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The connection between race and intelligence has been a subject of debate in both popular science and academic research since the inception of IQ testing in the early 20th century. The debate concerns the interpretation of research findings that American test takers identifying as "White" tend on average to score higher than test takers of African ancestry on IQ tests, and subsequent findings that test takers of East Asian background tend to score higher than whites. It is still not resolved what relation, if any, there is between group differences in IQ and race.
Large-scale tests of United States Army recruits in World War I showed that Blacks and non-English-speaking immigrants scored considerably lower than native-born whites. In the 1920s it was argued that this demonstrated that these groups were of inferior intellect to Anglo-Saxon whites due to innate biological differences. This was used as an argument in favor of immigration restriction and racial segregation. Soon, other studies appeared, contesting these conclusions and arguing instead that the Army tests had not adequately controlled for the environmental factors such as socio-economic and educational inequality between African-Americans and Whites. After a hiatus, the debate emerged again in the 1950s, and became prominent in the 1960s when the claim that Africans were less intelligent than whites and that compensatory education for African-American children was therefore doomed to be ineffective was dubbed "Jensenism," after Arthur Jensen. In 1994, the book The Bell Curve, which argued that social inequality in America could largely be explained as a result of IQ differences between races and individuals rather than being their cause, rekindled the public and scholarly debate with renewed force. During the debates following the book's publication the American Anthropological Association and the American Psychological Association (APA) published official statements regarding the issue, both highly skeptical of some of the book's claims, although the APA report called for more empirical research on the issue.
In subsequent decades much research has been published about the relationships between hereditary influences on IQ, group differences in intelligence, race, environmental influences on IQ. Particularly contentious in the ongoing debate has been the definition of both the concept "race" and the concept "intelligence", and especially whether they can in fact be objectively defined and operationalized. While several environmental factors have been shown to affect group differences in intelligence, it has not been demonstrated that they can explain the entire disparity. But on the other hand, no genetic factor has been conclusively shown to have a causal relation with group difference in intelligence test scores. Recent summaries of the debate call for more research into the topic to determine the relative contributions of environmental and genetic factors in explaining the apparent IQ disparity among racial groups.
Claims of races having different intelligence were used to justify colonialism, slavery, social Darwinism, and racial eugenics. In the late 19th and early 20th centuries, group differences in intelligence were assumed to be due to race and, apart from intelligence tests, research relied on measurements such as brain size or reaction times to demonstrate such differences. The first practical intelligence test was developed between 1905 and 1908 by Alfred Binet in France for school placement of children. Binet warned that results from his test should not be assumed to measure innate intelligence or used to label individuals permanently. Binet's test was translated into English and revised in 1916 by Lewis Terman (who introduced IQ scoring for the test results) and published under the name the Stanford–Binet Intelligence Scales. As Terman's test was published, there was great concern in the United States about the abilities and skills of recent immigrants. Different immigrant nationalities were sometimes thought to belong to different races, such as Slavs. A different set of tests developed by Robert Yerkes were used to evaluate draftees for World War I, and researchers found that people from southern and eastern Europe scored lower than native-born Americans.
In the 1920s, states like Virginia enacted eugenic laws, such as its 1924 Racial Integrity Act, which established the one-drop rule as law. On the other hand, many scientists reacted to eugenicist claims linking abilities and moral character to racial or genetic ancestry. They pointed to the contribution of environment to test results (such as speaking English as a second language). By the mid-1930s, mamy United States psychologists adopted the view that environmental and cultural factors played a dominant role in IQ test results. Discussion of the issue in the United States was also influenced by German Nazi claims of a "master race".
In the 1960s, William Shockley revived public debate on racial differences in intelligence. Arthur Jensen stimulated scholarly discussion of the issue with his Harvard Education Review article, "How Much Can We Boost IQ and Scholastic Achievement?" Jensen's article questioned remedial education for African-American children; he suggested their poor educational performance reflected an underlying genetic cause rather than lack of stimulation at home. Jensen continued to publish on the issue until his death in 2012.
Another revival of public debate followed the appearance of The Bell Curve (1994), a book by Richard Herrnstein and Charles Murray, who strongly emphasized the societal effects of low IQ (focusing in most chapters strictly on the white population of the United States). In 1994 a group of 52 researchers (mostly psychologists) signed an editorial statement "Mainstream Science on Intelligence" in response to the book. The Bell Curve also led to a 1995 report from the American Psychological Association, "Intelligence: Knowns and Unknowns", acknowledging a difference between mean IQ scores of whites and blacks as well as the absence of any adequate explanation of it, either environmental or genetic. The Bell Curve prompted the publication of several multiple-author books responding from a variety of points of view. They include The Bell Curve Debate (1995), Inequality by Design: Cracking the Bell Curve Myth (1996) and a second edition of The Mismeasure of Man (1996) by Steven J. Gould. Jensen's last book-length publication, The g Factor: The Science of Mental Ability was published a few years later in 1998.
