate since the 16th century. The contemporary debate focuses on the nature, causes, and importance, or lack of importance, of ethnic differences in intelligence test scores and other measures of cognitive ability, and whether race is a meaningful biological construct. The question of the relative roles of nature and nurture in causing individual and group differences in cognitive ability is seen as fundamental to understanding the debate.


The opinion that there are differences in the brain structures or sizes of different racial and ethnic groups was widely held and studied during the 19th and early 20th centuries. During this time period, research on race and intelligence was often used to claim that one race was superior to another, justifying the poor status and treatment of the "inferior" race.

The writings of Sir Francis Galton, a psychometrician and polymath (1822–1911), spurred interest in the study of mental abilities, particularly as they relate to heredity and eugenics. Galton claimed from his field observations in Africa that the African people were significantly below Anglo-Saxons' position in the normal frequency distribution of general mental ability; these claims continue to spark controversy in academia today.

The scientific debate on the contribution of nature versus nurture to individual and group differences in intelligence can be traced back to at least the mid-19th century. Beginning in the 1930s, race difference research and hereditarianism — the belief that genetics are the primary cause of differences in intelligence among human groups — began to fall out of favor in psychology and anthropology after major internal debates. By 1961, the mainstream view was that there were no race differences in intelligence, or if there were, they were solely the result of environmental factors.

Contemporary issues

The contemporary debate on race and intelligence is about the nature and the causes of racial and ethnic differences in IQ test scores; many hypotheses have been proposed to explain racial-ethnic group differences in IQ. Certain environmental factors, such as nutrition, are thought to modulate IQ in children, and other influences have been hypothesized, including education level, richness of the early home environment, and other social, cultural, or economic factors. The most controversial and most publicized part of the debate is whether group IQ differences also reflect a genetic component. Hereditarianism hypothesizes that a genetic contribution to intelligence could include genes linked to neuron structure or function, brain size or metabolism, or other physiological differences which could vary with biogeographic ancestry.

The 1970s debates

The modern debate was prompted by psychology professor Arthur Jensen's 1969 publication of the controversial article, "How Much Can We Boost IQ and School Achievement?".

All we are left with are various lines of evidence, no one of which is definitive alone, but which, viewed together, make it a not unreasonable hypothesis that genetic factors are strongly implicated in the average negro-white intelligence difference. The preponderance of evidence is, in my opinion, less consistent with a strictly environmental hypothesis than with a genetic hypothesis, which, of course, does not exclude the influence of environment or its interaction with genetic factors.
According to Peter Singer, the article was widely reported in the popular press "as an attempt to defend racism on scientific grounds.

Physicist William Shockley spent much of his later life focused on the questions of race, intelligence, and eugenics. He began giving speeches on population problems and published several controversial papers arguing that intelligence is primarily hereditary. He postulated that the higher reproduction rate of those with lower intelligence was having a dysgenic effect on society and proposed that individuals with IQs below 100 be paid to undergo voluntary sterilization. Biologists and geneticists criticized his theories and pointed out that eugenics was a rationale used by the Nazis. He was attacked in the media and in scientific journals. Shockley described his work on the issue as the most important of his career.

Population geneticist Luigi Luca Cavalli-Sforza on several occasions publicly debated Arthur Jensen and William Shockley arguing that environmental factors could explain the black-white IQ gap. In 1981, Stephen Jay Gould published The Mismeasure of Man, which argued against the concept of g, the general factor of cognitive ability.

The 1990s debates

The question of whether group differences in average IQ are purely social, economic, and cultural in nature or whether genetic factors may also be involved has continued to receive wide media coverage; this prompted a number of academic bodies to make public statements on their views. For example, the American Anthropological Association issued a declaration which dismissed the debate on the grounds that they view 'race' to be a biologically meaningless categorisation of human beings , while the American Psychological Association released a statement in which they acknowledged the reality of inter-group IQ differences, but said that the current evidence, while scarce, did not support a genetic cause. The claim that there are genetic causes for the differences in average IQ scores of different races has been forwarded by scholars such as Charles Murray, Michael Levin, and J. Philippe Rushton. Their research has been criticized by other scholars such as Stephen Jay Gould, Richard Levin, Richard Lewontin and Joseph L. Graves.

The publication of The Bell Curve, a best-selling 1994 book by American psychologist Richard Herrnstein and American Enterprise Institute political scientist Charles Murray, which included a discussion of racial differences in intelligence, received much attention in the popular press and ignited renewed debate within academia and amongst the general public. Its central point is that intelligence is a better predictor of many factors including financial income, job performance, unwed pregnancy, and crime than parents' socioeconomic status or education level. Also, the book argued that those with high intelligence (the "cognitive elite") are becoming separated from the general population of those with average and below-average intelligence, and that this was a dangerous social trend. Much of the controversy concerned Chapters 13 and 14, in which the authors wrote about the enduring racial differences in intelligence and discussed implications of those differences. The authors were reported throughout the popular press as arguing that racial IQ differences are genetic, although they state no position on the issue in the book, and write in the introduction to Chapter 13 that "The debate about whether and how much genes and environment have to do with ethnic differences remains unresolved."

