Study of the heritability of IQ is a controversial field of research that includes biology, psychology, philosophy, sociology, and anthropology. Heritability is a measure of the relative contribution of genotype to the variation of a phenotype on a given group in a specific environment.^[1] The debate about IQ heritability touches on the nature versus nurture divide,^[2] and there has been no consensus in the academic community about it even since research began in the 19th century.^[3] Most of the heritable variance in IQ appears to be carried by the general intelligence factor (or g). IQ is a polygenic trait under normal circumstances according to recent research.^[4] However, destructive mutation of individual genes associated with development can severely affect intelligence, with Phenylketonuria as an example.^[5]

Throughout the developed world, specific estimates in the academic research into the heritability of IQ have varied from relatively high, such as over 0.9 in a 1994 report,^[6] to relatively low, such as below 0.5 in a 1997 report.^[3] A general range of 0.4 to 0.8 was given by the "Mainstream Science on Intelligence", a 1994 declaration of 52 scientists in the field.^[7] A 1996 statement by the American Psychological Association gave about .45 for children and about .75 during and after adolescence.^[8] A 2004 meta-analysis of reports in Current Directions in Psychological Science gave an overall estimate of around three quarters.^[9] The New York Times Magazine has also listed about three quarters as a figure held by the majority of studies.^[10]

[edit] Methods and results

[edit] Heritability calculations

[edit] Background

Heritability is defined as the proportion of variance in a trait which is attributable to genotype within a defined population in a specific environment.^[1] Heritability takes a value ranging from 0 to 1; a heritability of 1 indicates that all variation in the trait in question is genetic in origin and a heritability of 0 indicates that none of the variation is genetic. The heritability of many traits can be considered primarily genetic under similar environmental backgrounds. For example, Visscher et al. (2006) found that adult height has a heritability estimated at 0.80, when a similar environmental background is present, to control for environment the study only looked at the contribution of heritability to variation within families. The paper stated that "one can never be sure that the estimates are correct, because nature and nurture can be confounded without one knowing it. The authors got around this problem by comparing the similarity between relatives as a function of the exact proportion of genes that they have in common, looking only within families."^[2] Other traits have low heritabilities, which indicate a large relative environmental influence. For example, a twin study on the heritability of depression in men calculated it as 0.29, while it was 0.42 for women in the same study.^[11]

Heritability for a trait is calculated by measuring how strongly traits covary in people of a given genetic and environmental similarity. The most common method is to consider identical twins reared apart, with any similarities which exists between such twin pairs attributed to genotype. In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share on average half of their alleles and the correlation of their scores would be 0.50 if IQ were affected by genes alone. Practically, however, the upper bound of these correlations are given by the reliability of the test, which tends to be 0.90 to 0.95 for typical IQ tests^[12] Thus, the actual heritability of IQ will tend to be slightly higher than attained by estimates derived from studies of monozygotic twins, though this effect is small.

In the case of the inheritance of IQ or a certain degree of giftedness, the relatives of probands with a high IQ exhibit a comparably high IQ with a much higher probability than the general population. In 1982, Bouchard and McGue reviewed such correlations reported in 111 original studies in the United States.^[13] The mean correlation of IQ scores between monozygotic twins was 0.86, between siblings, 0.47, between half-siblings, 0.31, and between cousins, 0.15. From such data the heritability of IQ was estimated at anywhere between 0.40 and 0.80 in the United States. The reason for this wide margin appeared to be that the heritability of IQ rises through childhood and adolescence, peaking at 0.68 and 0.78 in adults, leaving the overwhelming majority of IQ differences between individuals to be explained genetically.^[14]

The finding of rising heritability with age is counter-intuitive; it is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. However, that the opposite occurs is well documented. According to work by Robert Plomin,heritability estimates calculated on infant samples are as low as 20%, rising to around 40% in middle childhood, and ultimately as high as 80% in adult samples in the United States.^[15] This suggests that the underlying genes actually express themselves by affecting a person's predisposition to build, learn, and develop mental abilities throughout the lifespan.^{[citation needed]}

[edit] Estimates and caveats to them

In 2006, The New York Times Magazine listed about three quarters as a figure held by the majority of studies.^[10] A 2004 meta-analysis of reports in Current Directions in Psychological Science gave an estimate of around three quarters as well.^[9] As well, a 1996 statement by the American Psychological Association gave about .45 for children and about .75 during and after adolescence.^[8] The 2005 edition of Assessing adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger found correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings.^[16] A 1994 review in Behavior Genetics based on identical/fraternal twin studies found that it is as high as 0.92 in general cognitive ability but it also varies based on the trait, with .60 for verbal tests, .50 for spatial and speed-of-processing tests, and only .40 for memory tests.^[6]

There are a number of points to consider when interpreting heritability:

• A high heritability does not mean that the environment has no effect on the development of a trait, or that learning is not involved. Vocabulary size, for example, is very substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions.^[8]
• A common error is to assume that because something is heritable it is necessarily unchangeable. This is wrong. Heritability does not imply immutability. As previously noted, heritable traits can depend on learning, and they may be subject to other environmental effects as well. The value of heritability can change if the distribution of environments (or genes) in the population is substantially altered. For example, an impoverished or suppressive environment could fail to support the development of a trait, and hence restrict individual variation. This could affect estimates of heritability.^[8] Another example is Phenylketonuria which previously caused mental retardation for everyone who had this genetic disorder. Today, this can be prevented by following a modified diet.
• On the other hand, there can be effective environmental changes that do not change heritability at all. If the environment relevant to a given trait improves in a way that affects all members of the population equally, the mean value of the trait will rise without any change in its heritability (because the differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase.^[8]
• Even in developed nations, high heritability of a trait within a given group has no necessary implications for the source of a difference between groups.^[8][17]
• Heritability increases with education and with social class, such that the children of highly educated parents are more likely to inherent traits.^[16][10] Most of the studies done about heritability have used middle class families- not because of any intentional choice made by researchers but because, on average, lower income people are less likely to volunteer to be studied in the first place.^[10]
• Even among close groups such as families, different individuals- such as siblings- will still experience different environments, which matter in determining intelligence scores.^[7]

[edit] Test score differences

Intelligence tests measure many important abilities, such as verbal and quantitative reasoning, and can predict socially-relevant outcomes such as academic performance and occupational outcomes. However, intelligence test scores do not reflect all of the intricacies of the everyday meaning of intelligence, so researchers take care to distinguish between IQ test results and intelligence.^[18][19]

Some studies of intelligence tests use statistical methods to extract so-called latent variables from the IQ test scores. One such variable is the general intelligence factor, or g, which accounts for most of the differences in IQ test scores between individuals. There are other latent variables in addition to g, and IQ tests vary in their ability to measure these latent variables, if they measure them at all. IQ tests scores, while often summarized as a single overall number, are actually multidimensional in nature. Transforming IQ test scores into latent variables is an attempt to find one or dimensions on which to compare IQ test scores.

