Race and intelligence
|Genetics and differences|
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.
The first test showing differences in IQ test results between different population groups in the US was the tests of United States Army recruits in World War I. In the 1920s groups of eugenics lobbyists argued that this demonstrated that these groups were of inferior intellect to Anglo-Saxon whites due to innate biological differences, using this as an argument for policies of 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. The debate reemerged again in 1969, when Arthur Jensen championed the view that for genetic reasons Africans were less intelligent than whites and that compensatory education for African-American children was therefore doomed to be ineffective. In 1994, the book The Bell Curve, 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, and 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.
- 1 History of the debate
- 2 Ethics of research
- 3 Validity of race and IQ
- 4 Group differences
- 5 Potential environmental causes
- 6 Genetic arguments
- 7 Policy relevance
- 8 See also
- 9 References
- 10 Bibliography
- 11 External links
History of the debate
Claims of races having different intelligence were used to justify colonialism, slavery, racism, social Darwinism, and racial eugenics. Racial thinkers such as Arthur de Gobineau relied crucially on the assumption that black people were innately inferior to Whites in developing their ideologies of White supremacy. Even enlightenment thinkers such as Thomas Jefferson, a slave owner, believed Blacks to be innately inferior to Whites in physique and intellect.
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, That Americans from northern states had higher scores than Americans from southern states, and that Black Americans scored lower than white Americans. The results were widely publicized by a lobby of anti-immigration activists, including the New York patrician and conservationist Madison Grant, who considered the nordic race to be superior, but under threat of immigration by inferior breeds. In his influential work A Study of American Intelligence psychologist Carl Brigham used the results of the Army tests to argue for a stricter immigration policy, limiting immigration to countries considered to belong to the "nordic race".
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, many United States psychologists adopted the view that environmental and cultural factors played a dominant role in IQ test results, among them Carl Brigham who repudiated his own previous arguments, on the grounds that he realized that the tests were not a measure of innate intelligence. Discussion of the issue in the United States also influenced German Nazi claims of the "nordics" being a "master race", influenced by Grant's writings. As the American public sentiment shifted against the Germans, claims of racial differences in intelligence increasingly came to be regarded as problematic. Anthropologists such as Franz Boas, and Ruth Benedict and Gene Weltfish, did much to demonstrate the unscientific status of many of the claims about racial hierarchies of intelligence. Nonetheless a powerful eugenics and segregation lobby funded largely by textile-magnate Wickliffe Draper, continued to publicize studies using intelligence studies as an argument for eugenics, segregation, anti-immigration legislation.
As the de-segregation of the American South was begun in the 1950s the debate about Black intelligence resurfaced. Audrey Shuey, funded by Draper's Pioneer fund, published a new analysis of Yerkes' tests, concluding that blacks really were of inferior intellect to whites. This study was used by segregationists as an argument that it was to the advantage of Black children to be educated separately from the superior White children. In the 1960s, the debate was further revived as Nobel laureate William Shockley, publicly defended the argument that Black children were innately unable to learn as well as White children. 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.
Ethics of research
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.
Validity of race and IQ
The concept of intelligence and the degree to which intelligence is measurable is a matter of debate. While there is some consensus 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)|
The study of human intelligence is one of the most controversial topics in psychology. It remains unclear whether group differences in intelligence test scores are caused by heritable factors or by "other correlated demographic variables such as socioeconomic status, education level, and motivation."  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.
United States test scores
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]
North East Asians score relatively higher on visuospatial subtests with lower scores in verbal subtests while Ashkenazi jews score higher in verbal and reasoning subtests with lower scores in visuospatial 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. These data are invoked by psychologists to argue that intelligence among racial groups is flexible and can be heightened or depressed by environmental factors.
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.
Closing of the gap
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. Some studies reviewed by Hunt (2010:418) found that rise in the average achievement of African Americans was caused by a reduction in the number of African American students in the lowest range of scores without a corresponding increase in the number of students in the highest ranges. A 2012 review of the literature found that the IQ gap had diminished by 0.33 standard deviation since first reported.
Potential environmental causes
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 and minority status
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. Psychometrician Nicholas Mackintosh considers that there is little doubt that the effects of stereotype threat contribute to the IQ gap between blacks and whites.
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".
Different aspects of the Socioeconomic environment in which children are raised have been shown to correlate with part of the IQ gap, but they do not account for the entire gap. 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. Hunt (2010:428) points out that when controlling for both SES and parental IQ in populations of young children, the gap becomes so small as to be statistically unreliable, and the best predictors of IQ then becomes parental occupation status, mother's verbal comprehension score and nature of parental interaction with the child. Hunt also finds that the correlation between home environment and IQ becomes weaker with age.