The review article "Thirty Years of Research on Race Differences in Cognitive Ability" by Rushton and Jensen was published in 2005. The article was followed by a series of responses, some in support, some critical. Richard Nisbett, another psychologist who had also commented at the time, later included an amplified version of his critique as part of the book Intelligence and How to Get It: Why Schools and Cultures Count (2009). Rushton and Jensen in 2010 made a point-by-point reply to this thereafter. A comprehensive review article on the issue was published in the journal American Psychologist in 2012.
Some of the authors proposing genetic explanations for group differences have received funding from the Pioneer Fund which was headed by Rushton until his death in 2012. The Southern Poverty Law Center lists the Pioneer Fund as a hate group, citing the fund's history, its funding of race and intelligence research, and its connections with racist individuals. On the other hand, Ulrich Neisser writes that "Pioneer has sometimes sponsored useful research—research that otherwise might not have been done at all." Other researchers have criticized the Pioneer Fund for promoting scientific racism, eugenics and white supremacy.
The 1996 report of the APA commented on the ethics of research on race and intelligence. Gray & Thompson (2004) as well as Hunt & Carlson (2007) have also discussed different possible ethical guidelines.[non-primary source needed] Nature in 2009 featured two editorials on the ethics of research in race and intelligence by Steven Rose (against) and Stephen J. Ceci and Wendy M. Williams (for).
According to critics, research on group differences in IQ will run the risk of reproducing the negative effects of social ideologies (such as Nazism or social Darwinism) that were justified in part on claimed hereditary racial differences. Steven Rose maintains that the history of eugenics makes this field of research difficult to reconcile with current ethical standards for science.
Linda Gottfredson argues that suggestion of higher ethical standards for research into group differences in intelligence is a double standard applied in order to undermine disliked results. James R. Flynn has argued that had there been a ban on research on possibly poorly conceived ideas, much valuable research on intelligence testing (including his own discovery of the Flynn effect) would not have occurred.
The concept of intelligence and the degree to which intelligence is measurable is a matter of debate. While there is a general consensus within Western science about how to define intelligence, the concept of intelligence as something that can be unequivocally measured by a single figure is not universally accepted. A recurring criticism is that different societies value and promote different kinds of skills and that the concept of intelligence is therefore culturally variable and cannot be measured by the same criteria in different societies. Consequently, some critics argue that proposed relationships to other variables are necessarily tentative.
The concept of race as a biologically meaningful category of analysis is also hotly contested. Articles in the Encyclopædia Britannica and the Encyclopedia of Race, Ethnicity and Society state that the current mainstream view is that race is a social construction mainly based not in actual biological differences but rather in folk ideologies that construct groups based on social disparities and superficial physical characteristics. Sternberg, Grigorenko & Kidd (2005) state, "Race is a socially constructed concept, not a biological one. It derives from people's desire to classify." The concept of human "races" as natural and separate divisions within the human species has also been rejected by the American Anthropological Association. The official position of the AAA, adopted in 1998, finds that advances in scientific knowledge have made it "clear that human populations are not unambiguous, clearly demarcated, biologically distinct groups" and that "any attempt to establish lines of division among biological populations [is] both arbitrary and subjective." Others argue that this view is restricted to certain fields, while in other fields, race is still seen as a valid biological category.
Race in studies of human intelligence is almost always determined using self-reports, rather than based on analyses of the genetic characteristics of the tested individuals. According to psychologist David Rowe, self-report is the preferred method for racial classification in studies of racial differences because classification based on genetic markers alone ignore the "cultural, behavioral, sociological, psychological, and epidemiological variables" that distinguish racial groups. Hunt and Carlson write that "Nevertheless, self-identification is a surprisingly reliable guide to genetic composition. Tang et al. (2005) applied mathematical clustering techniques to sort genomic markers for over 3,600 people in the United States and Taiwan into four groups. There was almost perfect agreement between cluster assignment and individuals' self-reports of racial/ethnic identification as white, black, East Asian, or Latino." Sternberg and Grigorenko disagree with Hunt and Carlson's interpretation of Tang, "Tang et al.'s point was that ancient geographic ancestry rather than current residence is associated with self-identification and not that such self-identification provides evidence for the existence of biological race."