Latent variables are also sometimes called factors or constructs. The construct validity of an IQ test score is a key criteria for judging whether the IQ test score differences are meaningful. Tests which do not measure difference in latent variables for some group are said to have measurement bias. The construct validity of most commonly used IQ tests has been fairly well established within multiple racial-ethnic groups in developing countries such as the United States. That is, test score difference within each racial-ethnic group are valid indicators of differences in latent variables such as g. A related question is whether test-score differences between groups are valid. There is a consensus that test score differences between Black and White people in the United States have predictive validity (also called predictive invariance), meaning that test scores predict the same socially-relevant outcomes regardless of the race of the person being tested. To further address this question, three studies using sophisticated statistical techniques have shown that Black-White differences in IQ test scores are not a result of measurement bias (a criteria called measurement invariance).^[20][21][22] These studies imply that Black-White IQ differences reflect very general differences in some underlying latent variables, but they are unable to differentiate precisely which latent variables differ under a variety of models. These studies were performed in response to previous investigations which suggested that Black-White IQ differences are primarily differences in g in particular.^[23]

[edit] United States

There are observed differences in average test score achievement between racial-ethnic groups, which vary depending on the populations studied and the type of tests used. Self defined black and white United States citizens have been the subjects of the greatest number of studies. Black-White average IQ differences appear to increase with age, reaching an average of nearly 17 points by age 24, which is slightly more than 1 standard deviation.^[24] According to James Flynn and others, the overall average Black-White gap has reduced by one third over the course of the 20th century.^[24] For example, the black men inducted into the US armed forces during World War II averaged about 1.5 standard deviations below their white counterparts.^[25] This improvement is also reflected in Black-White differences on school achievement tests, which have shrunk from about 1.2 to about 0.8 standard deviations. However, these improvements may have stalled for people born after the early 1970s.^[26]

The average black-white IQ difference also varies depending on test content. For example, two subsections of the WISC IQ test, known as forward and reverse digit-span, ask children to repeat a long series of numbers either forwards or backwards. The black-white difference on forward digit span is relatively small, while the difference on reverse digit span is relatively large. Across a battery of tests, the size of the Black-White gap is correlated with the extent to which the tests measure the psychometric factor g, which also accounts for most of the variation in interindividual differences in IQ test performance.^[27] Gaps are seen in other tests of cognitive ability or aptitude, including university admission exams such as the SAT and GRE as well as employment tests for corporate settings and the military.^[28]

The IQ distributions of other racial and ethnic groups in the United States are less well studied. Hispanic and Native American populations, including Arctic Natives,^[29][30] tend to score worse on average than White populations but better on average than Black populations.^[31] East Asian populations may score higher on average than White populations in the United States as they do elsewhere.^[32] A 1960 study of 1236 American teenagers calculated six IQ measures for Jews relative to white gentiles. The results found that the relative IQ of American Jews varied from a low of 91.3 (visual reasoning) to a high of 109.7 (Mathematics).^[33][34] A recent review by Lynn (2004) used a 10 word vocabulary test to estimate the IQ of American Jews. The population of 150 Jews scored half a standard deviation above the 5300 white gentiles in verbal IQ.^[34][35]

For each of these populations, there is some evidence that the mixture of ability factors that distinguish individuals are differentially distributed between groups. For example, East Asian populations tend to outscore White populations in performance IQ, whereas the test score differences skew towards higher verbal IQ for Ashkenazi-White differences. However, the mixture of abilities within groups appears to be nearly identical across many ethnic groups.^[36] The stability of these differences is also less well studied than Black-White differences.

[edit] Worldwide

According to Richard Lynn, J. Philippe Rushton, and others, IQ test score differences are observed cross-culturally and around the world. Lynn has published three books summarizing IQ test scores from around the world.^[37] The inaccuracy of the cross cultural IQ scores is well documented, but many scholars use the results as an estimate of worldwide IQ scores.^{[38][39][40][41]} Lynn's meta-analysis lists East Asians (105), Europeans (99), Inuit (91), Southeast Asians and Amerindians (87 each), Pacific Islanders (85), South Asians/North Africans (84), Non-Bushmen sub-Saharan Africans (67), Australian Aborigines (62) and Bushmen (54).^[42][43][7] International achievement test scores, including TIMSS and PISA, have also been used to estimate average IQ worldwide with similar results where data is available.^{[41][44][45][46]}

The very low IQ scores reported for sub-Saharan African populations are especially controversial. For example, Wicherts argues that the average IQ of sub-Saharan Africans is poorly measured and is more likely 78.^[47] According to anthropologist Mark Cohen, the frequently reported African mean IQ of 70 is "preposterous". Using Western standards, this would mean that African countries evidencing such a low IQ would be largely dysfunctional. Given that individuals in these countries lead "vibrant artistic, symbolic and spiritual lives", this is, according to Cohen, clearly not the case. Thus, he concludes, the IQ test results from Africa do not reflect actual intelligence levels.^[48]

Differences in education, prolonged malnutrition, exposure to toxin, exposure to stress, and exposure to disease are all generally expected to contribute to the lower scores observed in developing countries. However, direct experimental evidence to confirm the role of individual factors is difficult to acquire in most cases because each of these factors tends to also be associated with one another and with unfavorable socioeconomic conditions.^[49] In the case of some toxins, such as lead, a negative effect on IQ scores has been established. Two other factors that have as well established negative association with IQ are severely premature birth and severe low birth weight.^[49]

[edit] Developing nations

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See also: Health and intelligence

Almost all studies on heritability have been in the developed world, mostly in the United States.^{[citation needed]} In developing nations there are many environmental factors affecting IQ which are much less important in developed nations.^{[citation needed]} Examples include nutrition, diseases, environmental toxins, and health care.^{[citation needed]} This likely affects heritability.^{[citation needed]}

[edit] Issues in the calculations

[edit] Family environment

In the developed world, nearly all personality traits show that, contrary to expectations, environmental effects actually cause non-related children raised in the same family ("adoptive siblings") to be as different as children raised in different families (Harris, 1998; Plomin & Daniels, 1987). There are some family effects on the IQ of children, accounting for up to a quarter of the variance. However, by adulthood, this correlation disappears, such that adoptive siblings are not more similar in IQ than strangers,^[50] while adult full siblings show an IQ correlation of 0.6. Twin studies reinforce this pattern: monozygotic (identical) twins raised separately are highly similar in IQ (0.86), more so than dizygotic (fraternal) twins raised together (0.6) and much more than adoptive siblings (~0.0).^[51]

The American Psychological Association's report Intelligence: Knowns and Unknowns (1995) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. Do differences between children's family environments (within the normal range) produce differences in their intelligence test performance? The problem here is to disentangle causation from correlation.

