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The most common view among intelligence researchers is that IQ differences among individuals of the same race reflect (1) real, (2) functionally/socially significant, and (3) substantially genetic differences in the general intelligence factor. Similarly, average IQ differences among races reflect (1) real and (2) significant differences in the same g factor. However, it is a matter of debate whether IQ differences among races in a given country are primarily environmental or partly genetic.

A recent review summarizing the arguments for a "partly-genetic" explanation can be found here. A critique can be found here.

Test bias

It has been suggested that IQ tests may be biased against minorities, and that this accounts for part or all of the IQ gap. Some claim that there is no evidence for test bias. IQ tests are equally good predictors of IQ-related factors (such as school performance) for U.S. Blacks and Whites. The performance differences persist in tests and testing situations in which care has been taken to eliminate bias. It has also been suggested that IQ tests are formulated in such a way as to disadvantage minorities. Controlled studies have shown that test construction does not substantially contribute to the IQ gap. Studies have shown that the race of the test administrator does not have an effect on the U.S. Black-White gap (Template:AYref).

The lack of test bias due to test construction or methodology is widely accepted in the research community. From the American Psychological Association's summary of their 1996 task force report, "Intelligence: Knowns and Unknowns": "The differential between the mean intelligence test scores of Blacks and Whites does not result from any obvious biases in test construction and administration, nor does it simply reflect differences in socio-economic status" (Template:AYref). From The Wall Street Journal: Mainstream Science on Intelligence (PDF): "Intelligence tests are not culturally biased against American Blacks or other native-born, English-speaking people in the U.S. Rather, IQ scores predict equally accurately for all such Americans, regardless of race or social class."

This does not mean that anyone is claiming that there is no bias in the tests whatsoever. Somewhat but not directly related to the idea of cultural bias, is the concept of "labeling bias", described by Jencks and Phillips (1998). They insist that there exists a labeling bias in the tests, stating that most psychologists agree that IQ tests measure developed ability rather than innate ability—although the tests supposedly measure innate ability. Their assertion is that non-cultural environmental factors cause gaps measured by the tests, rather than any possible innate difference based on genetics, and to use these tests as a measure of innate difference is misleading and improper. In a PBS interview, Jencks states, "If we change the names of the tests, they still measure the same thing but it wouldn't convey this idea that somehow you've gotten the potential of somebody when you measured their IQ. And I think that creates a big bias, because the people who do badly on the tests are labeled as people with low potential in many people's minds and they sometimes even believe that about themselves."

Since the U.S. Supreme Court outlawed employee selection, including testing, which is "fair in form, but discriminatory in operation" (Griggs v. Duke Power Co., 1971; see this page on disparate impact), American companies have had a strong incentive to construct valid tests which do not produce an IQ gap between ethnic groups, called "selection bias" in employment. Despite this incentive, these efforts have generally failed. For example, in one case regarding a police selection test in Nassau County, New York, a scandal ensued when tests which showed no "selection bias" (Black-White score gap) were found to have been denuded of their ability to measure intelligence (Template:AYref, pp. 24-26 PDF).

While the existence of average IQ test score differences has been a matter of accepted fact for decades, during the 1960s and 1970s a great deal of controversy existed among scholars over the question of whether these score differences reflected real differences in cognitive ability. The critics of testing argued that IQ tests are predominantly measures of cultural variables, and that these variables have nothing to do with intelligence, but they do differ between racial-ethnic and socio-economic groups. If the culture of one group prevents them from developing the skills and abilities that IQ tests measure, then that group is culturally disadvantaged. In contrast, if people from different cultures score differently despite possessing the relevant skills and abilities to the same extent, then the IQ test is culturally biased.

Cultural bias can be distinguished from cultural disadvantage. To do this, a precise definition of bias is required. Bias as mean differences merely begs the question. IQ tests must be standardized against a representative sample of people, and so one suggestion was bias as improper standardization. This has been ruled out in part because re-standardization does not affect the IQ gap. Bias as content could exist if test items presuppose knowledge that is more common in one group. This has been ruled out in part because test item difficulty levels are nearly identical for Blacks and Whites. Arguably the most important condition is bias as differential predictive validity. If test scores predict performance on some other criteria (e.g., school grades) less well for one group than another, then the test shows differential validity. IQ tests have equivalent predictive validity for Blacks and Whites across a range of criteria (Template:AYref). Other kinds of bias have been proposed, but none have been found. Of these, bias as motivation is the most difficult to resolve because empirical tests are difficult. It is argued that Blacks are less motivated than Whites to perform well on IQ tests, but if equally motivated the IQ score gap would disappear. At least two views exist on the question of motivation. One view is that intelligence is an abstract concept related to potential, and the possibility of motivation affecting IQ scores is a problem. The second and more common view is that intelligence is a behavior, and performance is more relevant than potential. Under this view, the equal predictive validity of IQ for Blacks and Whites implies that IQ tests are right to be sensitive to motivation if motivation affects performance in important life outcomes.