The notions that cluster analysis and the correlation between self-reported race and genetic ancestry supports a view of race as primarily based in biology is contradicted by C. Loring Brace and geneticist Joseph Graves. They argue that while it is possible to find biological and genetic variation corresponding roughly to the groupings normally defined as races, this is true for almost all geographically distinct populations. The cluster structure of the genetic data is dependent on the initial hypotheses of the researcher and the populations sampled. When one samples continental groups, the clusters become continental; if one had chosen other sampling patterns, the clusters would be different. Kaplan 2011 therefore concludes that, while differences in particular allele frequencies can be used to identify populations that loosely correspond to the racial categories common in Western social discourse, the differences are of no more biological significance than the differences found between any human populations (e.g., the Spanish and Portuguese).
Earl B. Hunt agrees that racial categories are defined by social conventions, though he points out that they also correlate with clusters of both genetic traits and cultural traits. Hunt explains that, due to this, racial IQ differences are caused by these variables that correlate with race, and race itself is rarely a causal variable. Researchers who study racial disparities in test scores are studying the relationship between the scores and the many race-related factors which could potentially affect performance. These factors include health, wealth, biological differences, and education.
|The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (February 2013)|
Hunt and Carlson outlined four contemporary positions on differences in IQ based on race or ethnicity. The first is that these reflect real differences in average group intelligence, which is caused by a combination of environmental factors and heritable differences in brain function. A second position is that differences in average cognitive ability between races are caused entirely by social and/or environmental factors. A third position holds that differences in average cognitive ability between races do not exist, and that the differences in average test scores are the result of inappropriate use of the tests themselves. Finally, a fourth position is that either or both of the concepts of race and general intelligence are poorly constructed and therefore any comparisons between races are meaningless.
Rushton & Jensen (2005) write that, in the United States, self-identified blacks and whites have been the subjects of the greatest number of studies. They state that the black-white IQ difference is about 15 to 18 points or 1 to 1.1 standard deviations (SDs), which implies that between 11 and 16 percent of the black population have an IQ above 100 (the general population median). The black-white IQ difference is largest on those components of IQ tests that are claimed best to represent the general intelligence factor g.[non-primary source needed] The 1996 APA report "Intelligence: Knowns and Unknowns" and the 1994 editorial statement "Mainstream Science on Intelligence" gave more or less similar estimates. Roth et al. (2001), in a review of the results of a total of 6,246,729 participants on other tests of cognitive ability or aptitude, found a difference in mean IQ scores between blacks and whites of 1.1 SD. Consistent results were found for college and university application tests such as the Scholastic Aptitude Test (N = 2.4 million) and Graduate Record Examination (N = 2.3 million), as well as for tests of job applicants in corporate sections (N = 0.5 million) and in the military (N = 0.4 million).
A 2006 study by Dickens and Flynn estimated that the difference between mean scores of blacks and whites closed by about 5 or 6 IQ points between 1972 and 2002, which would be a reduction of about one-third. However this was challenged by Rushton & Jensen who claim the difference remains stable.[non-primary source needed] In a 2006 study, Murray agreed with Dickens and Flynn that there has been a narrowing of the difference; "Dickens' and Flynn's estimate of 3–6 IQ points from a base of about 16–18 points is a useful, though provisional, starting point". But he argued that this has stalled and that there has been no further narrowing for people born after the late 1970s. Murray found similar results in a 2007 study.[non-primary source needed]
The IQ distributions of other racial and ethnic groups in the United States are less well-studied. The Bell Curve (1994) stated that the average IQ of African Americans was 85, Latinos 89, whites 103, East Asians 106, and Ashkenazi Jews 113. Asians score relatively higher on visuospatial than on verbal subtests. The few Amerindian populations who have been systematically tested, including Arctic Natives, tend to score worse on average than white populations but better on average than black populations.
The racial groups studied in the United States and Europe are not necessarily representative samples for populations in other parts of the world. Cultural differences may also factor in IQ test performance and outcomes. Therefore, results in the United States and Europe do not necessarily correlate to results in other populations.
Raw scores on IQ tests had until recently been rising throughout the world for a century. This score increase is known as the "Flynn effect," named after Jim Flynn, who did much to document it and promote awareness of its implications. In the United States, the increase had been continuous and approximately linear from the earliest years of testing to about 1998. For example, in the United States the average scores of blacks on some IQ tests in 1995 were the same as the scores of whites in 1945.
Te Nijenhuis and van der Flier (2013), in a meta analysis of 5 papers, concluded that the Flynn effect and group differences in intelligence have different causes. They stated that the Flynn effect is caused primarily by environmental factors and that it's unlikely these same environmental factors play an important role in explaining group differences in IQ.