Shortly after publication, many people rallied both in criticism and defense of the book. Some critics denounced the book and its authors as supporting scientific racism. A number of critical texts, including The Bell Curve Debate and The Mismeasure of Man (second edition), were published in response to the book. Several collective statements were released by groups of scholars.

The American Psychological Association's Board of Scientific Affairs in 1995 established a special task force to publish an investigative report on the research presented in the book. The report, titled "Intelligence: Knowns and Unknowns" and published in the February 1996 issue of American Psychologist, agreed with some of the book's statements on intelligence; however, regarding the book's genetic implications, it states:

It is sometimes suggested that the Black/White differential in psychometric intelligence is partly due to genetic differences (Jensen, 1972). There is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis.
The January 1997 issue of American Psychologist published an article with eleven critical responses to the American Psychological Association (APA) report, most of which agreed with the substance of the report, but suggested ways in which the report could have been improved. According to the report's lead author, only Lynn and Rushton "dispute[d] the task force's conclusion that there is no direct evidence for a genetic interpretation of the Black—White IQ difference. The APA report concluded with a call for more reflection in debates on intelligence and for a "shared and sustained effort" in more research to answer the many unanswered questions that remain.

A collective statement titled "Mainstream Science on Intelligence" was published in the Wall Street Journal on December 13, 1994. It was written by psychology professor Linda Gottfredson, and signed by Gottfredson and 51 other professors specializing in intelligence and related fields. It listed 25 statements which claimed to uphold findings on the subject of intelligence research discussed in the The Bell Curve. This statement was reprinted in the psychology journal Intelligence in 1997 with additional information and a bibliography. Regarding the cause of racial-ethnic group differences in IQ, the statement says:

There is no definitive answer to why IQ bell curves differ across racial-ethnic groups. The reasons for these IQ differences between groups may be markedly different from the reasons for why individuals differ among themselves within any particular group (whites or blacks or Asians). In fact, it is wrong to assume, as many do, that the reason why some individuals in a population have high IQs but others have low IQs must be the same reason why some populations contain more such high (or low) IQ individuals than others. Most experts believe that environment is important in pushing the bell curves apart, but that genetics could be involved too.

In 1994, the American Anthropological Association published a statement, declaring the organization was:

deeply concerned by recent public discussions which imply that intelligence is biologically determined by race. Repeatedly challenged by scientists, nevertheless these ideas continue to be advanced. Such discussions distract public and scholarly attention from and diminish support for the collective challenge to ensure equal opportunities for all people, regardless of ethnicity or phenotypic variation.

Policy debates

See also: Intelligence and public policy
Public policy implications of IQ and race research are one of the greatest sources of controversy surrounding this issue. Regardless of the source of the IQ gap, most educators agree that it must be addressed; they often advocate equitable funding for education. An achievement gap refers to the observed disparity on a number of educational measures between the performance of groups of students, including groups defined by race or ethnicity.

Some proponents of a genetic interpretation of the IQ gap, such as Rushton and Jensen (2005a) and Gottfredson (2005b), have sometimes argued that their interpretation does not in itself demand any particular policy response: while a conservative/libertarian commentator may feel the results justify, for example, reductions in affirmative action, a liberal commentator may argue from a Rawlsian point of view (that genetic advantages are undeserved and unjust) for substantial affirmative action. Since all races have representatives at all levels of the IQ curve, this means any policy based on low IQ affects members of all races.

In the book Practical Ethics, philosopher Peter Singer wrote that:

Let us suppose that the genetic hypothesis turns out to be correct... I believe that the implications of this supposition are less drastic than they are often supposed to be... First, the genetic hypothesis does not imply that we should reduce our efforts to overcome other causes of inequality between people... Perhaps we should put special efforts into helping those who start from a position of disadvantage, so that we end with a more egalitarian result. Second, the fact that the average IQ of one racial group is a few points higher than that of another does not allow anyone to say that all members of the higher IQ group have higher IQs... The point is that these figures are averages and say nothing about individuals... The third reason... is simply that, as we saw earlier, the principle of equality is not based on any actual equality that all people share. I have argued that the only defensible basis for the principle of equality is equal consideration of interests... Equal status does not depend on intelligence. Racists who maintain the contrary are in peril of being forced to kneel before the next genius they encounter.

Policies focused on geographical regions or nations may have disproportionate influences on certain racial groups and on cognitive development. Differences in health care, nutrition, regulation of environmental toxins, and geographic distribution of diseases and control strategies between the developing world and developed nations have all been subjects of policies or policy recommendations (see Health and intelligence).