Motivation

One environmental source of the IQ gap which has been suggested is poor motivation among low scorers. This hypothesis has been disputed by the researcher Arthur Jensen (1998). For example, one such test asks the subject to lift a finger from a depressed button to strike a light when it flashes. When more than one light is offered as a target the task involves a decision of which to hit (i.e. the one which is lit). These tests measure both reaction time (from when the bulb illuminates to when the subject lifts their finger) and movement time (from when the subject lifts their finger to when the subject reaches the bulb). While movement time measurements show no difference, reaction time measurements negatively correlate with IQ scores and show the same performance gaps between races (Template:AYref; Template:AYref). Jensen argues that it is difficult to imagine that people could be motivated during one part of each segment of the test but not motivated during the other, although no correlation between movement speed and intelligence is claimed. The correlation between IQ and reaction time is low (from .20 to .40).

Socio-economic factors

IQ is correlated with economic factors. Blacks and Hispanics suffer poorer economic conditions than Whites. It has been suggested that the effects of poverty are responsible for some or all of the IQ gap. However, in the American Psychological Association report Template:A(Y)ref argue that economics cannot be the whole explanation. First, see the discussion in "Shared and nonshared environmental effects" below. Second, to the moderate extent that IQ and income are related, it appears that IQ determines income, and not the other way around (Template:AYref). (Note there are many other potential environmental factors beside income.) Third, there are gaps in IQ scores that are slightly smaller but still persist for individuals with similar family income and parental education.

Work by Template:A(Y)ref on average Black-Hispanic-White differences in IQ, education, and income casts doubt on conventional explanations of Black-White differences:

Hispanic children start with cognitive and noncognitive deficits similar to those of black children. They also grow up in similarly disadvantaged environments and are likely to attend schools of similar quality. Hispanics complete much less schooling than blacks. Nevertheless, the ability growth by years of schooling is much higher for Hispanics than for blacks. By the time they reach adulthood, Hispanics have significantly higher test scores than do blacks. Conditional on test scores, there is no evidence of an important Hispanic-white wage gap. Our analysis of the Hispanic data illuminates the traditional study of black-white differences and casts doubt on many conventional explanations of these differences because they do not apply to Hispanics, who also suffer from many of the same disadvantages. The failure of the Hispanic-white gap to widen with schooling or age casts doubt on the claim that poor schools and bad neighborhoods are the reasons for the slow growth rate of black test scores.

Researchers have found that many American Blacks and Hispanics are not given sufficient opportunity to learn language and thinking skills during the first three years of life, possibly due to economic status. The first three years are especially critical years for neural development of the brain, and previous studies have shown that when human children were deprived of most or all language skills at an early age, they never developed the ability to master language at a later age; if they only mastered a small amount of language and thinking skills at a young age, then they could only make small improvements in later years. A recent study has shown that many American Blacks and Hispanics are raised in homes where their parents speak relatively few sentences, and the sentences usually show only simple grammar. As a result, their children never hear millions of words during the time when their brains are developing linguistic skills. Without this linguistic input during their developing years, many are observed to quickly fall behind, and they can never catch up. Children in poorer welfare families, which includes a higher percentage of many minority populations, apparently hear up to 30 million fewer words by age three than children in higher income, usually White, families. (Source: The Early Catastrophe: The 30 Million Word Gap by Age 3)

Cultural explanations

Many anthropologists have argued that intelligence is a cultural category; some cultures emphasize speed and competition more than others, for example. During WWI African-Americans from the north tested higher than those from the south. This could be because African-Americans in the north had received more formal education (see Race: Science and Politics, written by Ruth Benedict in 1940). Thousands of ethnographic studies indicate that innate capacities for cultural evolution are equal among all human populations. The American Anthropological Association has endorsed a statement deriding all studies of race and intelligence .

Speculations about innate differences in intelligence between ethnic groups have occurred throughout history. Aristotle in the 4th century B.C. and Cicero in the 1rst. century B.C. disparaged the intelligence of the northern Europeans of the time, as did the Moors in Iberia in the 11th century.

It has been suggested that Black culture disfavors academic achievement and fosters an environment that is damaging to IQ (Template:AYref). Likewise, it is argued that a persistence of racism reinforces this negative effect. John Ogbu has developed a hypothesis that the condition of being a "caste-like minority" affects motivation and achievement, depressing IQ. However, Arthur Jensen has criticized these arguments on the grounds that they cannot explain the higher scores of East Indians and East Asians. Even proponents of the view that the IQ gap is caused partly by genetic differences, such as Arthur Jensen, recognize that non-genetic factors are likely involved. Indeed, one author has compiled a list of over one hundred possible causes of the Black-White IQ gap. These include the following:

• lack of reading material in the home
• poor cultural amenities in the home
• weak structural integrity of the home
• foreign language in the home
• low preschool attendance
• no encyclopedia in the home
• low level of parental education
• little time spent on homework
• low parental educational desires for child
• low parental interest in school work
• negative child self-concept (self-esteem)
• low child interest in school and reading

However, such factors have not been found to have an effect on IQ that lasts to adulthood among members of the same race (see below).