Adoption studies have shown that children adopted from lower-class homes to middle-class homes experience a 12 - 18 pt gain in IQ.
Health and nutrition
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. Mackintosh points out that for American Blacks infant mortality is about twice as high as for for whites, and low birthweight is twice as prevalent. At the same time white mothers are twice as likely to breastfeed their infants, and breastfeeding is highlycorrelated with IQ for low birthweight infants. In this way a wide number of health related factors that influence IQ are unequally distributed between the two groups.
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.
Some recent research has argued that argue that the retardation caused in brain development by infectious diseases, many of which are more prevalent in non-White populations, may be an important factor in explaining the differences in IQ between different regions of the world. The findings of this research, showing the correlation between IQ, race and infectious diseases was also shown to apply to the IQ gap in the US, suggesting that this may be an important environmental factor .
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 series of studies by Fagan and Holland, measured the effect of prior exposure to the kind of cognitive tasks posed in IQ tests on test performance. Assuming that the IQ gap was the result of lower exposure to tasks using the cognitive functions usually found in IQ tests among African American test takes, they prepared a group of African Americans in this type of tasks before taking an IQ test. The researchers found that there was no subsequent difference in performance between the African-Americans and White test takers. 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". A similar argument is made by David Marks who argues that IQ differences correlate well with differences in literacy suggesting that developing literacy skills through education causes an increase in IQ test performance.
The American Anthropological Association stated in 1994 that intelligence is not biologically determined by race. In the same year, the American Psychological Association 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. A 2012 review article published by the American Psychological Association commented on recent research, and a reply to a response to that article concludes that "Group differences in IQ are best understood as environmental in origin."
Genetics of race and intelligence
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 nor 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.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 2005 literature review article by Sternberg, Grigorenko and Kidd stated that no gene has been shown to be linked to intelligence, "so attempts to provide a compelling genetic link of race to intelligence are not feasible at this time". Hunt (2010:447) and Mackintosh (2011:344) concurred, both scholars noting that while several environmental factors have been shown to influence the IQ gap, the evidence for a genetic influence has been circumstantial, and according to Mackintosh negligible. Mackintosh however suggests that it may never become possible to account satisfyingly for the relative contributions of genetic and environmental factors. The 2012 review by the Nisbett, Aronson, Blair, Dickens, Flynn, Halpern and Turkheimer concluded that "Almost no genetic polymorphisms have been discovered that are consistently associated with variation in IQ in the normal range". Hunt and and several other researchers however maintain that genetic causes cannot be ruled out and that new evidence may yet show a genetic contribution to the gap. Hunt concurs with Rushton and Jensen who considered the 100% environmental hypothesis to be impossible. Nonetheless, Nisbett and colleagues (2012) consider the entire IQ gap to be explainabed by the environmental factors that have thus far been demonstrated to influence it, and Mackintosh does not find this view to be unreasonable.
Heritability within and between groups
Intelligence as tested by IQ tests is generally considered to be highly heritable. Psychometricians have found that intelligence is substantially heritable within populations, with 30–50% of variance in IQ scores in early childhood being attributable to genetic factors in analyzed US populations, increasing to 75–80% by late adolescence. In biology heritability is defined as the ratio of variation attributable to genetic differences in an observable trait to the trait's total observable variation. The heritability of a trait describes the proportion of variation in the trait that is attributable to genetic factors within a particular population. A heritability of 1 indicates that variation correlates fully with genetic variation and a heritability of 0 indicates that there is no correlation between the trait and genes at all. In psychological testing heritability tends to be understood as the degree of correlation between the results of a test taker and those of their biological parents. However, since high heritability is simply a correlation between traits and genes, it does not describe the causes of heritability which in humans can be either genetic or environmental.
Therefore, a high heritability measure does not imply that a trait is genetic or unchangeable, however, as environmental factors that affect all group members equally will not be measured by heritability and the heritability of a trait may also change over time in response to changes in the distribution of 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). The figure to the left demonstrates how heritability works. In both gardens the difference between tall and short cornstalks is 100% heritable as cornstalks that are genetically disposed for growing tall will become taller than those without this disposition, but the difference in height between the cornstalks to the left and those on the right is 100% environmental as it is due to different nutrients being supplied to the two gardens. Hence the causes of differences within a group and between groups may not be the same, even when looking at traits that are highly heritable.