Studies have shown that the disparity between African-American and non-Hispanic white test scores diminished considerably in the period from 1972 to 2002. According to one survey by Dickens and Flynn it diminshed with almost 6 points from a size of approximately 15 pts to approximately 9 in this period. In the same period the educational achievement disparity also diminished. Using a different data set, Charles Murray has argued for the same period that the IQ gap remained stable. A recent review of the literature has found that the IQ gap has diminished by 0.33 standard deviation since first reported.
The following environmental factors are some of those suggested as explaining a portion of the differences in average IQ between races. These factors are not mutually exclusive with one another, and some may in fact contribute directly to others. Furthermore, the relationship between genetics and environmental factors may be complicated. For example, the differences in socioeconomic environment for a child may be due to differences in genetic IQ for the parents, and the differences in average brain size between races could be the result of nutritional factors.
A number of studies have reached the conclusion that IQ tests may be biased against certain groups. The validity and reliability of IQ scores obtained from outside the United States and Europe have been questioned, in part because of the inherent difficulty of comparing IQ scores between cultures. Several researchers have argued that cultural differences limit the appropriateness of standard IQ tests in non-industrialized communities. In the mid-1970s, for example, the Soviet psychologist Alexander Luria concluded that it was impossible to devise an IQ test to assess peasant communities in Russia because taxonomy was alien to their way of reasoning.
A 1996 report by the American Psychological Association states that controlled studies show that differences in mean IQ scores were not substantially due to bias in the content or administration of the IQ tests. Furthermore, the tests are equally valid predictors of future achievement for black and white Americans. This view is reinforced by Nicholas Mackintosh in his 1998 book IQ and Human Intelligence, and by a 1999 literature review by Brown, Reynolds & Whitaker (1999).
Stereotype threat is the fear that one's behavior will confirm an existing stereotype of a group with which one identifies or by which one is defined; this fear may in turn lead to an impairment of performance. Testing situations that highlight the fact that intelligence is being measured tend to lower the scores of individuals from racial-ethnic groups who already score lower on average or are expected to score lower. Stereotype threat conditions cause larger than expected IQ differences among groups.
According to the report of a 1996 APA task force, socioeconomic status (SES) cannot account for all the observed racial-ethnic group differences in IQ in the US. Their first reason for this conclusion is that the difference between mean test scores of blacks and whites is not eliminated when individuals and groups are matched on SES. Second, excluding extreme conditions, nutritional and biological factors that may vary with SES have shown little effect on IQ. Third, the relationship between IQ and SES is not simply one in which SES determines IQ, but differences in intelligence, particularly parental intelligence, also cause differences in SES, making separating the two factors difficult. Adoption studies have shown that children adopted from lower-class homes to middle-class homes experience a 12 - 18 pt gain in IQ
Fryer & Levitt (2006) found in tests of children aged eight to twelve months only minor differences (0.06 SD) between blacks and whites that disappeared with the inclusion of a limited set of controls including social-economic status. Flynn has argued that the United States black-white gap appears gradually which suggests environmental causes. "At just 10 months old, the average score is only one point behind; by the age of 4, it is 4.6 points behind, and by the age of 24, the gap is 16.6 points. This could be due to genes, but the steady rate after the age of 4 (about 0.6 IQ points lost every year) suggests otherwise, since genetically driven differences such as height differences between males and females tend to kick in at a certain age."[non-primary source needed]
Rushton and Jensen argue that the black-white IQ difference of one standard deviation is present at the age of 3 and does not change significantly afterward. Murray argues that the heritability of IQ increases with age which is reflected in the racial IQ gaps gradually increasing.
Environmental factors including lead exposure, breast feeding, and nutrition can significantly affect cognitive development and functioning. For example, iodine deficiency causes a fall, on average, of 12 IQ points. Such impairments may sometimes be permanent, sometimes be partially or wholly compensated for by later growth. The first two years of life is the critical time for malnutrition, the consequences of which are often irreversible and include poor cognitive development, educability, and future economic productivity. The African American population of the United States is statistically more likely to be exposed to many detrimental environmental factors such as poorer neighborhoods, schools, nutrition, and prenatal and postnatal health care.
The Copenhagen consensus in 2004 stated that lack of both iodine and iron has been implicated in impaired brain development, and this can affect enormous numbers of people: it is estimated that one-third of the total global population are affected by iodine deficiency. In developing countries, it is estimated that 40% of children aged four and under suffer from anaemia because of insufficient iron in their diets.
Other scholars have found that simply the standard of nutrition has a significant effect on population intelligence, and that the FLynn effect may be caused by increasing nutrition standards across the world. James Flynn has himself argued against this view.