Eugenics advocates the improvement of human hereditary traits through various forms of intervention. After World War II, both the public and the scientific communities associated eugenics with Nazi abuses, such as enforced racial hygiene, human experimentation, and the extermination of undesired population groups. Beginning in the 1980s, the history and concept of eugenics were again widely discussed as knowledge about genetics advanced significantly; developments in genetic, genomic, and reproductive technologies raised many new questions and concerns about eugenics and its ethical and moral status in the modern era.

Test score difference

United States

The observed differences in average test score achievement between ethnic groups varies depending on the populations studied and the type of tests used. Self defined black and white United States citizens have been the subjects of the greatest number of studies. Black-White average IQ differences appear to increase with age, averaging nearly 17 points by age 24. According to James Flynn, the overall average Black-White gap has reduced by one third over the last 30 years. A large (21,260 children) and probably the most recent (1998) study found that the Black-White gap for young children in reading and math scores was much smaller than in earlier studies. Less controversially, Black-White differences on school achievement tests shrunk over the last 30 years, but some of the improvements did not continue through the 1990s. The average black-white IQ difference also varies depending on test content. For example, two subsections of the WISC IQ test, known as forward and reverse digit-span, ask children to repeat a long series of numbers either forwards or backwards. The black-white difference on forward digit span is relatively small, while the difference on reverse digit span is relatively large. Across a battery of tests, the size of the Black-White gap is correlated with the extent to which the tests measure the psychometric factor g, which also accounts for most of the variation in interindividual differences in IQ test performance. Using a variety of statistical techniques, Dolan and colleagues have found that the black-white IQ gap can be accounted for by differences in g and the other interindividual ability factors measured by IQ tests, and also that IQ tests measure roughly the same mix of abilities in both black and white populations. Gaps are seen in other tests of cognitive ability or aptitude, including university admission exams such as the SAT and GRE as well as employment tests for corporate settings and the military.

The IQ distributions of other racial and ethnic groups in the United States are less well studied. Hispanic and Native American populations, including Arctic Natives, tend to score worse on average than White populations but better on average than Black populations. East Asian populations may score higher on average than White populations in the United States as they do elsewhere. Studies such as that of Backman (1972), Levinson (1959) and Romanoff (1976) estimate Ashkenazi Jews to score 0.75 to 1.0 standard deviations above the general European average in verbal or crystalized IQ, corresponding to an IQ of 112-115. A recent review by Lynn (2004) concludes that the advantage is slightly less, only half a standard deviation. For each of these populations, there is some evidence that the mixture of ability factors that distinguish individuals are differentially distributed between groups. For example, East Asian populations tend to outscore White populations on tests of visuospatial ability, whereas the test score differences skew towards higher verbal ability for Ashkenazi-White differences. However, the mixture of abilities within groups appears to be nearly identical across many ethnic groups. The stability of these differences is also less well studied than Black-White differences.


According to Richard Lynn, J. Philippe Rushton, and others, IQ test score differences are observed cross-culturally and around the world. Lynn has published three books summarizing IQ test scores from around the world. The accuracy of the cross cultural IQ scores is highly controversial, but many scholars use the results as an estimate of world-wide IQ scores. Lynn's meta-analysis lists East Asians (105), Europeans (99), Inuit (91), Southeast Asians and Amerindians (87 each), Pacific Islanders (85), Middle Easterners (including South Asians and North Africans) (84), sub-Saharan Africans (67), and Australian Aborigines (62). International achievement test scores, including TIMSS and PISA, have also been used to estimate average IQ world-wide with similar results where data is available. The very low IQ scores reported for sub-Saharan African populations is especially controversial. For example, Wicherts argues that the average IQ of sub-Saharan Africans is poorly measured and is more likely 78. Sternberg, a critic of Rushton and Lynn, reported an average IQ of 70 for "young adolescents" in Kenya. Current scientific consensus is that IQ scores in developing countries are depressed to some extent by environmental conditions, such as macronutrient and micronutrient deficiencies.

Genetic and Environmental Factors

The cause(s) of group average IQ test score differences are not known but hypotheses have been proposed. Many scholars have offered descriptions of the variety of hypotheses that have been proposed. These descriptions usually distinguish between those hypotheses which invoke a contribution of genetic factors and those which solely invoke environmental (i.e., non-genetic) factors. Some descriptions of the positions are themselves controversial. In a review published in 2007, Hunt and Carlson listed four positions. The first position, attributed to Jensen and Rushton, is that group differences in IQ reflect differences in intelligence that are "due in substantial part to genetically determined differences in brain structure and/or function" The second position, attributed to Ogbu and Sowell, is that the differences in intelligence test scores are due to social factors. The third view, attributed to Sternberg and colleagues, is that the use of IQ scores to argue for differences in intelligence is an inappropriate use of tests in different groups. The fourth position, attributed to Fish and others, is that there is no such thing as race: "a term motivated by social concerns and not a scientific concept".