There is no doubt that such variables as resources of the home and parents' use of language are correlated with children's IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors. But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the family environment is substantial in early childhood, it becomes quite small by late adolescence. These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children's lives make little long-term difference for the skills measured by intelligence tests. It also stated

"We should note, however, that low-income and non-white families are poorly represented in existing adoption studies as well as in most twin samples. Thus it is not yet clear whether these studies apply to the population as a whole. It remains possible that, across the full range of income and ethnicity, between-family differences have more lasting consequences for psychometric intelligence."^[52]

A study of French children adopted between the ages of 4 and 6 shows the continuing interplay of nature and nurture. The children came from poor backgrounds with IQs that initially averaged 77, putting them near retardation. Nine years later after adoption, they retook the IQ tests, and all of them did better. The amount they improved was directly related to the adopting family’s status. "Children adopted by farmers and laborers had average IQ scores of 85.5; those placed with middle-class families had average scores of 92. The average IQ scores of youngsters placed in well-to-do homes climbed more than 20 points, to 98."^[10]

[edit] Biased older studies?

Stoolmiller (1999) found that the range restriction of family environments that goes with adoption, that adopting families tend to be more similar on for example SES than the general population, means that role of the shared family environment have been underestimated in previous studies. Corrections for range correction applied to adoption studies indicate that SE could account for as much as 50% of the variance in IQ.^[53] However, the effect of restriction of range on IQ for adoption studies was examined by Matt McGue and colleagues, who write that "restriction in range in parent disinhibitory psychopathology and family SES had no effect on adoptive-sibling correlations [in] IQ".^[54]

Eric Turkheimer and colleagues (2003), not using an adoption study, included impoverished US families. Results demonstrated that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared family environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.^[55] They suggest that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.^[56]

When comparing late 1970s to pre-1963 recorded data, researches DeFries and Plomin found that IQ correlation between parent and child living together fell significantly, from 0.50 to 0.35.^[16]

[edit] Maternal (fetal) environment

A meta-analysis, by Devlin and colleagues in Nature (1997), of 212 previous studies evaluated an alternative model for environmental influence and found that it fits the data better than the 'family-environments' model commonly used. The shared maternal (foetal) environment effects, often assumed to be negligible, account for 20% of covariance between twins and 5% between siblings, and the effects of genes are correspondingly reduced, with two measures of heritability being less than 50%. They argue that the shared maternal environment may explain the striking correlation between the IQs of twins, especially those of adult twins that were reared apart.^[3]

Bouchard and McGue reviewed the literature in 2003, arguing that Devlin's conclusions about the magnitude of heritability is not substantially different than previous reports and that their conclusions regarding prenatal effects stands in contradiction to many previous reports.^[57] They write that:

Chipuer et al. and Loehlin conclude that the postnatal rather than the prenatal environment is most important. The Devlin et al. (1997a) conclusion that the prenatal environment contributes to twin IQ similarity is especially remarkable given the existence of an extensive empirical literature on prenatal effects. Price (1950), in a comprehensive review published over 50 years ago, argued that almost all MZ twin prenatal effects produced differences rather than similarities. As of 1950 the literature on the topic was so large that the entire bibliography was not published. It was finally published in 1978 with an additional 260 references. At that time Price reiterated his earlier conclusion (Price, 1978). Research subsequent to the 1978 review largely reinforces Price’s hypothesis (Bryan, 1993; Macdonald et al., 1993; Hall and Lopez-Rangel, 1996; see also Martin et al., 1997, box 2; Machin, 1996).^[57]

[edit] The Dickens and Flynn model

Dickens and Flynn (2001) argue that the arguments regarding the disappearance of the shared family environment should apply equally well to groups separated in time. This is contradicted by the Flynn effect. Changes here have happened too quickly to be explained by genetics. This paradox can be explained by observing that the measure "heritability" includes both a direct effect of the genotype on IQ and also indirect effects where the genotype changes the environment, in turn effecting IQ. That is, those with a higher IQ tend to seek out stimulating environments that further increase IQ. The direct effect can initially have been very small but feedback loops can create large differences in IQ. In their model an environmental stimulus can have a very large effect on IQ, even in adults, but this effect also decays over time unless the stimulus continues (the model could be adapted to include possible factors, like nutrition in early childhood, that may cause permanent effects). The Flynn effect can be explained by a generally more stimulating environment for all people. The authors suggest that programs aiming to increase IQ would be most likely to produce long-term IQ gains if they taught children how to replicate outside the program the kinds of cognitively demanding experiences that produce IQ gains while they are in the program and motivate them to persist in that replication long after they have left the program.^[58][59]

[edit] Regression towards the mean

Any large enough group of IQ scores can be modeled using the Gaussian distribution, with a mean score of 100.

The heritability of IQ measures the extent to which the IQ of a child is measurably influenced by the IQ of its parents. As IQ is a quantifiable phenotype, one can estimate the expected IQ of child using the equation , where

• is the expected IQ of the child,
• is the mean IQ of the population to which the parents belong,
• h² is the heritability of IQ,
• m and f are the IQs of the mother and father, respectively.^[60]

The equation asserts that, on average, the IQ of a child tends to the mean IQ of the population. For instance, if the heritability of IQ is 50% and the mean IQ of a population is 100, then a couple with an average IQ of 120 will, on average, have a child with an IQ of 110. Similarly, a couple with an average IQ of 80 will, on average, have a child with an IQ of 90.

It is noted that the above equation relates only statistical averages and is not deterministic. Furthermore, the equation is a general equation based in the inheritance of genetically-based characteristics (in this case, phenotypes), and so it is implicitly assumed that environmental factors are, for the sake of correctly assessing the genetic contribution to IQ, the same across the population.