Eppig, Fincher & Thornhill (2010) argue that "From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks" and that differences in prevalence of infectious diseases (such as malaria) may be an important explanation for differences in IQ between different regions of the world. They tested other hypotheses as well, including genetic explanations, concluding that infectious disease was "the best predictor". Christopher Hassall and Thomas Sherratt repeated the analysis, and concluded "that infectious disease may be the only really important predictor of average national IQ".
In order to mitigate the effects of education on IQ, Eppig, Fincher & Thornhill (2010) repeated their analysis across the United States where standardized and compulsory education exists. The correlation between infectious disease and average IQ was confirmed, and they concluded that the "evidence suggests that infectious disease is a primary cause of the global variation in human intelligence".
Several studies have proposed that a large part of the gap can be attributed to differences in quality of education. Racial discrimination in education has been proposed as one possible cause of differences in educational quality between races. According to a paper by Hala Elhoweris, Kagendo Mutua, Negmeldin Alsheikh and Pauline Holloway, teachers' referral decisions for students to participate in gifted and talented educational programs were influenced in part by the students' ethnicity.
The Abecedarian Early Intervention Project, an intensive early childhood education project, was also able to bring about an average IQ gain of 4.4 points at age 21 in the black children who participated in it compared to controls. Arthur Jensen agreed that the Abecedarian project demonstrates that education can have a significant effect on IQ, but also said that no educational program thus far has been able to reduce the black-white IQ gap by more than a third, and that differences in education are thus unlikely to be its only cause.
Rushton and Jensen argue that long-term follow-up of the Head Start Program found large immediate gains for blacks and whites but that these were quickly lost for the blacks although some remained for whites. They argue that also other more intensive and prolonged educational interventions have not produced lasting effects on IQ or scholastic performance. Nisbett argues that they ignore studies such as Campbell & Ramey (1994) which found that at the age 12, 87% black of infants exposed to an intervention had IQs in the normal range (above 85) compared to 56% of controls, and none of the intervention-exposed children were mildly retarded compared to 7% of controls. Other early intervention programs have shown IQ effects in the range of 4–5 points, which are sustained until at least age 8–15. Effects on academic achievement can also be substantial. Nisbett also argues that not only early age intervention can be effective, citing other successful intervention studies from infancy to college.
A large number of studies have shown that systemically disadvantaged minorities, such as the African American minority of the United States generally perform worse in the educational system and in intelligence tests than the majority groups or less disadvantaged minorities such as immigrant or "voluntary" minorities. The explanation of these findings may be that children of caste-like minorities, due to the systemic limitations of their prospects of social advancement, do not have "effort optimism", i.e. they do not have the confidence that acquiring the skills valued by majority society, such as those skills measured by IQ tests, is worthwhile. They may even deliberately reject certain behaviors seen as "acting white".
This argument is also explored in the book Inequality by Design: Cracking the Bell Curve Myth (1996) which argues that it is not lower average intelligence that leads to the lower status of racial and ethnic minorities, it is instead their lower status that leads to their lower average intelligence test scores. One example being Jews in the early 20th century in the US who, the authors argue, scored low on IQ tests. To substantiate this claim, the book presents a table comparing social status or caste position with test scores and measures of school success in several countries around the world. Examples include Koreans, Peruvians and Brazilians in Japan, Burakumin in Japan, Australian Aborigines, Romani in Czechoslovakia, Maori in New Zealand, Afro-Brazilians, Indigenous Brazilians, Pardos and Rural Exiles (as, but not limited to, people from Northeast in Brasília, São Paulo and Rio de Janeiro metropolitan areas, and including a minority of European descent) in Brazil, Afrikaners in South Africa, Catholics in North Ireland, Irish and Scottish in Great Britain, Arabs and Sephardi Jews in Israel, and Dalit, low caste, and tribal people in India. The authors note, however, that the comparisons made in the table do not represent the results of all relevant findings, that sometimes studies have shown more mixed findings, that the tests and procedures varied greatly from study to study, and that there is no simple way to compare the size of group differences. The statement regarding Arabs in Israel, for example, is based on a news report that, in 1992, 26% of Jewish high school, predominantly Ashkenazim, students passed their matriculation exam as opposed to 15% of Arab students. Stephen Jay Gould in the The Mismeasure of Man also argued that Jews in the early 20th century scored low on IQ tests. Rushton as well as Cochran, Hardy & Harpending have argued that this is a misrepresentation of the studies and that also the early testing support a high average Jewish IQ.[non-primary source needed]
Murray replies that purely sociocultural factors like this cannot explain the gap, because the size of the gap on any test is dependent on that test's degree of g-loading. As an example, Murray notes that the test of reciting a string of digits backwards is much more g-loaded than reciting it forwards, and the black-white gap is around twice as large on the first test as on the second. According to Murray, there is no way that culture or motivation could systematically encourage black performance on one test while decreasing it on another, when both tests are provided by the same examiner in the same setting.[non-primary source needed]
One theory of Intelligence formulated by Psychologists Joseph Fagan and Cynthia Holland argues that what IQ tests really measure is not innate ability but a form of knowledge. From this theoretical perspective they predicted that if prior exposure to the kinds of knowledge that are typically found on an IQ tests is not equal between African-Americans and Whites that could explain the difference in performance. They then tested these predictions by providing one group of African-American test subjects with prior exposure to the task types. The researchers found that there was no subsequent difference in performance between the African-Americans and White test takers. Daley and Onwugbuezie conclude that Fagan and Holland demonstrate that "differences in knowledge between Blacks and Whites for intelligence test items can be erased when equal opportunity is provided for exposure to the information to be tested".