Cultural explanations for the IQ deficit among Blacks and Hispanics compared to Whites and East Asian minorities are complemented – and sometimes challenged – by the observation that East Asian minorities score well on IQ tests and on average enjoy greater economic success than other minorities. Along these lines, East Asians are sometimes referred to as "model minorities". East Asian and Jewish populations have suffered past discrimination and persecution which some argue is evidence against the importance of discrimination for IQ differences. While the severe discrimination against Jews and East Asians have today diminished, many argue that discrimination continue against blacks and that this is impacting the IQ scores of Blacks.

Language

Some argue that the higher IQ test scores in East Asian nations are in part attributed to some IQ tests' inherent bias towards testing spatial reasoning. They argue that logographic writing systems, like those used by Chinese and Japanese, develop spatial reasoning better than the alphabetic writing systems prevalent in Europe and America, though there are no studies that support this hypothesis. The same reasoning has been used to explain why students from some Asian countries (e.g., Singapore) tend to score better than average in tests of mathematics. Some argue that the East Asian advantage can also be explained by more rigorous education programs. However, even though few native-born Asian Americans learn to read and write Chinese characters, their performance is above-average on IQ tests.

A direct comparative test between Greek and Chinese students showed no difference in IQ or g, contradicting earlier studies which do not take the finer architecture of mental processing into account. The Chinese did outperform the Greeks in visuo/spatial ability, but this difference was smaller at earlier ages, grew during the first years of schooling and decreased later. The authors suggest that this pattern can be explained as follows: the Chinese students train their visuo/spatial ability during their early school years, as they have to learn many characters of the Chinese writing system. Later in life, the Greek students adopt compensating strategies to deal with visuo/spatial information, and therefore the difference decreases in this realm.

The Flynn effect

The Flynn effect consists of large documented worldwide increases in IQ scores for at least several decades. Attempted explanations have included improved nutrition, a trend towards smaller families, better education, greater environmental complexity, and heterosis.

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. As demonstrated by Flynn, the average IQ scores in several countries have increased about 3 points per decade during the 20th century, which he and others attribute predominantly to environmental causes. This means, given the same test, the mean Black American performance today could be higher than the mean White American performance in 1920, though the gains causing this appear to have occurred predominantly in the lower half of the IQ distribution. If changes in environment can cause such large changes in IQ over time, they argue, then contemporary differences between groups could also be due to an unknown environmental factor. On the supposition that the effect started earlier for Whites, because their social and economical conditions began to improve earlier than did those of Blacks, they anticipate that the IQ gap among races might change in the future or is even now changing. An added complication to this hypothesis is the question of whether the secular IQ gains can be predominantly a change in real intelligence. Flynn's face-value answer to this question is "No", and other researchers have found reason to concur. Responding to such concerns, Template:AYref have proposed a solution which rests on genotype-environment correlation, hypothesizing that small initial differences in environment cause feedback effects which magnify into large IQ differences. Such differences would need to develop before age 3, when the Black-White IQ gap can be first detected.

Comparing the Flynn effect (IQ differences within races over time) to contemporary IQ differences between races is contested; for example, one report concludes "the nature of the Flynn effect is qualitatively different from the nature of black-white differences in the United States," and that "the implications of the Flynn effect for black-white differences appear small" (Template:AYref).

A recent theory hypothesizes that fluid cognition (gF') may be separable from general intelligence, and that gF' may be very susceptible to environmental factors, in particular early childhood stress. Some IQ tests, especially those used with children, are poor measures of gF', which means that the effect of the environment on intelligence regarding racial differences, the Flynn effect, early childhood intervention, and life outcomes may have been underestimated in many studies. The article has received numerous peer commentaries for and against.

A recent, newly available, large, and nationally representative data set find only very small (0.06 SD between whites and blacks) racial differences on measures for mental function for children aged eight to twelve months. These differences disappear when controlling for a limited set of factors such as differences in SES. "These findings pose a substantial challenge to the simplest, most direct, and most often articulated genetic stories regarding racial differences in mental function." "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."

Nongenetic biological factors

Other researchers have come across what they see as additional reasons for the IQ gap. The paper Poverty and Brain Development in Early Childhood holds that there is a large amount of neural damage in many American Black and Hispanic children due to inadequate nutrition, substance abuse of the children's parents, a high incidence of maternal depression, exposure to environmental toxins, psychological trauma, and the neural effects of physical abuse. Template:A(Y)ref has proposed a "neurotoxity hypothesis" where pre- and post-natal exposure to heavy metal poisons differentially impacts Blacks. Drug abuse during pregnancy (e.g., alcohol and phenobarbital) can negatively affect IQ.