Prominent proponents of the genetic hypothesis are predominately psychologists or experts in intelligence testing, such as Charles Spearman, Hans Eysenck, Arthur Jensen, J. Philippe Rushton, Linda Gottfredson, Richard Herrnstein, and David C. Rowe. A few prominent proponents have come from other fields, such as political scientist Charles Murray, evolutionary psychologist Satoshi Kanazawa, and philosophers Michael Levin and Neven Sesardic. Not all proponents share the same views, nor have all of their views been constant throughout their careers. The genetic hypothesis has also received support from scholars whose primary research is in other fields, such as William Shockley and James D. Watson. Prominent critics come from many backgrounds, and hold a diversity of views. They include psychologists and experts in intelligence testing such as James Flynn (prior to his work on IQ Flynn was a political scientist), Ulric Neisser, Robert Sternberg, and a number of less well known psychometricians who criticize the underlying IQ test data. Other critics include economists Thomas Sowell and Roland G. Fryer Jr. anthropologists such as Ashley Montagu, Leonard Lieberman, and Jonathan Marks argue that "race" is a social and not a biological concept. For example Marks (1996) argues that the eugenics movement of the 1920s identified Ashkenazi Jews from Europe as intellectually inferior due to their genetics, but that Murray and Herrnstein claim this group as a "cognitive elite" in The Bell Curve, concluding that "race" is a social construct that does not reflect underlying biology. Sociologists John Ogbu, Claude S. Fischer and Troy Duster, paleontologist Stephen Jay Gould, geneticist Richard C. Lewontin and evolutionary biologist and physiologist Jared Diamond also oppose the genetic hypothesis.

The genetic hypothesis has been characterized as inherently racist. For example, Turkheimer argues that proponents of a genetic contribution, whom he calls "racialists", deserve "vigorous disapprobation" and do not deserve "respect". Turkheimer writes that he disagrees with his colleagues, such as Flynn and Ceci, who "agree that the question is a legitimate matter for scientific inquiry, to be settled by cool-headed evaluation of the empirical evidence". Flynn does not accept Turkheimer's arguments. Rushton and Jensen argue that the policy issues should be regarded as separate from the academic inquiry.

What is heritability?

The consensus among intelligence researchers is that IQ differences between individuals of the same racial-ethnic group reflect real, functionally and socially significant, and substantially heritable differences in intelligence.

Concordance rates for IQ from twin studies and other study designs consistently fall in the range of 50% to 80%, with the estimated heritability in young (preschool) children in the lower range and adults in the higher range.

Critics have also questioned the interpretation of heritability as a whole. Lewontin suggests that some genotypes are more influenced by environments than others, leading to the possibility that populations that have similar genetic variance in the same environment can have different heritabilities because of their different genotypes. David Layzer (1974) contends that the development of a trait can be influenced by genetic differences qualitatively and that heritability estimates cannot measure such qualitative differences, as such it is possible that even with a heritability of close to 100% it is possibly for phenotypic variance to be due largely to environment.

Genetic hypothesis

Much of the research on explaining group differences stems from an observation promoted first by Arthur Jensen and later James Flynn and others regarding an environmental explanation for group differences. According to Jensen and Flynn, the very high within-group heritability of IQ (within both white and black populations) presents a problem for environmental explanations of group differences in IQ. They consider two general classes of environmental factors: common environmental factors and X-factors. Common environmental factors vary within and between populations. X-factors vary between populations, but do not vary substantially within populations. They first consider common environmental factors. To account for a 1 SD B-W IQ gap only in terms of common environmental factors would require very large environmental differences. For example, if the within-group heritability of IQ is 80%, then a B-W IQ difference of 2.24 SD in common environmental factors is required. For a heritability of 40%, a difference of 1.29 SD is required. Jensen and Flynn agree that it is an empirical question whether common environmental factors that influence IQ differ between whites and blacks to such an extent, and both agree that most commonly suggested environmental factors do not. Jensen believes that empirical evidence supports the view that the B-W IQ gap is caused by both common environmental factors and genetic factors. Flynn disagrees and believes that empirical evidence supports the view that the B-W IQ gap is caused by yet unrecognized environmental factors.

The alternative to common environmental factors is to hypothesize that X-factors account for the B-W IQ gap. A frequently-cited example from Lewontin describes the effect of a hypothetical X-factor. Imagine that the height of "ordinary genetically varied corn" is 100% heritable when grown in a uniform environment. Further imagine that two populations of corn are grown: one in a normal nutrient environment and the other in a deficient nutrient environment. Consequently, the average height of the corn grown in the deficient nutrient environment is less than the average height of the corn grown in the normal environment. In such a scenario, the within-group heritability of height is 100% in both populations, but the substantial difference between groups are due entirely to environmental factors. The quality of the nutrient is an "X-factor" in the language of Jensen and Flynn. With respect to the B-W IQ gap, Jensen suggests that effects associated with racism (both overt and institutionalized racism) might be X-factors. Flynn believes that attributing the B-W gap to the effects of racism is incorrect, because the most plausible ways in which discrimination could affect IQ are themselves common environmental factors. These may include psychological effects such as stereotype threat; biological effects such as poor nutrition, health care and living close to toxic environments; and educational effects such as a lack of good schools. Instead, Flynn and his colleague William Dickens have developed more complicated models to explain the black-white gap in terms of environmental factors. One initial motivation of the Dickens-Flynn theory was Flynn's observation that IQ test scores have been rising over time in countries around the world – termed the Flynn effect. Flynn and others believe an explanation for the Flynn effect may elucidate the cause of the B-W gap. Jensen and others disagree.