Psychologist David Marks correlated Armed Forces Intelligence tests with literacy tests taken by the same groups and found correlation of .96 and .997. Marks concluded that "On the basis of the studies summarized here, there can be little doubt that the Armed Forces Qualifications Test is a measure of literacy." He also argued that cross-national IQ disparities correlated closely with literacy rates.
The American Anthropological Association in 1994 stated that intelligence is not biologically determined by race. The American Psychological Association in 1994 stated that there is little evidence to support environmental explanations, certainly no support for genetic interpretations, and that presently the cause of the black-white IQ gap is unknown. It is well-established that intelligence is highly heritable for individuals, and many different kinds of genetically caused intelligence impairments are known. But the possible relations between genetic differences in intelligence within the normal range are not established. Ongoing research aims to understand the contribution of genes to individual differences in intelligence. It remains unknown if there is a possible genetic contribution to the racial disparity in IQ test results.
The decoding of the human genome has enabled scientists to search for sections of the genome that may contribute to cognitive abilities. However the geneticist, Alan R. Templeton suggests this question is muddled by the general focus on "race" rather than on populations defined by gene frequency or by geographical proximity, and by the general insistence on phrasing the question in terms of heritability. Templeton points out that racial groups neither represent sub-species or distinct evolutionary lineages, and that therefore there is no basis for making claims about the general intelligence of races. He also finds that phrasing the question in terms of heritability not helpful because heritability "by definition is not applicable to between-population phenotypic differences" and is therefore "completely irrelevant to the question of genetic differentiation for any trait, including intelligence, among human populations." Templeton says that the only way to design a study of the genetic contribution to intelligence is to the correlation between degree of geographic ancestry and cognitive abilities. He states that this would require a Mendelian "common garden" design where specimens with different hybrid compositions are subjected to the same environmental influences, and that when this design has been carried out, it has shown no significant correlation between any cognitive and the degree of African or European ancestry.
Intelligence is a polygenic trait. This means that intelligence is under the influence of several genes, possibly several thousand. The effect of most individual genetic variants on intelligence is thought to be very small, well below 1% of the variance in g. Current studies using quantitative trait loci have yielded little success in the search for genes influencing intelligence. Robert Plomin is confident that QTLs responsible for the variation in IQ scores exist, but due to their small effect sizes, more powerful tools of analysis will be required to detect them. Others assert that no useful answers can be reasonably expected from such research before an understanding of the relation between DNA and human phenotypes emerges.
A 2005 literature review article on the links between race and intelligence in American Psychologist stated that no gene has been shown to be linked to intelligence, "so attempts to provide a compelling genetic link of race to intelligence are not feasible at this time". Several candidate genes have been proposed to have a relationship with intelligence. However, a review of candidate genes for intelligence published in Deary, Johnson & Houlihan (2009) failed to find evidence of an association between these genes and general intelligence, stating "there is still almost no replicated evidence concerning the individual genes, which have variants that contribute to intelligence differences". A 2012 review in American Psychologist concluded that "Almost no genetic polymorphisms have been discovered that are consistently associated with variation in IQ in the normal range".
Heritability is defined as the ratio of variation attributable to genetic differences in an observable trait to the trait's total observable variation. The heritability of a trait describes the proportion of variation in the trait that is attributable to genetic factors within a particular population. Heritability is not the proportion of a trait that is genetic and it does not describe what role, if any, genes play in producing the trait itself. A heritability of 1 indicates that variation correlates fully with genetic variation and a heritability of 0 indicates that there is no correlation between the trait and genes at all. There is broad agreement that individual variation in intelligence is neither fully genetic nor fully environmental, but there is little agreement on the relative contribution of genes and environment on individual intelligence.