Infant mortality may be an indicator of environmental conditions that are sublethal but damaging to health. The rate of infant mortality in the U.S. Black population is twice that of the White population, which in turn is twice the rate of infant mortality among Asians. The rates of low birth weight (LBW), defined as less than 5.5 pounds, are correlated with infant death. LBW is different than premature birth; LBW can occur in full-term babies. LBW babies are at risk for many developmental, behavioral and cognitive abnormalities, including mental retardation. LBW (and premature birth) affect Blacks at twice the overall rate for the U.S. population. Mother's age is the strongest predictor of LBW, where teenagers are especially susceptible. Most of the Black-White differences in LBW are not account for by other environmental variables such as socioeconomic status, poverty status, mother's age, and education; but differential prenatal care explains some of the gap (Template:AYref). Thus, the cause of the Black-White gap in LBW is a mystery. Environmental intervention has strong but short-lasting effects on IQ among LBW babies (Template:AYref). Studies of LBW Black and White babies matched for birth weight and gestational age still find a one standard deviation IQ gap (Template:AYref).

A study of LBW babies indicates that breastfeeding can significantly improve their IQ scores tested at 8 years old (Template:AYref). After controlling for possible confounding factors, an improvement of 8.3 IQ points was found in the breastfed group as compared to the formula fed group. Black mothers are known to breastfeed infants less and for a shorter time than White mothers (Template:AYref; Template:AYref). Studies have shown IQ gains lasting into adulthood with increased duration of breastfeeding. Several recent studies shows that the intake of certain micronutrients, like those present in breast milk or fish oil, affects IQ scores even in developed nations. Template:A(Y)ref have shown larger head size at birth and higher IQ scores at 4 years of age when mothers took fish oil supplements during pregnancy and lactation. Template:A(Y)ref believes that dietary supplementation is a promising avenue of research for raising Black children's levels of g. Template:A(Y)ref has proposed a nutritional hypothesis for the Flynn effect.

Genetics

Part of the gap may well be genetic; there is no a priori reason to believe that every ethnic group or race has precisely the same distribution of genes that affect intelligence; a small amount of random variation early in human evolution may have later crystallized into differences seen today. Also there might have been smaller evolutionary pressure towards greater intelligence in some environments. The partly genetic hypothesis is often ignored or disregarded in primary research on group differences. It has been studied by researchers doing meta-analyses that combine multiple sources of primary materials, although such meta-analyses have been harshly criticized..

Arthur Jensen and others have concluded that the Black-White IQ gap is partly genetic. That is, they argue that the same mix of genetic and environment factors that cause IQ differences among individuals or between families of the same race also causes the differences seen between races. In this view, the genetic contribution to average intelligence differences among races are like average skin color differences: a product of different allelic frequencies within each population. Others are critical of Jensen's methods and evaluation (Template:AYref; Template:AYref; Template:AYref).

The results of most (indirect) analyses used to test the genetic hypothesis do not logically contradict a primarily environmental explanation of the lower IQ of Blacks. That is, a plausible (but some argue ad hoc) environmental explanation for the lower mean IQ in Blacks can be offered in most cases.

Shared and nonshared environmental effects

see also IQ: genetics vs environment

The heritability of intelligence within groups is high. It is widely recognized that within-group heritability does not in itself indicate that between-group differences are genetic in origin, although it is likely a necessary condition. Different kinds of evidence are needed to address the question of between-group heritability. As Herrnstein and Murray explain in The Bell Curve:

As we discussed in Chapter 4, scholars accept that IQ is substantially heritable, somewhere between 40 and 80 percent, meaning that much of the observed variation in IQ is genetic. And yet this tells us nothing for sure about the origin of the differences between races in measured intelligence. This point is so basic, and so commonly misunderstood, that it deserves emphasis: That a trait is genetically transmitted in individuals does not mean that group differences in that trait are also genetic in origin. Anyone who doubts this assertion may take two handfuls of genetically identical seed corn and plant one handful in Iowa, the other in the Mojave Desert, and let nature (i.e., the environment) take its course. The seeds will grow in Iowa, not in the Mojave, and the result will have nothing to do with genetic differences. (Template:AYref, p. 298.)

In most studies, measured heritabilities for intelligence are the same for Blacks as for Whites. A review by Template:AYref found some evidence suggesting lower heritability in Blacks than Whites (e.g., Template:AYref), but a larger body of evidence suggested equal heritabilities for both races. An analysis of the Georgia Twin Study by Template:AYref found equal heritabilities for both Blacks and Whites.

Two kinds of environmental effects can be distinguished: shared and nonshared effects (see nature versus nurture). Twin and adoption studies, used to measure heritability, can also be used to quantify the two types of environmental effects (Template:AYref). Shared environmental effects are due to factors experienced in common by all children raised in the same family but that differ among families. Examples of shared environmental effects include socio-economic factors, family cultural practices, and parental influences on children. Nonshared effects are unique for each child, and thus differ among families. Examples include chance events such as accidents, illness, and childhood friends. Anything that happens to one sibling and not to the other contributes to nonshared effects.