Operating under the assumption that child and parent are raised in the exact same environment (unlikely, but usually closer to the truth than in the completely dissimilar environment that the previous equation assumes), h² can be replaced by h, which is simply the correlation between parent and offspring IQ. In this case, regression towards the mean is no longer partially caused by environmental differences and therefore only by random genetic variation.

Finally, it is important to note that the expected IQ of the offspring is normally distributed around the mean calculated using the above equation, so in many cases regression towards the mean does not actually occur; as the values are normally distributed, there is a chance that offspring IQ will be more deviant from the mean than that of the parental average.

[edit] Historical research

As early as 1869, Francis Galton replaced mere speculations by statistical data through his book, Hereditary Genius:

Highly Gifted Men and the Percentage of their Highly Gifted Male Relatives

(classified by occupation and achievement ^{[disambiguation needed]})

Despite the differences in methods and societies, there is a notable parallelism in the published statistics. The ITO-method by Li and Sacks (1954) allows from this set of data the estimation of the underlying number of genes and their allele frequencies.

[edit] The inheritance of cognitive deficits

There are many genetic variants known to cause lower IQ. The number of such mutations already known is in the hundreds. For example, an allele of the gene GDI1 is associated with an IQ below 70.^{[citation needed]}

Copy number variation has also been associated with idiopathic learning disability. ^[61]

There are number of known cases where the homozygotes have severe cognitive deficits and the heterozygotes show a small decrease of IQ. In such cases further alleles are investigated to estimate their influence on IQ. For example, one minor allele of the gene ALDH5A1 is associated with an IQ difference of around 1.5 points.^[62]

Interindividual (between individuals) differences in learning ability are also known in mice, dogs and other animals, and the achievements of pure strains can be improved by selective breeding.^{[clarification needed]} In such a way also behaviour genetics is contributing to our knowledge of the inheritance of mental traits. There is an open question to which degree differences of animal behaviour have any meaning for differences in human intelligence.^{[clarification needed]}

[edit] The search for specific genes

Unfortunately, most of the research done about the heritability of intelligence have focused on children and young adults. Thus, the role of genetic factors to intelligence, which most likely increases over time, is mostly unknown.^[6] Many studies attempting to find loci in the genome relating to IQ have had little success. For example, a study by Robert Plomin using groups of around 100 people investigated 1,842 DNA markers in a high-IQ group and in an average-IQ control group. The study used a five-step replication process to eliminate false positives, and no gene met this rigid criterion for replicability^[63]

The failure to find a specific gene associated with IQ indicates that cognitive abilities are very complex and are likely to involve several genes (polygenic). Some estimate that as much as 40% of all genes may contribute to IQ.^[64] The more genes that contribute to a trait the more the trait will be continuous instead of discrete. A 2008 study of 500,000 single nucleotide polymorphisms (SNPs) from 7,089 children did not substantially improve on earlier studies. The study did not find any any SNPs that accounted for more than 0.5% of the variance in general intelligence.^[65]

A recent study did find that a gene called FADS2 along with breastfeeding adds about 7 IQ points to those with the "C" version of the gene. Those with the "G" version see no advantage.^[66][67]

There is "a highly significant association" between the CHRM2 gene and intelligence according to a 2006 Dutch family study. The study concluded that there was an association between the CHRM2 gene on chromosome 7 and Performance IQ, as measured by the Wechsler Adult Intelligence Scale-Revised. The Dutch family study used a sample of 667 individuals from 304 families.^[68] A similar association was found independently in the Minnesota Twin and Family Study (Comings et al. 2003) and by the Department of Psychiatry at the Washington University.^[69] Microcephalin and ASPM are two genes that are associated with brain development. Mutations in these genes are associated with microcephaly, and hence they were initially associated with general intelligence. However recent studies have found no association with general cognitive abilities.^[70]

[edit] Between-group heritability

Although IQ differences between individuals are shown to have a large genetic component, it does not automatically follow that mean group-level disparities (between-group differences) in IQ can be assumed to have a genetic basis. An analogy, attributed to Richard Lewontin,^[71] illustrates this point:

Suppose two handfuls are taken from a sack containing a genetically diverse variety of corn, and each grown under carefully controlled and standardized conditions, except that one batch is lacking in certain nutrients that are supplied to the other. After several weeks, the plants are measured. There is variability of growth within each batch, due to the genetic variability of the corn. Given that the growing conditions are closely controlled, nearly all the variation in the height of the plants within a batch will be due to differences in their genes. Thus, within populations, heritabilities will be very high. Nevertheless, the difference between the two groups is due entirely to an environmental factor - differential nutrition. Lewontin didn't go so far as to have the one set of pots painted white and the other set black, but you get the idea. The point of the example, in any case, is that the causes of between-group differences may in principle be quite different from the causes of within-group variation.^[72]

However, differences in IQ between reportedly discreet genetic groups (such as races) has been observed, and some researchers, such as Arthur Jensen, maintain that environmental differences are too small to account for these differences. They propose that therefore genetic differences must provide the primary explanation.^[73] James Flynn ^{[disambiguation needed]} spends much of his book What Is Intelligence rebutting Jensen's argument.

The issue of observed between-group IQ differences is controversial vis-a-vis considerations on both the nature of race and the meaning and measurement of intelligence.^{[74][75][76][77][78][18]} Important related questions include whether intelligence can be accurately described by a single number, and whether the nature of intelligence is the same across cultures, as well as the emerging consensus that "races" as they are commonly understood are a social rather than a biological category.^[79]

[edit] Overview

At the base of the debate is the observation that significant differences in IQ scores have been measured in different population groups, particularly US Blacks and Whites (racially self-identified for the most part). No consensus exists regarding the meaning and/or relevance of these differences, and numerous interpretations have been proposed to clarify them.

No set populations of humans have been found to have significant genetic differences that would affect the average group intelligence, and part of the debate is whether measures such as IQ can meaningfully quantify such differences and whether it would be beneficial or ethical to study differences if they did exist.^[18] Various explanations for racial group differences have been proposed, such as the effect of a population's health, differences in education quality, sub-conscious psychological factors, and institutional racial discrimination. While environmental differences could statistically account for differences in test scores, no specific environmental factor has been identified as a definitive cause, and no direct evidence exists for genetic factors.^[80]

In 1995, a task force of the American Psychological Association (APA) headed by psychologist Ulric Neisser published a report entitled Intelligence: Knowns and Unknowns which intended to present a summary of the findings of scientific research regarding intelligence, which also commented upon the topic of race and intelligence.^[80] According to this report, psychometric testing, despite being one of the most fruitful approaches to studying intelligence, has yet to produce answers to many questions regarding intelligence. Though psychometricians have devised ways to measure the distinct yet intercorrelated abilities believed to play an important role in the development of intelligence, the correlations between those abilities remain largely unclear. As intelligence test scores correlate moderately well with measures of educational success, it is apparent that such tests measure important skills. However, noted the authors, educational achievement is not primarily determined by intelligence, though intelligence test scores do correlate significantly with occupational status later in life.