It has been argued that intelligence is substantially heritable within populations, with 30–50% of variance in IQ scores in early childhood being attributable to genetic factors in analyzed US populations, increasing to 75–80% by late adolescence. High heritability does not imply that a trait is genetic or unchangeable, however, as environmental factors that affect all group members equally will not be measured by heritability (see the figure) and the heritability of a trait may also change over time in response to changes in the distribution of genes and environmental factors. High heritability also doesn't imply that all of the heritability is genetically determined, but can also be due to environmental differences that affect only a certain genetically defined group (indirect heritability).
Jensen and Rushton have argued that there may be environmental factors ("X factors") that are not measured by the heritability figure, but such factors must have the properties of not affecting whites while at the same time affecting all blacks equally, but, the hereditarians argue, no such plausible factors have been found and other statistical tests for the presence of such an influence in the United States are negative.
Dickens and Flynn argue that the conventional interpretation ignores the role of feedback between factors, such as those with a small initial IQ advantage, genetic or environmental, seeking out more stimulating environments which will gradually greatly increase their advantage, which, as one consequence in their alternative model, would mean that the "heritability" figure is only in part due to direct effects of genotype on IQ. The 2012 review by Nisbett et al. concluded that heritability of IQ varies between social classes with a higher heritability among populations with low SES. This has been taken as a suggestion that children who grow up in poverty do not get to develop their full genetic potential.
Spearman's hypothesis states that the magnitude of the black-white difference in tests of cognitive ability is entirely or mainly a function of the extent to which a test measures general mental ability, or g. The hypothesis, first formalized by Arthur Jensen in the 1980s based on Charles Spearman's earlier comments on the topic, argues that differences in g are the sole or major source of differences between blacks and whites observed in many studies of race and intelligence.
Studies on Spearman's hypothesis have often confirmed that racial IQ differences between Blacks and Whites are in line with Spearman's hypothesis.[non-primary source needed] Some have criticized these results. Hunt and Carlson argues Spearman's hypothesis is just one of several models that could explain the observed distributions in test scores.
Several studies have been done on the effect of similar rearing conditions on children from different races.
The Minnesota Transracial Adoption Study (1976) examined the IQ test scores of 122 adopted children and 143 nonadopted children reared by advantaged white families. The children were restudied ten years later. The study found higher IQ for whites compared to blacks, both at age 7 and age 17. Three other studies found opposing evidence with none finding higher intelligence in white children than in black children. However, unlike the Minnesota Transracial Adoption Study, these studies did not retest the children post-adolescence when heritability of IQ would be much higher. Moore (1986) compared black and mixed-race children adopted by either black or white middle-class families in the United States. Moore observed that 23 black and interracial children raised by white parents had a significantly higher mean score than 23 age-matched children raised by black parents (117 vs 104), and argued that differences in early socialization explained these differences. Eyferth (1961) studied the out-of-wedlock children of black and white soldiers stationed in Germany after World War 2 and then raised by white German mothers and found no significant differences. Tizard et al. (1972) studied black (African and West Indian), white, and mixed-race children raised in British long-stay residential nurseries. Three out of four tests found no significant differences. One test found higher scores for non-whites.
Studies on Korean infants adopted by European families have consistently shown a higher IQ than the European average. Frydman and Lynn (1989) showed a mean IQ of 119 for Korean infants adopted by Belgium families. After correcting for the Flynn effect, the IQ of the adopted Korean children was still 10 points higher than the indigenous Belgian children. Lynn and Rushton claim that a Stams et al. (2000) dataset shows a mean IQ of 115 for Korean infants adopted in the Netherlands. The higher IQ for Korean adoptees is in line with the higher IQ average of South Korea compared to Western nations.
Many people have an ancestry from different geographic regions. For example, African Americans typically have ancestors from both Africa and Europe, with, on average, 20% of their genome inherited from European ancestors. If racial IQ gaps have a partially genetic basis, blacks with a higher degree of European ancestry should on average have higher IQ, because the genes inherited from European ancestors would likely include some genes with a positive effect on IQ.
Witty and Jenkins (1936) compared a group of 63 black students all with an IQ of 125 and above and compared their white ancestry level to Herskovits (1930) national sample of blacks and concluded no significant difference in the level of white ancestry between their sample of gifted students and the national sample of Herskovits (1930).