A few of the notable proponents of the partly genetic hypothesis are Raymond B. Cattell, Arthur Jensen and Hans Eysenck.

Rushton and Jensen examined 10 categories of research evidence from around the world to contrast "a hereditarian model" (50% genetic-50% cultural) and a culture-only model (0% genetic-100% cultural). Their article "Thirty Years of Research on Race Differences in Cognitive Ability" was published in the APA journal Psychology, Public Policy and Law showing evidence that they believe supports the hereditarian model. Rushton and Jensen (2005a) believe that the best explanation for the gap is that 50%-80% of the group differences in average US IQ is genetic.

Other evidence, such as the Minnesota Transracial Adoption Study, certain racial admixture studies, behavior genetic modeling of group differences, "life-history" traits, and evolutionary explanations have also been proposed to indicate a genetic contribution to the IQ gaps and explain how these arose..

Environmental effects

Critics of the significantly genetic view, such as Robert Sternberg, argue that these studies are either flawed and thus inconclusive, or else that they support a primarily environment (<20% genetic) hypothesis. For example, Dolan and Hamaker (2001) argue that the statistical methods linking the Black-White gap to g are insufficient. They reanalyzed the data from several earlier studies and concluded that Spearman's hypothesis is not an "empirically established fact" (i.e., that Black-White IQ differences may be due to differences in common factors other than g) due to insufficient power in the data to choose between alternative models. "This leaves the validity of Spearman's hypothesis, considered a central justification for the genetic explanation, an unresolved question." However, they did confirm that the Black-White IQ gap is not due to measurement artifacts, and is instead due to some measured factor that varies both within and between groups.

Nisbett (2005) argues that many studies find results that do not support the genetic hypothesis. They include studies on IQ and skin color that reported that the average correlation between skin color and IQ is .1 (the average correlation between IQ and judged “Negroidness” of features is even lower); IQ and self-reported European ancestry; IQ and blood groups showing degree of European Ancestry; IQ among children in post WWII Germany born to black and white American soldiers; and IQ among mixed-race children born to either a black or a white mother. He argues that these are direct tests of the genetic hypothesis and of more value than indirect variables, such as skull size and reaction time. He argues that "There is not a shred of evidence in this literature, which draws on studies having a total of five very different designs, that the gap has a genetic basis." He argues further that many intervention and adoption studies also find results that do not support the genetic hypothesis. He also argues "that the Black-White IQ gap has lessened considerably in recent decades. Hunt and Carlson argue that Nisbett's interpretations are far too strong in light of problems with these studies that have been recognized for decades. Gottfredson writes that the studies Nisbett cites "actually lack the ability to rule out any hypothesis at all, genetic or not.

Dickens (2005) states that "Although the direct evidence on the role of environment is not definitive, it mostly suggests that genetic differences are not necessary to explain racial differences. Advocates of the hereditarian position have therefore turned to indirect evidence...The indirect evidence on the role of genes in explaining the black-white gap does not tell us how much of the gap genes explain and may be of no value at all in deciding whether genes do play a role. Because the direct evidence on ancestry, adoption, and cross-fostering is most consistent with little or no role for genes, it is unlikely that the black-white gap has a large genetic component.

Fryer and Levitt (2006), with data from "the first large, nationally representative sample" of its kind, report finding only a very small racial difference when measuring mental function for children aged eight to twelve months, and that even these differences disappear when including a "limited set of controls". "On tests of intelligence, Blacks systematically score worse than Whites, whereas Asians frequently outperform Whites. Some have argued that genetic differences across races account for the gap. Using a newly available nationally representative data set that includes a test of mental function for children aged eight to twelve months, we find only minor racial differences in test outcomes (0.06 standard deviation units in the raw data) between Blacks and Whites that disappear with the inclusion of a limited set of controls. The only statistically significant racial difference is that Asian children score slightly worse than those of other races." They argue that their report poses "a substantial challenge to the simplest, most direct, and most often articulated genetic stories regarding racial differences in mental function." They conclude that "to the extent that there are any genetically-driven racial differences in intelligence, these gaps must either emerge after the age of one, or operate along dimensions not captured by this early test of mental cognition.