Template:A(Y)ref found (among a population of people studied in the U.S.) that the nonshared environmental effects on IQ remain approximately constant throughout life. Shared environmental effects in their study remained approximately constant (40% to 30%) from 4 to 20 years of age but then drop to zero in adulthood. Genetic factors increase throughout development (from 40% to 50%) but especially after 20 years of age (from 50% to 80%). Template:A(Y)ref corroborates these results. Environmental factors usually proposed to explain the Black-White gap are shared effects (e.g. social class, religion, cultural practices, father absence, and parenting styles). Template:A(Y)ref argues that because these effects account for little variance within a race, they are unlikely to account for the differences among races in developed nations.

However, others studies do support that shared environmental factors in developed nations can affect IQ, including IQ gains lasting into adulthood (Capron and Duyme, 1989). However, many such studies measure IQ in children (those shared effects that have disappeared in studies don't disappear until adulthood) or, some critics claim, do not have the controls needed to differentiate genetic and environmental effects. Others argue that some IQ gains disappear exactly because the interventions cease, continuing interventions like Head Start have showed that the IQ gains then remain.

In a re-analysis of adoption data from Template:AYref, Template:AYref found that the IQ gains that result from being adopted into high socioeconomic-status homes do not produce gains in g, but only in non-g factors. Jensen also found that the g factor scores of the adopted children reflected the socioeconomic level of their biological parents, not their adopted parents. This is consistent with Jensen's theory that g is the predominant genetic component of IQ scores; see Spearman's hypothesis below from the relationship between g and racial difference in IQ.

The recent paper Socioeconomic status modifies heritability of IQ in young children finds that the role of the environment is more important in poorer families. "The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse." This suggests that the role of shared environmental factors may have been underestimated in older studies which often only studied affluent middle class families.

Only shared environmental effects captured in heritability studies disappear in adulthood; more extreme environmental deprivation may likely have a lasting impact on IQ in adults. Heritability only tells us what is the contribution of genes to variation in a trait, not what it could be (Template:AYref). Thus, heritability measures in the U.S. population cannot be extrapolated to populations in developing nations.

Spearman's hypothesis

Individual differences in the general intelligence factor, g, and its various biological correlates (e.g., the volume of gray matter in the frontal cortex) are partly caused by genetic differences between individuals. g has the highest measured heritability of any cognitive ability factor. Jensen formulated a hypothesis now referred to as Spearman's hypothesis which states that the degree of difference between black and white cognitive test scores will be correlated with the degree to which the test measures g (called the test's g-loading). Spearman's hypothesis has a strong form, which says that all test-score differences can be traced to g, and a weak form, which claims that some but not all differences are due to g.

Jensen found that black-white cognitive test score differences and test g-loadings correlate with a correlation coefficient of 0.6 (Template:AYref), and concluded that the weak form of Spearman's hypothesis was thus confirmed. Jensen's study combined scores on 149 psychometric tests obtained from 15 independent samples totaling 43,892 Blacks and 243,009 Whites (Template:AYref). Template:AYref have reanalyzed the data from several previous studies (Template:AYref; Template:AYref) that used the statistical method invented by Jensen (the method of correlated vectors) with a more recent and improved method (multigroup confirmatory factor analysis). "On the basis of the present, as well as other results (Dolan, 2000), we are convinced that the Spearman correlation cannot be used to demonstrate the importance of g in b-w differences with any confidence." and "It is possible that the analysis of all available data sets (perhaps using an appropriate meta-analytic procedure) will demonstrate that a model incorporating the weak version of Spearman's hypothesis provides the best description of the data. However, until this work is undertaken, we cannot accept Spearman's hypothesis as an "empirically established fact" This leaves the validity of Spearman's hypothesis, considered a central justification for the genetic explanation, an unresolved question.

Gene-environment interactions

Minority-specific effects on intelligence arising from cultural background differences between the races would be expected to affect the correlations between the measures of environmental background variables and outcome measures. Rowe et al. (1994) compared cross-sectional correlation matrices using both independent variables (e.g., home environment, peer characteristics) and developmental outcomes (e.g., achievement, delinquency). Template:A(Y)ref compared correlations between academic achievement and family environment. They found that the covariance matrix of each group were equal. That is, they failed to find evidence for distortions in the correlations between the background variables and the outcome measures that would suggest a minority-specific developmental factor. Similarly, Template:A(Y)ref, Template:A(Y)ref, and Template:A(Y)ref, Template:A(Y)ref, Template:A(Y)ref) found nearly identical statistical structure on psychometric variables in each group. The factor structure of cognitive ability is nearly identical for Blacks and for Whites; there were no race-specific factors.

Using structural equation modeling Template:A(Y)ref estimated the genetic architecture for Black and White siblings. They found that the best-fitting model for the source of differences between and within races was the same: both genetic and environmental factors. Template:AYref (p. 465) reanalyzed a subset of this data. This analysis found that the Black-White IQ difference was best explained by a model of both genetic and environmental factors, and that the genetic-only and the environmental-only models were inadequate.