The APA report further noted that, while both genetic and environmental variables are involved in the manifestation of intelligence, the role of genetics has been shown to increase in importance with age. Why this happens in not yet understood, and the question as to what role the environment plays in this increase remains unanswered. Nonetheless, there are several important environmental factors which are known to affect the development of intelligence, such as formal education and general health. The much-discussed "Flynn effect", which refers to the striking worldwide mean IQ increase of 15+ points over the last 50 years, may be the result of similar environmental factors such as improved nutrition, cultural changes, improvement in the administration of tests, changes in educational practices or some other hitherto unrecognized factor.

On the topic of race and intelligence, the APA task force wrote that, as the measured differences in intelligence between various ethnic groups is the result of complex patterns, any conclusions which require broad generalizations run the risk of oversimplifying the issue as well as misrepresenting the available data. At the same time, intelligence test scores in some minority populations are reasonably good indicators of educational achievement levels in later life. The long-standing 15+/- point difference between the intelligence test scores of African Americans and White Americans, though it might have narrowed somewhat in recent years, remains unaccounted for despite proposed explanations claiming systematic bias, differences in culture or socioeconomic status (SES), or genetics as the sole underlying cause.

[edit] Suitability for study

Some scholars have expressed the view that the study of race differences in intelligence is meaningless, unethical or both. This is an on-going debate with proponents and opponents of this view making statements in the academic literature in support of their respective positions.^[74][75]

For example, according to Robert Sternberg and colleagues^[18], intelligence (often approximated using IQ) is not a well defined construct, and IQ tests do not provide definitive measures of intelligence. They argue that race and ethnicity are socially defined groups—rather than biological observations—whose membership isn't homogeneous; races and ethnicities are often defined by affiliation with very large geographical areas (Asian) or common language (Hispanic). For these reasons, they conclude that discussions of correlations between race and intelligence which extrapolate a genetic causation are fundamentally flawed.

In another example, Richard E. Nisbett^[81] and James Flynn^{[82][citation needed]}, who each believe that the available evidence favors an entirely environmental explanation for differences in test scores between blacks and whites, nonetheless reject the argument that studying these differences is meaningless and also reject the argument that the research is unethical. Rather, each has endorsed the view that the cause of group differences can be resolved with empirical study. Flynn gives particular credit to Arthur Jensen for stimulating more rigorous research on the topic.^[83]

[edit] Utility of research

Theories of race and intelligence have been challenged on grounds of their utility. Critics want to know what purpose such research could serve and why it has been an intense an area of focus for a few researchers. Some defend the research, saying it has egalitarian aims. IQ is, after all, a predictor of educational achievement and special needs. Accordingly, as a matter of public policy, resources can be better allocated by reducing the data to better understand the challenges; it is considered a pure science. Still others say that the true motivation for the research is the same as that of the eugenics movement and other forms of scientific racism.^[84][84] Even supporters of intelligence research have described such research as analogous to "working with dynamite" or "dangerous play" in sports.^[85]

Jensen and Rushton have justified their research in this area as being necessary to answer the question of how much white racism should be held responsible for ethnic groups’ unequal performance in certain areas. They maintain that when racism is blamed for disparities which are the result of biological differences, the result is mutual resentment, and unjustified punishment of the more successful group. They state:

[T]he view that one segment of the population is largely to blame for the problems of another segment can be even more harmful to racial harmony, by first producing demands for compensation and thereby inviting a backlash. Equating group disparities in success with racism on the part of the more successful group guarantees mutual resentment. As overt discrimination fades, still large racial disparities in success lead Blacks to conclude that White racism is not only pervasive but also insidious because it is so unobservable and “unconscious.” Whites resent that nonfalsifiable accusation and the demands to compensate Blacks for harm they do not believe they caused.^[86]

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

[edit] Hypotheses

The cause(s) of group average IQ test score differences are not known but hypotheses have been proposed. Many scholars have offered descriptions of the variety of hypotheses that have been proposed. These descriptions usually distinguish between those hypotheses which invoke a major contribution of genetic factors (hereditarian) and those which invoke mainly environmental (i.e., non-genetic) factors. Some descriptions of the positions are themselves controversial.

In a review published in 2007, Hunt and Carlson categorized four explanations for observed differences in IQ scores between groups.^[87] The strongest hereditarian position, attributed to Jensen and Rushton, is that group differences in IQ reflect differences in intelligence that are "due in substantial part to genetically determined differences in brain structure and/or function"^[87] A second position, attributed to Ogbu and Sowell, is that the differences in intelligence test scores are due to social factors. Third, Sternberg and colleagues are attributed with the view that the use of IQ scores to argue for differences in intelligence is an inappropriate use of tests in different groups. The fourth position, attributed to Fish and others, is that there is no such thing as race: "a term motivated by social concerns and not a scientific concept".^[87]

[edit] Socioeconomic factors

Socioeconomic status (SES) varies both between and within populations, but Black-White differences in IQ persist among the children of parents matched for SES, and the gap is largest among the children of wealthiest and best educated parents.^[88]

According to the report of a 1996 APA task force, socioeconomic factors (SES) cannot be the whole explanation for racial-ethnic group differences in IQ. Their first reason for this conclusion is that that the black-white test score gap 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 little effect on IQ. Third, the relationship between IQ and SES is not simply one in which SES determines IQ, rather it is more likely that intelligence causes differences in SES than the other way around. Lastly, they argue that income and education simply fail to capture important categories of cultural experience which differ between racial and ethnic groups.

[edit] Stereotype threat

Stereotype threat is the fear that one's behavior will confirm an existing stereotype of a group with which one identifies; this fear may in turn lead to an impairment of performance.^[89] Testing situations that highlight the fact that intelligence is being measured tend to lower the scores of individuals from racial-ethnic groups that already score lower on average. Stereotype threat conditions cause larger than expected IQ differences among groups but do not explain the gaps found in non-threatening test conditions.