The Flynn effect

The secular, international increase in test scores, commonly called the Flynn effect, is seen by Flynn and others as reason to expect the eventual convergence of average black and white IQ scores. Flynn argues that the average IQ scores in several countries have increased about 3 points per decade during the 20th century, which he and others attribute predominantly to environmental causes. This means, given the same test, the mean performance of Black Americans today could be higher than the mean for White Americans in 1920, though the gains causing this appear to have occurred predominantly in the lower half of the IQ distribution. If an unknown environmental factor can cause changes in IQ over time, they argue, then contemporary differences between groups could also be due to an unknown environmental factor. An added complication to this hypothesis is the question of whether the secular IQ gains can be predominantly a real change in cognitive ability. Flynn's face-value answer to this question is "No", and some other researchers have found reason to concur. In terms of the mixture of ability factors that IQ tests were designed to measure, such as g and verbal and mathematical ability, changes in IQ scores over time are different than either within-group individual differences and between group differences. For example, there has been little increase over time in performance on either the forward digit-span or reverse digit-span subtests, and tests of school achievement have been less affected than tests of abstract reasoning. Other recent studies have found that g has improved substantially. Cranial vault size has increased and the shape changed during the last 150 years in the US; these changes must occur by early childhood because of the early development of the vault.


Numerous explanations beside genetics have been proposed to account for the IQ gaps in the U.S. High rates of low birth-weight babies, lower rates of breastfeeding, and exposure to toxins are some factors. The Flynn effect is often cited as evidence that average IQ scores have changed greatly and rapidly, for reasons poorly understood, thus the IQ gap between races could change in the future or is changing, especially if the Flynn effect started earlier for Whites.

High levels of lead at an early age may affect intelligence; studies indicate that black and hispanic children have measurably higher levels than white children. A 10 µg/dL increase in blood lead at 24 months of age is associated with a 5.8-point lower IQ later in life. In 1976 77.8% of all children had at least this much lead in their blood.

Exposure to lead is frequently attributed to housing conditions including lead based paint, which is no longer used but has accumulated in older buildings; people of lower economic means are more frequently exposed to lead from housing.

Stereotype threat

Stereotype threat is the fear that one's behavior will confirm an existing stereotype of a group with which one identifies; this fear may in turn lead to an impairment of performance. Testing situations that highlight the fact that intelligence is being measured tend to lower the scores of individuals from racial-ethnic groups that already score lower on average. Stereotype threat conditions cause larger than expected IQ differences among groups but do not fully explain the gaps found in non-threatening test conditions.

Quality of education

Some researchers have written that studies that find test performance gaps between races even after adjusting for education level, such as the analysis found in The Bell Curve, fail to adjust for the quality of education. Not all high school graduates or college graduates have received the same quality of education. A 2006 study reported that years of education is an inadequate measure of the educational experience among multicultural elders, and that adjusting for quality of education greatly reduced the overall effect of racial differences on the tests. A 2004 study reported that quality of education and cultural experience influence how older African Americans approach neuropsychological tasks and concluded that adjustment for these variables may improve specificity of neuropsychological measures. Yet another study reported that, although significant differences were observed between the ethnic groups when matched for years of education, equating for literacy level eliminated all performance differences between African Americans and European Americans on both cancellation tasks which assess visual scanning (like reaction time tests, cancellation task tests are sometimes regarded as "culture free" tests of intelligence). Eric A. Hanushek and Steven G. Rivkin wrote in their 2006 book that unequal distributions of inexperienced teachers and of racial concentrations in schools can explain all of the increased achievement gap between grades 3 and 8.

A 2004 study in South Africa found highly significant effects for both level and quality of education within the black African first language groups taking the Wechsler IQ tests. The scores of black African first language groups with advantaged education were comparable with the US standardization, whereas scores for black African first language participants with disadvantaged education were significantly lower than this. The study cautioned that faulty conclusions may be drawn about the effects of ethnicity and the potential for neuropsychological misdiagnosis.

Racial discrimination in education

Roslyn Arlin Mickelson writes that racial discrimination in education arises from actions of institutions or individual state actors, their attitudes and ideologies, or processes that systematically treat students from different racial/ethnic groups disparately or inequitably. Despite advancement in education reform efforts, to this day African American students continue to experience inequities within the educational system. Hala Elhoweris, Kagendo Mutua, Negmeldin Alsheikh and Pauline Holloway conducted a study of the effect of students' ethnicity on teachers' educational decision making. The results of this study indicated that the student's ethnicity did make a difference in the teachers' referral decisions for gifted and talented educational programs.Recently, a number of scholars have examined the issue of disproportionate representation of minority students in special education programs

Teachers' perceptions of a students cultural background may effect school achievement. African American students with African American cultural backgrounds, for example, have been found to benefit from culturally responsive teaching. In a 2003 study researchers found that teachers perceived students with African American culture-related movement styles as lower in achievement, higher in aggression, and more likely to need special education services than students with standard movement styles irrespective of race or other academic indicators.