Template:A(Y)ref using differential heritabilities among Blacks and Whites and later Template:A(Y)ref using inbreeding depression calculated in Japan found that the Black-White gap is least on IQ subtests most affected by the environment, and greatest on subtests that are least affected by the environment. It is difficult to attribute the relationship between inbreeding depression from Japan with the Black-White IQ gap in the U.S. to an environmental (not-genetic) cause.

Rushton's application of r-K theory

In his controversial 1995 work Race, Evolution and Behavior, J. Philippe Rushton argued that racial differences in IQ, as well as a number of other racial differences, could be explained by r/K selection theory, where one evolutionary strategy favors reproduction potential over other strengths (r-selection) and one favors traits that are highly adaptive in a stable environment (K-selection). Rushton claims that humans are extremely K-selected, and that it was less extreme (there was more r-selection) in the African environment than elsewhere. He posits that the comparatively cold and harsh environment of Europe caused the evolution of those who migrated there slightly more to a K-selected pattern than those who remained in Africa, and the even harsher environment of Northeastern Asia forced the evolution of East Asians to an even higher level of K-selective behavior. This theory has been severely criticized.

Richard Lynn has developed similar theories and argues that the ice age that took place in East Asia from about 28,000 to 12,000 years ago acted as a selection force on East Asians to increase intelligence by requiring the building of shelter, making clothes, and making fires, and selected especially strongly for spatial skills such as those needed to hunt large prey and build the tools necessary to do so. (Template:AYref). Template:A(Y)ref has cited the fact that the order in Blacks, Whites, and East Asians appeared is the same as the order of their respective brain sizes as additional evidence. This theory, however, has difficulty explaining why Native Americans, who appeared even later and emigrated from the northernmost parts of Asia, do not currently have high scores on IQ tests. On the other hand, Template:A(Y)ref argues that lower scores of Native Americans can be attributed to the evolutionary relaxation of cognitive demands due to the more temperate environment and comparative ease with which North American fauna could be hunted. But it can be argued that life along the fertile river plains in China was not particularly harsh. It is also questionable that conditions in deserts are no less harsh but people living there do not currently score high on IQ tests.

The theory is directly contradicted by the only comparative study on IQ scores in different European nations that showed a statistically insignificant association between the average IQ and latitude of various European nations. In contrast, Template:AYref (p. 309) found a correlation of 0.62 (p=0.00001) between latitude and cranial capacity in samples worldwide and reported that each degree of latitude was associated with an increase of 2.5 cm³ in cranial volume. A more recent study finds this pattern only when including a Siberian population living in extremely cold condition. The explanation may be natural selection for a thermoregulatory capacity in extremely cold environments, resulting in brachycephalization, rather than a selection for intelligence.

Rushton sources, such as s "semi-pornographic book" and the Penthouse magazine, have been dismissed by other researchers, or have been criticized as extremely biased and inadequate reviews of the literature, as misreporting the results, or as simply false. There have also been many other criticisms of the theory. More recent studies contradicts Rushton's claims. A meta-analysis shows that blacks are not more psychopathic, nor do they differ in from whites when testing for the big five personality traits, differences in sex hormones between whites and East Asians are best explained by environmental differences, and the fundamental prediction of the theory that blacks have a higher frequency of twins is incorrect. However, the rate of twin births in the US has doubled since 1971, the time of the study Rushton cited, due to older moms (for which twin births are naturally more common) and fertility treatments, both demographic characteristics that are more common among Whites.

Other evolutionary explanations for putative genetic differences

There are two mainstream theories of the evolution of contemporary humans. The single-origin hypothesis proposes that modern humans evolved in Africa and later replaced hominids in other parts of the world. The multiregional hypothesis proposes that modern humans evolved to some degree from independent hominid populations. An emerging synthesis theory proposes that the genes of contemporary human are predominantly descended from a recent African origin, but that interbreeding with other hominids may have contributed genes to local populations (Template:AYref). Template:A(Y)ref speculate that "as much as 80% of the nuclear genome is significantly affected by assimilation from archaic humans (i.e., 80% of loci may have some archaic admixture, not that the human genome is 80% archaic)."

Populations within continents are more closely related to one another than to populations on other continents (Template:AYref; Template:AYref; Template:AYref; Template:AYref). Thus, to the extent that racial labels correspond to ancient ancestry, racial groups (especially in the U.S.) are statistically distinguishable on the basis of genetics (Template:AYref).

The Imperial examination system in China and similar systems in other East Asians nations have been proposed as an explanation for the higher average IQ, compared for example with the caste system in India which made if much more difficult for the intelligent but poor to gain SES. Celibacy for priests have been invoked as an explanation for claimed lower IQ in Catholic countries, although this also seems to be contradicted by the equal IQ in northern and southern Europe. The earlier mentioned comparative European IQ study found that there was a larger variation in IQ scores in southern Europe. Possible explanations for the earlier mentioned difference for this include sample selection, larger environmental differences affecting IQ scores between urban and rural areas in southern Europe at the time of the test (1981), and/or that northern Europe became socially stratified later in history, causing less genetic variation in IQ.