[edit] The Flynn effect

The secular, international increase in test scores, commonly called the Flynn effect, is seen by Flynn and others as reason to expect the eventual convergence of average black and white IQ scores. Flynn argues that the average IQ scores in several countries have increased about 3 points per decade during the 20th century, which he and others attribute predominantly to environmental causes.^[90] This means, given the same test, the mean performance of African Americans today could be higher than the mean for white Americans in 1920, though the gains causing this appear to have occurred predominantly in the lower half of the IQ distribution.^[91] If an unknown environmental factor can cause changes in IQ over time, they argue, then contemporary differences between groups could also be due to an unknown environmental factor.

Nichols (1987)^[92] critically summarized the argument as follows:

(1) We do not know what causes the test score changes over time. (2) We do not know what causes racial differences in intelligence. (3) Since both causes are unknown, they must, therefore, be the same. (4) Since the unknown cause of changes over time cannot be shown to be genetic, it must be environmental. (5) Therefore, racial differences in intelligence are environmental in origin (p. 234).

Flynn and other research have found reason to doubt the construct validity of secular increases in IQ scores.^[93] In terms of the latent variables that IQ tests were designed to measure, such as g and verbal and mathematical ability, changes in IQ scores over time are different than either within-group individual differences and between group differences.^[94] For example, there has been little increase over time in performance on either the forward digit-span or reverse digit-span subtests, and tests of school achievement have been less affected than tests of abstract reasoning.^[95] At least one study has found that measurement bias contributes to the Flynn effect, which is not seen in black-white IQ differences.^[96]

[edit] Heritability

An environmental factor that varies between groups but not within groups can cause group differences in a trait that is otherwise 100%heritable. The height of this "ordinary genetically varied corn" is 100% heritable, but the difference between the groups is totally environmental. This is because the nutrient solution varies between populations, but not within populations.^[97]

There is a consensus among intelligence researchers that IQ differences between individuals within the same population (usually self identified "Black" or "White" in studies) are significantly heritable. It should be noted that heritability is a property of a population and may vary significantly between populations.^{[80][98][99][100][101][102][103][104][105]}

Concordance rates for IQ from twin studies and other study designs consistently fall in the range of 30% to 80%, with the estimated heritability in young (preschool) children in the lower range and adults in the higher range.^[106]

Much of the research on explaining group differences stems from an observation promoted first by Arthur Jensen and later James Flynn and others regarding an environmental explanation for group differences. According to Jensen^[107] the very high within-groupheritability of IQ (within both white and black populations) presents a problem for environmental explanations of group differences in IQ. They consider two general classes of environmental factors: common environmental factors and X-factors. Common environmental factors vary within and between populations. X-factors vary between populations, but do not vary substantially within populations. They first consider common environmental factors. To account for a 1 SD B-W IQ gap only in terms of common environmental factors would require very large environmental differences. For example, if the within-group heritability of IQ is 80%, then a B-W IQ difference of 2.24 SD in common environmental factors is required. For a heritability of 40%, a difference of 1.29 SD is required. Jensen and Flynn agree that it is an empirical question whether common environmental factors that influence IQ differ between whites and blacks to such an extent, and both agree that most commonly suggested environmental factors do not. Jensen believes that empirical evidence supports the view that the B-W IQ gap is caused by both common environmental factors and genetic factors. Flynn disagrees and believes that empirical evidence supports the view that the B-W IQ gap is caused by yet unrecognized environmental factors.^[108]

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

Critics have also questioned the interpretation of heritability as a whole. Lewontin suggests that some genotypes are more influenced by environments than others, leading to the possibility that populations that have similar genetic variance in the same environment can have different heritabilities because of their different genotypes.^[109] David Layzer (1974) contends that the development of a trait can be influenced by genetic differencesqualitatively and that heritability estimates cannot measure such qualitative differences, as such it is possible that even with a heritability of close to 100% it is possibly forphenotypic variance to be due largely to environment.^[110] As a comparison, schizophrenia is estimated to be at least 70% heritable, of which 30% of the actual genes have been accounted for.^{[111][112][113][114]}

[edit] Black and biracial children raised by white parents

Studies in which white parents raise black and biracial children have been variously regarded as inconclusive, supportive of an environmental interpretation, or supportive of a hereditarian interpretation. Three studies are commonly cited: the Minnesota Transracial Adoption Study^[115][116], Moore (1986), and Eyferth (1961). The Moore and Eyferth studies have been criticized due to concern that children and parents in these studies are not representative for reasons such as their age and circumstances that led their inclusion in the studies.^[117] A similar study on black, white, and mixed-race children raised in an educationally-enriched nursery groups^[118] is cited by Nisbett; the study found IQs at age 5 of 108 for black children, 103 for white, and 106 for mixed-race.^[81]:224-5 Like Moore (1986), the IQ of the parents is unknown, and the results could be distorted by the selective migration of the West Indian parents of the black children.^[81]

[edit] African ancestry and IQ

African Americans typically have ancestors from both African and Europe, with on average 20% their genomes inherited from European ancestors. Several studies performed without the use of DNA-based ancestry estimation attempted to correlated estimates of African or European ancestry with IQ. These studies have been variously regarded as inconclusive, supportive of an environmental interpretation, or supportive of a hereditarian interpretation. These studies are generally criticized for using unreliable methods to estimate ancestry and for their small samples sizes.

Rowe (2005) and others have suggested using DNA-based methods to reproduce these studies with reliable estimates of ancestry. Such experiments have never been published, although the requirements for such a study have been discussed in the academic literature.

[edit] Molecular genetic studies

The decoding of the human genome has enabled scientists to search for sections of the genome that contribute to cognitive abilities. 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 that more powerful tools of analysis will be required to detect them.^[119] Some researchers have expressed reluctance to investigate possible links between genes and intelligence, due to the controversy it can produce.^[120]

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".^[18] Two 2007 studies found that DTNBP1 and CHRM2 appear to influence intelligence depending on which allele of it a person carries.^[121][122] However, a study published in 2009 by Deary et al.. 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".^[123]

[edit] Health

Percentage of children aged 1-5 with blood lead levels at least 10 µg/dL. Black and Hispanic children have measurably higher levels than white children. High levels of lead at an early age may affect intelligence.

Numerous explanations beside genetics have been proposed to account for the IQ gaps in the U.S.^[124] High rates of low birth-weight babies, lower rates of breastfeeding, and exposure to toxins are some factors. The Flynn effect is often cited as evidence that average IQ scores have changed greatly and rapidly, for reasons poorly understood, thus the IQ gap between races could change in the future or is changing, especially if the Flynn effect started earlier for Whites.