Ellis Cose writes that low expectations may have a negative impact on the achievement of minorities. He writes that black people did not need to read The Bell Curve to be aware of the low expectations held for them by the majority culture. He recalls examples of low expectations from his teachers in school who regarded his use of AAVE as "laziness" and teachers who did not feel it was important to purchase new text books because they did not expect the students to be able to read anything complex. He contrasts these low expectations with the high expectations philosophy of Xavier University where, using the ideas Whimbey articulated in his book Intelligence can be Taught teachers created a program called SOAR. SOAR raised the performance of black students and lead Xavier to become the university that sends the greatest number of black students to medical school in the United States. The SOAR program produced gains equivalent to 120 points on an SAT test. Cose writes that "..we must treat people, whatever their color, as if they have unlimited intellectual capacity.

Caste-like minorities

The book Inequality by Design: Cracking the Bell Curve Myth claims that it is not lower average intelligence that leads to the lower status of ethnic minorities, it is instead their lower status that leads to their lower average intelligence test scores. The following table from the same book compares social status or caste position to test scores and school success in nations around the world.

Group Differences Around the World
  Status or Caste Position Test Scores, School Success
Country High Low High Low
United States Whites Blacks Whites Blacks
  Asians Latinos Asians Latinos
  Whites American Indians Whites American Indians
Great Britain English Irish, Scottish English Irish, Scottish
Northern Ireland Protestants Catholics Protestants Catholics
Australia Whites Aborigines Whites Aborigines
New Zealand Whites Maoris Whites Maoris
South Africa English Afrikaners English Afrikaners
Belgium Flemish French Flemish French
Israel Jews Arabs Jews Arabs
  Western Jews Eastern Jews Western Jews Eastern Jews
India Nontribals Tribal people Nontribals Tribal people
  Brahmin Dalit Brahmin Dalit
  High caste Low caste High caste Low caste
Czechoslovakia Slovaks Gypsies Slovaks Gypsies

  1. Czechoslovakia split into the Czech Republic and Slovakia in 1993.
  2. The Flemish are not a minority in Belgium. The PISA 2006 results for schoolchildren 15 years of age showed that the Flemish outperformed the Walloons (French).
  3. Afrikaans speakers make up the majority of the white population; most South Africans, however, identified themselves in the census as black, and speak one of the Bantu languages as their home language

Viewpoints of notable scientists and researchers

Given the observed differences in IQ scores between certain groups, a great deal of debate revolves around the significance of these observations. Various interpretations of test data lead to a multitude of conflicting conclusions as to which specific explanations the data support.

The range of views among contemporary scholars include:

  • Arthur Jensen argues that differences in cognitive abilities between races exist and are caused by both social/environmental disparities and also genetic differences between races.
  • James Flynn argues that differences in cognitive abilities between races exist and are of social/environmental origin.
  • Robert Sternberg argues that the use of differences in test scores to argue for race differences in intelligence is a case of the inappropriate use of tests in different groups.
  • Ashley Montagu and Joseph L. Graves, amongst others, argue that race is not a biological concept, but rather is socially constructed, and hence measurements of IQ differences between races cannot reflect an underlying intrinsic biological cause.

Researchers who believe that there is no significant genetic contribution to race differences in intelligence include:

Flynn (1980), Brody (1992), Neisser et al. (1996), Nisbett (1998), Mackintosh (1998), Jencks and Phillips (1998), and Fish (2002).

Some scientists who emphasize cultural explanations do not necessarily exclude a small genetic influence. Reynolds (2000) suggests up to 20% genetic influence be included in the cultural explanation.

Researchers who believe that there are significant genetic contributions to race differences in intelligence include:

McGurk (1953), Garrett (1961), Shuey (1966), Shockley (1968), Eysenck (1971), Baker (1974), Loehlin et al. (1975), Vernon (1979), Lynn (1991a), Waldman et al. (1994), Scarr (1995), Levin (1997), Jensen (1998b), Rushton (2000), and Gottfredson (2005b).

Coming advances in genetics and genomics are expected to soon provide the ability to test hypotheses about group differences more rigorously than has as yet been possible.

Jared Diamond's Guns, Germs and Steel instead argues that historical differences in economic and technological development for different geographic areas can be explained by differences in geography (which affects factors like population density and spread of new technology) and differences in available crops and domesticatable animals. Richard Nisbett argues in his 2004 The Geography of Thought that some of these regional differences shaped lasting cultural traits, such as the collectivism required by East Asian rice irrigation, compared with the individualism of ancient Greek herding, maritime mercantilism, and money crops wine and olive oil However, it has been suggested that these environmental differences may operate in part by selecting for higher levels of IQ.

J. Philippe Rushton, a professor of psychology at the University of Western Ontario and the current head of the Pioneer fund, has written a controversial book called Race, Evolution, and Behavior: A Life History Perspective. Rushton claims in the book that race is a valid biological concept and that racial differences frequently arrange in a continuum of Mongoloids (Orientals, East Asians) at one extreme, Negroids (blacks, Africans) at the opposite extreme, and Caucasoids (whites, Europeans) in the middle. It has been heavily criticized.