This study has been criticized as the study's control sample of Herskovitz (1930) was not considered representative. 50 percent of the Herskovitz (1930) sample were composed of Howard University undergraduates and “well to do” professions with higher than average SES. Blacks in both the control sample of Herskovitz (1930) and the test sample by Witty and Jenkins (1936) had higher percentage of white ancestry (approximately 30%) than the national average.
The frequency of different blood types vary with ancestry. Correlations between degree of European blood types and IQ have varied between 0.05 and -0.38 in two studies from 1973 and 1977. Nisbett writes that one problem with these studies is that white blood genes are very weakly associated with one another in the black population, so they are not a reliable method of estimating ancestry. T. Edward Reed, an expert on blood groups, argues that the methodology used in these studies would have been unable to detect any difference, regardless of whether or not the hereditarian hypothesis is correct.
Some authors have suggested that new studies of the relationship ancestry and IQ should be performed using modern DNA-based ancestry estimations, which would provide a more reliable measure of ancestry than is possible based on skin tone or blood groups.
Mental chronometry measures the elapsed time between the presentation of a sensory stimulus and the subsequent behavioral response by the participant. This reaction time (RT) is considered a measure of the speed and efficiency with which the brain processes information. Scores on most types of RT tasks tend to correlate with scores on standard IQ tests as well as with g, and no relationship has been found between RT and any other psychometric factors independent of g. The strength of the correlation with IQ varies from one RT test to another, but Hans Eysenck gives 0.40 as a typical correlation under favorable conditions. According to Jensen individual differences in RT have a substantial genetic component, and heritability is higher for performance on tests that correlate more strongly with IQ. Nisbett argues that some studies have found correlations closer to 0.2, and that the correlation is not always found.
Several studies have found differences between races in average reaction times. These studies have generally found that reaction times among black, Asian and white children follow the same pattern as IQ scores. Rushton and Jensen have argued that reaction time is independent of culture and that the existence of race differences in average reaction time is evidence that the cause of racial IQ gaps is partially genetic instead of entirely cultural. Responding to this argument in Intelligence and How to Get It, Nisbett has pointed to the Jensen & Whang (1993) study in which a group of Chinese Americans had longer reaction times than a group of European Americans, despite having higher IQs. Nisbett also mentions findings in Flynn (1991) and Deary (2001) suggesting that movement time (the measure of how long it takes a person to move a finger after making the decision to do so) correlates with IQ just as strongly as reaction time, and that average movement time is faster for blacks than for whites.
Harry Jerison writes that there are real race differences in brain size, if races are defined sociologically rather than genetically, but it is difficult to know what to make of these differences. Within a race, sex, or age group, brain size has about a 0.4 correlation with IQ. He also writes that because of politics, it is almost impossible to discuss these differences in any scientific forum.
Earl Hunt states that because brain size is found to have a correlation of about .35 with intelligence among whites and is almost entirely genetically determined, race differences in average brain size are therefore an important argument for a possible genetic contribution to racial IQ gaps. However as brain images are expensive to obtain, much of the research in this area is based on external measures of head size, which only measures brain size indirectly and thus makes the data less reliable. Hunt notes that Rushton's head size data would account for a difference of .09 standard deviations between Black and White average test scores, only a portion of the 1.0 standard deviation gap in average scores that is observed.
Hunt and Carlson write that whether there is a relationship between race, genetics, brain size, and IQ is at present unknown. However, they say that it is an area both feasible and reasonable to study. Summarising Rushton's hypothesis that the black-white IQ gap can be explained by larger average brain size among whites, they say that because this argument presents a biological mechanism known to influence intelligence that could potentially help explain racial IQ gaps, discussing ideas like this is more likely to be informative than arguing about heritability statistics.
Jensen and Rushton argued that the existence of biological group differences does not rule out, but raises questions about the worthiness of policies such as affirmative action or placing a premium on diversity. They also argued for the importance of teaching people not to overgeneralize or stereotype individuals based on average group differences, because of the significant overlap of people with varying intelligence between different races.
The environmentalist viewpoint argues for increased interventions in order to close the gaps. Nisbett argues that schools can be greatly improved and that many interventions at every age level are possible. Flynn, arguing for the importance of the black subculture, writes that "America will have to address all the aspects of black experience that are disadvantageous, beginning with the regeneration of inner city neighbourhoods and their schools. A resident police office and teacher in every apartment block would be a good start." Researchers from both sides agree that interventions should be better researched.
Especially in developing nations, society has been urged to take on the prevention of cognitive impairment in children as of the highest priority. Possible preventable causes include malnutrition, infectious diseases such as meningitis, parasites, cerebral malaria, in utero drug and alcohol exposure, newborn asphyxia, low birth weight, head injuries, lead poisoning and endocrine disorders.