High levels of lead at an early age may affect intelligence; studies indicate that black and Hispanic children have measurably higher levels than white children. A 10 µg/dL increase in blood lead at 24 months of age is associated with a 5.8-point lower IQ later in life.^[125] In 1976 77.8% of all children had at least this much lead in their blood.^[126]

Exposure to lead is frequently attributed to housing conditions including lead based paint, which is no longer used but has accumulated in older buildings; people of lower economic means are more frequently exposed to lead from housing.^[127]

[edit] Quality of education

Some researchers have written that studies that find test performance gaps between races even after adjusting for education level, such as the analysis found in The Bell Curve, fail to adjust for the quality of education. Not all high school graduates or college graduates have received the same quality of education. A 2006 study reported that years of education is an inadequate measure of the educational experience among multicultural elders, and that adjusting for quality of education greatly reduced the overall effect of racial differences on the tests.^[128] A 2004 study reported that quality of education and cultural experience influence how older African Americans approach neuropsychological tasks and concluded that adjustment for these variables may improve specificity of neuropsychological measures.^[129] Yet another study reported that, although significant differences were observed between the ethnic groups when matched for years of education, equating for literacy level eliminated all performance differences between African Americans and European Americans on both cancellation tasks which assess visual scanning^[130] (like reaction time tests, cancellation task tests are sometimes regarded as "culture free" tests of intelligence). Eric A. Hanushek and Steven G. Rivkin wrote in their 2006 book that unequal distributions of inexperienced teachers and of racial concentrations in schools can explain all of the increased achievement gap between grades 3 and 8.^[131] In recent years there have also been studies into the degrees in which many minorities, especially blacks, have internalized pathologies about their supposed lack of intelligence and the effects it has in their self-confidence, quality of learning and achievement.^[132] Additionally, Jensen's studies (Jensen, 1974b) show that 7% of black children of black professionals have mean IQs below that of white children from low-income families, yet this seems to have little to no detriment on the black children's success.^[133]

A 2004 study in South Africa found highly significant effects for both level and quality of education within the black African first language groups taking the Wechsler IQ tests. The scores of black African first language groups with advantaged education were comparable with the US standardization, whereas scores for black African first language participants with disadvantaged education were significantly lower than this. The study cautioned that faulty conclusions may be drawn about the effects of ethnicity and the potential for neuropsychological misdiagnosis.^[134]

[edit] Racial discrimination in education

Roslyn Arlin Mickelson writes that racial discrimination in education arises from actions of institutions or individual state actors, their attitudes and ideologies, or processes that systematically treat students from different racial/ethnic groups disparately or inequitably.^[135] Despite advancement in education reform efforts, to this day African American students continue to experience inequities within the educational system. Hala Elhoweris, Kagendo Mutua, Negmeldin Alsheikh and Pauline Holloway conducted a study of the effect of students' ethnicity on teachers' educational decision making. The results of this study indicated that the student's ethnicity did make a difference in the teachers' referral decisions for gifted and talented educational programs.^[136] Recently, a number of scholars have examined the issue of disproportionate representation of minority students in special educationprograms^[137][138]

Teachers' perceptions of a students cultural background may affect school achievement. African American students with African American cultural backgrounds, for example, have been found to benefit from culturally responsive teaching.^[139] In a 2003 study researchers found that teachers perceived students with African American culture-related movement styles as lower in achievement, higher in aggression, and more likely to need special education services than students with standard movement styles irrespective of race or other academic indicators.^[140]

Ellis Cose writes that low expectations may have a negative impact on the achievement of minorities. He writes that black people did not need to read The Bell Curve to be aware of the low expectations held for them by the majority culture. He recalls examples of low expectations from his teachers in school who regarded his use of AAVE as "laziness" and teachers who did not feel it was important to purchase new text books because they did not expect the students to be able to read anything complex. He contrasts these low expectations with the high expectations philosophy of Xavier University where, using the ideas Whimbey articulated in his book Intelligence can be Taught teachers created a program called SOAR. SOAR raised the performance of black students and lead Xavier to become the university that sends the greatest number of black students to medical school in the United States. The SOAR program produced gains equivalent to 120 points on an SAT test. Cose writes that "..we must treat people, whatever their color, as if they have unlimited intellectual capacity."^[141]

Structural equation models have been used to test for possible uncommon factors in the development of children belonging to different ethnic groups, which would include the results of racial discrimination. However, these tests have concluded that black, white, Hispanic and Asian children follow developmental processes which are "nearly identical."^[142][143]

[edit] Caste-like minorities

The book Inequality by Design: Cracking the Bell Curve Myth (1996) claims 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. 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.^[144] The authors note, however, that the comparisons made in the table do not represent the results of all relevant findings, nor do they reflect the fact that the tests and procedures varied greatly from study to study. The comparison of Jews and Arabs, for example, is based on a news report that, in 1992, 26% of Jewish high school students passed their matriculation exam, as opposed to 15% of Arab students.^[144]

[edit] Other views

Jared Diamond's Guns, Germs and Steel argues that historical differences in economic and technological development for different geographic areas can be explained by differences in geography (which affects factors like population density and spread of new technology) and differences in available crops and domesticatable animals. Richard Nisbettargues in his 2004 The Geography of Thought that some of these regional differences shaped lasting cultural traits, such as the collectivism required by East Asian rice irrigation, compared with the individualism of ancient Greek herding, maritime mercantilism, and money crops wine and olive oil^[145] However, it has been suggested that these environmental differences may operate in part by selecting for higher levels of IQ.^[146]

Differing rates of economic growth have also been attributed to numerous factors other than racial IQ gaps such as local availability of resources, climate, and sociopolitical factors. See for example the Global Competitiveness Report, the Ease of Doing Business Index, and theIndex of Economic Freedom or works by Kenneth Pomeranz,^[147] Eric Jones,^[148] Joel Mokyr,^[149] and Douglass C. North.^[150]

[edit] Debating the hereditarian position

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

Rushton and Jensen examined 10 categories of research evidence from around the world to contrast "a hereditarian model" (50% genetic-50% cultural) and a culture-only model (0% genetic-100% cultural). Their article "Thirty Years of Research on Race Differences in Cognitive Ability" was published in the APA journal Psychology, Public Policy and Law showing evidence that they believe supports the hereditarian model.^[151][152] Rushton and Jensen (2005a) believe that the best explanation for the gap is that 50%-80% of the group differences in average US IQ is genetic.^[153]

Other evidence, such as the Minnesota Transracial Adoption Study, certain racial admixture studies, behavior genetic modeling of group differences, "life-history" traits, and evolutionary explanations have also been proposed to indicate a genetic contribution to the IQ gaps and explain how these arose.^[154]

[edit] Outdated methodology

A 2006 paper by Professor Denny Borsboom argues that mainstream contemporary test analysis does not reflect substantial recent developments in the field and "bears an uncanny resemblance to the psychometric state of the art as it existed in the 1950s."^[155] It also claims that some of the most influential recent studies on group differences in intelligence, in order to show that the tests are unbiased, use outdated methodology. In particular the reliance onclassical test theory rather than more sophisticated measurement models as found in item response theory. In response to criticism proponents of the genetic hypothesis claim they use a standard for intelligence known as g. g is measured by performance on test items without the influence of language or math.