Differing rates of economic growth have also been attributed to numerous factors other than racial IQ gaps such as local availability of resources, climate, and sociopolitical factors. See for example the Global Competitiveness Report, the Ease of Doing Business Index, and the Index of Economic Freedom or works by Kenneth Pomeranz, Eric Jones, Joel Mokyr, and Douglass C. North.


Outdated methodology

A 2006 paper by Professor Denny Borsboom argues that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s.It also claims that some of the most influential recent studies on group differences in intelligence, in order to show that the tests are unbiased, use outdated methodology. In particular the reliance on classical test theory rather than more sophisticated measurement models as found in item response theory.

Utility of research

Theories of race and intelligence have been challenged on grounds of their utility. Critics want to know what purpose such research could serve and why it has been an intense an area of focus for a few researchers. Some defend the research, saying it has egalitarian aims or that it is pure science; others say that the true motivation for the research is the same as that of the eugenics movement and other forms of scientific racism. Even supporters of intelligence research have described such research as analogous to "working with dynamite" or "dangerous play" in sports.

As to whether research in this area is desirable, John C. Loehlin wrote in 1992, "Research on racial differences in intelligence is desirable if the research is appropriately motivated, honestly done, and adequately communicated." [emphasis original] Defenders of the research suggest that both scientific curiosity and a desire to draw benefits from the research are appropriate motivations. Researchers such as Richard Lynn have suggested that conclusions from the research can help make political decisions, such as the type of educational opportunities and expectations of achievement policy makers should have for people of different races. Charles Murray, a political scientist of the American Enterprise Institute has used their conclusions to criticize social programs based on racial equality that fail, he claims, to recognize the realities of racial differences.

Test construction

While the existence of average IQ test score differences has been a matter of accepted fact for decades, a great deal of controversy exists among scholars over the question of whether these score differences reflected real differences in cognitive ability. Some claim that there is no evidence for test bias since IQ tests are equally good predictors of IQ-related factors (such as school performance) for U.S. Blacks and Whites. The performance differences persist in tests and testing situations in which care has been taken to eliminate bias. It has also been suggested that IQ tests are formulated in such a way as to disadvantage minorities. Controlled studies have shown that test construction does not substantially contribute to the IQ gap. However, some psychometricians are not satisfied that the question of test bias is fully answered by these results. Also, all commenters reject the common misconception that IQ is meant to measure "innate" differences in intelligence, and they agree that average IQ scores on their own say nothing about the cause of the differences.

The preponderance of evidence indicates that IQ tests measuring general intelligence are crossculturally valid. There is little or no evidence of population-specific cultural effects apart from the obvious example of language bias. For example, Robert Sternberg et al. found that the IQ of 12- to 15-year-old Kenyans predicted school grades at about the same level as they do in the West. IQ also predicted university performance equally well in African and non-African engineering students in South Africa in a 2004 study. Salgado et al. (2003) demonstrated the international generalizability of general mental ability across 10 member countries of the European Community and differences in a nation’s culture, religion, language, socioeconomic level or employment legislation did not affect the predictive validity of IQ tests.

However, other studies have found evidence for bias. A 2005 study finds some evidence that the WAIS-R is not culture-fair for Mexican Americans. Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.

Source of funding

Proponents of genetic explanations of race/IQ correlation, such as Rushton, Lynn, and Jensen, have often been criticized for receiving funding from the Pioneer Fund, a group that had ties to Nazis and the eugenics of the early 20th century. The Southern Poverty Law Center considers the Pioneer Fund to be a hate group. Rushton is the current head of the Pioneer Fund and has spoken at conferences of the American Renaissance magazine, in which he has also published articles. Anti-racist Searchlight Magazine described one of these articles as a "veritable 'who’s who' of American white supremacy."

Proponents of genetic explanations of race-IQ correlation have in turn accused their critics of suppressing scientific debate in the name of political correctness. They claim harassment and interference with both their work and funding. The Pioneer Fund, whose stated purpose is "to advance the scientific study of heredity and human differences", makes "no grants to individuals but only to research institutions, mainly universities, mostly for specialized 'niche' projects, which have difficulty attracting funds from government sources or from larger foundations".

See also

















  • Purdey, J. E., University of Melbourne (2002).
  • Reynolds, C. R., Chastain, R. L., Kaufman, A. S., & McLean, J. E. (1987). "Demographic characteristics and IQ among adults: Analysis of the WAIS-R standardization sample as a function of the stratification variables". Journal of School Psychology 25 323–342.
  • *

















  • Wade, Nicholas (2006). Before the Dawn: Recovering the Lost History of Our Ancestors. New York: Penguin Press. ISBN 1-59420-079-3.
  • *

External links

Collective statements

  • APA Task Force Examines the Knowns and Unknowns of Intelligence
  • Statement on "Race" and Intelligence American Anthropological Association. Adopted December 1994.
  • Mainstream Science on Intelligence Intelligence, v24 n1 p. 13–23 Jan–Feb 1997
  • Review papers


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