In regards to the IQ gap the question becomes whether racial groups can be shown to be influenced by different environmental factors that may account for the observed differences between them. Jensen originally argued that given the high heritability of IQ the only way that the IQ gap could be explained as caused by the environment would be if it could be shown that all blacks were subject to a single "x-factor" which affected no white populations while affecting all black populations equally. Jensen considered the existence of such an x-factor to be extremely improbable, but Flynn's discovery of the Flynn effect showed that inspite of high heritability environmental factors could cause considerable disparities in IQ between generations of the same population, showing that the existence of such an x-factor was not only possible but real. Today researchers such as Hunt (2010), Nisbett, Flynn et al. (2012) and Mackintosh (2011) consider that rather than a single factor accounting for the entire gap, probably many different environmental factors differ systematically between the environments of White and Black people converge to create part of the gap and perhaps all of it.
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, Flynn et al. concluded that heritability of IQ varies between social classes, with a lower heritability among populations with low SES. This they interpret to mean 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 was first formalized by Arthur Jensen in the 1980s based on Charles Spearman's earlier comments on the topic. Jensen holds that if Spearman's hypothesis holds true then some cognitive tasks have a higher g-load than others, and that these tasks are exactly the tasks in which the gap between Black and White test takers are greatest. This he and other psychometricians such as Rushton and Lynn takes to show that the cause of G and the cause of the gap are the same - in his view genetic differences. Hunt (2011:415) states that many of the conclusions of Jensen and his colleagues rest on the validity of Spearman's hypothesis, and the method of correlated vectors used to test it. He points out that other researchers have found this method of calculation to produce false positive results, and that other statistical methods should be used instead. According to Hunt, Jensen and Rushton's frequent claim that Spearman's hypothesis should be regarded as empirical fact does not hold, and that rather new studies based on better statistical methods would be required to confirm or reject the hypothesis that there is a correlation between g, heritability and g-loading of test items and the IQ gap.
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 Belgian 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.
Reviewing the evidence from adoption studies Mackintosh considers the studies by Tizard and Eyferth to be inconclusive, and the Minnesota study to to be consistent only with a partial genetic hypothesis. On the whole he finds that environmental and genetic variables remain confounded and considers evidence from adoption studies inconclusive on the whole, and fully compatible with a 100% environmental explanation.
Racial admixture studies
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, one might expect blacks with a higher degree of European ancestry to score higher on IQ tests than blacks with less European ancestry, because the genes inherited from European ancestors would likely include some genes with a positive effect on IQ. Reviewing the evidence from admixture studies Hunt (2010) considers it to be inconclusive because of too many uncontrolled variables, Mackintosh (2011:338) quotes a statement by Nisbett to the effect that admixture studies have not provided a shred of evidence in favor of a genetic basis for the gap.
Mental chronometry measures the elapsed time between the presentation of a sensory stimulus and the subsequent behavioral response by the participant. This reaction time (RT) is considered a measure of the speed and efficiency with which the brain processes information. 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. Mackintosh considers reaction time evidence unconvincing and points out that other cognitive tests that also correlate well with IQ show no disparity at all, for example the habituation/dishabituation test. And he points out that studies show that rhesus monkeys have shorter reaction times than American college students, suggesting that different reaction times may not tell us anything useful about intelligence.
A number of studies have reported that there is a statistical correlation between differences in IQ and brain size between individuals in the same group. And some scholars have reported differences in average brain sizes between Africans, Europeans and Asians. A prominent proponent of the argument that there is a causal relation between brain size, IQ and race is J. P. Rushton who has argued that Africans on average have smaller brain cases and brains than Europeans, and that this is evidence that the gap is biological in nature. Some scholars have argued that most of Rushton's arguments rest on outdated data collected by unsound methods and should be considered invalid. However newer studies seems to have replicated Rushton's findings and recent reviews by Nesbitt, Flynn et al. (2012) and Mackintosh (2011) consider that current data does show an average difference in brain size and head-circumference between American Blacks and Whites. Nesbitt et al. point out that crude brain size is unlikely to be a good measure of IQ, for example brain size also differs between men and women, but without documented differences in IQ. At the same time newborn Black children have the same average brain size as Whites, suggesting that the difference in average size could be accounted for by differences in postnatal environment. Several factors that reduce brain size have been demonstrated to disproportionately affect Black children.
Earl Hunt states that because brain size is found to have a correlation of about .35 with intelligence among whites and cites studies showing that genes may account for as much as 90% of individual variation in brain size. According to Hunt, race differences in average brain size could therefore be an important argument for a possible genetic contribution to racial IQ gaps. Nonetheless, Hunt notes that Rushton's head size data would account for a difference of .09 standard deviations between Black and White average test scores, less than a tenth of the 1.0 standard deviation gap in average scores that is observed.
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.
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