[edit] Test construction

While the existence of average IQ test score differences has been a matter of accepted fact for decades, a great deal of controversy exists among scholars over the question of whether these score differences reflected real differences in cognitive ability. Some claim that there is no evidence for test bias since IQ tests are equally good predictors of IQ-related factors (such as school performance) for U.S. Blacks and Whites.^[156] The performance differences persist in tests and testing situations in which care has been taken to eliminate bias.^[156] It has also been suggested that IQ tests are formulated in such a way as to disadvantage minorities.^[156] Controlled studies have shown that test construction does not substantially contribute to the IQ gap.^[156] However, some psychometricians are not satisfied that the question of test bias is fully answered by these results.^{[157][158][159]}

The preponderance of evidence indicates that IQ tests measuring general intelligence are crossculturally valid. There is little or no evidence of population-specific cultural effects apart from the obvious example of language bias.^[160] For example, Robert Sternberg et al. found that the IQ of 12- to 15-year-old Kenyans predicted school grades at about the same level as they do in the West.^[161] IQ also predicted university performance equally well in African and non-African engineering students in South Africa in a 2004 study.^[162] Salgado et al. (2003) demonstrated the international generalizability of general mental ability across 10 member countries of theEuropean Community and differences in a nation’s culture, religion, language, socioeconomic level or employment legislation did not affect the predictive validity of IQ tests.^[163]

However, other studies have found evidence for bias. A 2005 study finds some evidence that theWAIS-R is not culture-fair for Mexican Americans.^[164] Other recent studies have questioned the culture-fairness of IQ tests when used in South Africa.^[165][166]

[edit] Lack of direct evidence

Nisbett (2005) argues that many studies find results that do not support the genetic hypothesis. They include studies on IQ and skin color that reported that the average correlation between skin color and IQ is .1 (the average correlation between IQ and judged “Negroidness” of features is even lower); IQ and self-reported European ancestry; IQ and blood groups showing degree of European Ancestry; IQ among children in post WWII Germany born to black and white American soldiers; and IQ among mixed-race children born to either a black or a white mother. He argues that these are direct tests of the genetic hypothesis and of more value than indirect variables, such as skull size and reaction time. He argues that "There is not a shred of evidence in this literature, which draws on studies having a total of five very different designs, that the gap has a genetic basis." He argues further that many intervention and adoption studies also find results that do not support the genetic hypothesis. He also argues "that the Black-White IQ gap has lessened considerably in recent decades."^[167] Hunt and Carlson^[32] argue that Nisbett's interpretations are far too strong in light of problems with these studies that have been recognized for decades.^[168] Gottfredson writes that the studies Nisbett cites "actually lack the ability to rule out any hypothesis at all, genetic or not".^[169]

Dickens (2005) states that "Although the direct evidence on the role of environment is not definitive, it mostly suggests that genetic differences are not necessary to explain racial differences. Advocates of the hereditarian position have therefore turned to indirect evidence...The indirect evidence on the role of genes in explaining the black-white gap does not tell us how much of the gap genes explain and may be of no value at all in deciding whether genes do play a role. Because the direct evidence on ancestry, adoption, and cross-fostering is most consistent with little or no role for genes, it is unlikely that the black-white gap has a large genetic component."^[170]

Fryer and Levitt (2006), with data from "the first large, nationally representative sample" of its kind, report finding only a very small racial difference when measuring mental function for children aged eight to twelve months, and that even these differences disappear when including a "limited set of controls". "On tests of intelligence, Blacks systematically score worse than Whites, whereas Asians frequently outperform Whites. Some have argued that genetic differences across races account for the gap. Using a newly available nationally representative data set that includes a test of mental function for children aged eight to twelve months, we find only minor racial differences in test outcomes (0.06 standard deviation units in the raw data) between Blacks and Whites that disappear with the inclusion of a limited set of controls. The only statistically significant racial difference is that Asian children score slightly worse than those of other races." They argue that their report poses "a substantial challenge to the simplest, most direct, and most often articulated genetic stories regarding racial differences in mental function." They conclude that "to the extent that there are any genetically-driven racial differences in intelligence, these gaps must either emerge after the age of one, or operate along dimensions not captured by this early test of mental cognition."^[171]

[edit] Source of funding

Vocal proponents of partially genetic explanations of race/IQ correlation, such as Rushton, Lynn, and Jensen, have been criticized for receiving funding from the Pioneer Fund, a group that has been reported as having had ties to German eugenicists working under theNazi regime as well as to other US eugenicists of the early 20th century.^[172][173] The Southern Poverty Law Centerconsiders the Pioneer Fund to be a hate group. Rushton is the current head of the Pioneer Fund and has spoken at conferences of the American Renaissance magazine, in which he has also published articles.^[174]

Proponents of genetic explanations of race-IQ correlation have in turn accused their critics ofsuppressing scientific debate in the name of political correctness. They claim harassment and interference with both their work and funding. The Pioneer Fund, whose stated purpose is "to advance the scientific study of heredity and human differences", makes "no grants to individuals but only to research institutions, mainly universities, mostly for specialized 'niche' projects, which have difficulty attracting funds from government sources or from larger foundations".^[175]

[edit] See also

• Race and intelligence
• Cyril Burt

[edit] Notes and references

[edit] Further reading

• McGuffin P (2000). "The quantitative and molecular genetics of human intelligence". Novartis Found. Symp. 233: 243–55; discussion 255–9, 276–80. doi:10.1002/0470870850.ch15. PMID 11276906. 
• G. Meisenberg: Genes for intelligence. A review of recent progress. Mankind Quarterly 36 (Winter 2005) 139-164. abstract
• Payton A (2006). "Investigating cognitive genetics and its implications for the treatment of cognitive deficit". Genes, Brain and Behavior 5 Suppl 1: 44–53. doi:10.1111/j.1601-183X.2006.00194.x. PMID 16417617.