Constant persecutions favoring a high IQ have been proposed as an explanation for the higher average Ashkenazi IQ, but other persecuted groups like the Romani do not score highly on IQ tests. Another theory suggests that there was selective breeding for Talmudic scholarship, but this seems unlikely to have been important because there weren't very many professional rabbis. A selective force that only affects a tiny fraction of the population can never be strong enough to cause important evolutionary change in tens of generations. A more plausible, but difficult to evaluate without detailed demographic information, variant of this is that achievement in Talmudic scholarship had high status and that rich families therefore preferred to marry their daughters to males who excelled in this. Yet another explanation, according to a 2005 study, the most likely, is that they mostly worked jobs in which increased IQ strongly favored economic success, in contrast with other populations, who were mostly peasant farmers. (See "Natural History of Ashkenazi Intelligence")

Comparison of explanations

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The Black-White IQ gap in the U.S. may be explained by a variety of explanations. In order to differentiate between alternatives, additional evidence is needed. The "partly-genetic" and "primarily-environmental" interpretations offer contrasting points of view on this evidence. Typically, however, the argument is over the amount of genetic contribution. "Hereditarian" scientists typically assert anywhere from 50% - 80% genetic determinism, whereas "environmental" scientists generally accept 20% or less genetic determinism in intelligence.

Other interpretations

The two most widely-known works concerning race and intelligence are The Mismeasure of Man by Stephen Jay Gould, originally published in 1981, and The Bell Curve by Richard Herrnstein and Charles Murray, published in 1994. Media controversy surrounding The Bell Curve motivated Gould to revise and expand The Mismeasure of Man to respond to arguments from The Bell Curve, publishing the book's second edition in 1996. Many current researchers think that both books are outdated due to new research.

A recent paper in the Psychological Review, "Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved" by William T. Dickens of The Brookings Institution and James R. Flynn presents a mechanism by which environmental effects on IQ may be magnified by feedback effects. This work may provide a resolution of the contradiction between the viewpoint of The Bell Curve's authors and the 'nurture' effects observed by others. A latter paper responded to objections.

For additional relevant discussion, see: http://www.skeptic.com/eskeptic/05-02-18.html Two book reviews, by Paul R. Gross and Alondra Oubre, of Sarich’s and Miele’s book, Race: The Reality of Human Differences.

Some cite research that they believe indicates that discriminated or lower-status minorities do tend to have lower IQ, some without apparent genetic differences. Like Blacks and Hispanics in the U.S., minorities in some societies show achievement gaps (such as the Māori in New Zealand, aboriginals in Australia, scheduled castes ("untouchables") in India, non-European Jews in Israel, and the Burakumin in Japan). The most prominent finding cited is that Northern Irish Catholics used to score about 15 points lower than Protestants. Similarly, Irish, Italian and Polish immigrants in the U.S. are reported to have all scored about 80 in the beginning of the 19th century, but now tend to reach 100. The same is true of persons from rural versus urban areas in general (see e.g., this article by conservative columnist and economist Thomas Sowell and this page on European and Greek IQ. More arguments of the kind are to be found here).

Opinions of scholars and others

A survey was conducted in 1987 of a broad sample of 1,020 scholars (65% replied) in specialties that would give them reason to be knowledgeable about IQ (but not necessarily about race; Snyderman & Rothman, 1987). The survey was given to members of the American Education Research Association, National Council on Measurement in Education, American Psychological Association, American Sociological Association, Behavior Genetics Association, and Cognitive Science Society. Political and social opinions, reported in the same survey, accounted for less than 10% of the variation in responses. (Respondents on average called themselves slightly left of center politically.) Measures of expertise or eminence accounted for little or no variation in responses.

One question was "Which of the following best characterizes your opinion of the heritability of the Black-White difference in I.Q.?" (emphasis original). The responses were divided into five categories:

• The difference is entirely due to environmental variation: 15%.
• The difference is entirely due to genetic variation: 1% (8 respondents).
• The difference is a product of both genetic and environmental variation: 45%.
• The data are insufficient to support any reasonable opinion: 24%.
• No response (or not qualified): 14%.

The age of the survey and the anonymity of the respondents could constrain its interpretation.

In a 1988 survey, journalists, editors, and IQ experts were asked their "opinion of the source of the black-white difference in IQ" Template:AYref

The view of the American Psychological Association

In response to the controversy surrounding The Bell Curve, the American Psychological Association's Board of Scientific Affairs in 1995 established a special task force to publish an investigative report on the research presented in the book.

The task force agrees that there do exist large differences between the average IQ scores of blacks and whites, and that these differences cannot be attributed to biases in test construction, nor does it "simply reflect differences in socio-economic status". While they admit there is no empirical evidence supporting it, the APA task force suggests that explanations based on social status and cultural differences may be possible. Regarding genetic causes, they noted that there is not much direct evidence on this point, but what little there is fails to support the genetic hypothesis. The January 1997 issue of American Psychologist included eleven critical responses to the APA report, most of which criticized the report's failure to examine all of the evidence for or against the partly-genetic interpretation of racial differences in IQ.

See also

• Biological determinism
• Cultural determinism
• Genetic determinism
• Nature versus nurture
• Social determinism
• Twin study
• Norms of reaction

Notes

1. ^ Language by Template:AYref, p. 311. For consensus statements see Template:AYref and Template:AYref. For survey data see Template:AYref.
2. http://www.ssc.uwo.ca/psychology/faculty/rushtonpdfs/PPPL1.pdf
3. http://taxa.epi.umn.edu/~mbmiller/journals/pppl/200504/2/302-2.html
4. ^ Template:AYref
5. Jencks, C., & Phillips, M. (1998). The black-white test score gap: An introduction. In C. Jencks and M. Phillips (Eds.), The black-white test score gap (pp. 1-51). Washington, DC: Brookings Institution. ". . . we find it hard to see how anyone reading these studies with an open mind could conclude that innate ability played a large role in the black-white gap."
6. Aristotle: "Having spoken of the number of the citizens, we will proceed to speak of what should be their character. This is a subject which can be easily understood by any one who casts his eye on the more celebrated states of Hellas, and generally on the distribution of races in the habitable world. Those who live in a cold climate and in Europe are full of spirit, but wanting in intelligence and skill; and therefore they retain comparative freedom, but have no political organization, and are incapable of ruling over others. Whereas the natives of Asia are intelligent and inventive, but they are wanting in spirit, and therefore they are always in a state of subjection and slavery. But the Hellenic race, which is situated between them, is likewise intermediate in character, being high-spirited and also intelligent. Hence it continues free, and is the best-governed of any nation, and, if it could be formed into one state, would be able to rule the world." (Aristotle, Politics, ch. 7).
   Cicero: "Do not obtain your slaves from Britain because they are so stupid and so utterly incapable of being taught that they are not fit to form a part of the household of Athens." Attributed to Cicero's Epistulae ad Atticum (Letters to Atticus), 68 BC-43 BC (latin text). Translation: Template:AYref.
   "Races north of the Pyrenees are of cold temperament and never reach maturity; they are of great stature and of a white colour. But they lack all sharpness of wit and penetration of intellect." Attributed to "Said of Toledo (a moorish savant)" by Template:AYref (p.34), originally quoted in Template:AYref.
7. Template:AYref; Template:AYref; Template:AYref
8. Template:AYref, p. 512
9. Joel Wiesen, "An Annotated List of Many Possible Reasons for the Black-White Mean Score Differences Seen With Many Cognitive Ability Tests: Notes to File," Applied Personnel Research, March 18, 2005.
10. Template:AYref, p. 510; Template:AYref
11. Greek IQ
12. Template:AYref, Template:AYref, Template:AYref, Template:AYref
13. Template:AYref
14. Template:AYref
15. Template:AYref concluded that "the gains cannot be explained solely by increases at the level of the latent variables (common factors), which IQ tests purport to measure"; in other words, some of the inter-generational difference in IQ is attributable to bias or other artifacts, and not real gains in general intelligence or higher-order ability factors, unlike the B-W IQ gap. An analysis by Template:AYref found that the IQ increases associated with the Flynn effect did not produce changes in g, which Rushton compares to the finding by Template:AYref that IQ increases associated with adoption likewise do not increase g. Template:AYref disagrees with Rushton's analysis.
16. Template:AYref and others find this hypothesis unsupported by the available evidence. Template:AYref respond to these criticisms.
19. National Center for Health Statistics: Vital Statistics of the United States (1988).
20. Scientific American, April 1996, p.25.
21. "Fat, Fitness And Performance," Peak Performance; I.B. Helland et al., "Maternal supplementation with very-long-chain n-3 fatty acids during pregnancy and lactation augments children's IQ at 4 years of age," Pediatrics 111, no. 1 (January 2003): 39–44.
22. "Myth: Social intervention cannot raise IQ; Intelligence Quotient, The Encyclopedia of Adoption.
23. Papers relating to group difference in IQ test scores & Spearman's hypothesis.
24. Joseph L. Graves, "What a tangled web he weaves: Race, reproductive strategies and Rushton's life history theory," Anthropological Theory 2, no. 2 (2002): 131–54; Leonard Lieberman et al., "How 'Caucasoids' Got Such Big Crania and Why They Shrank,"; Current Anthropology 42 (2001): 69–95; Zack Cernovsky, "On the similarities of American blacks and whites: A reply to J.P. Rushton," Journal of Black Studies 25 (1995): 672.
25. Greek IQ
26. Charles C. Roseman, "Detecting interregionally diversifying natural selection on modern human cranial form by using matched molecular and morphometric data" Proceedings of the national Academy of Sciences of the United States vol. 101 no. 35 (August 31, 2004):12824-12829.
27. http://www.mugu.com/cgi-bin/Upstream/People/Rushton/rushton-black-reply.html
28. http://ant.sagepub.com/cgi/content/abstract/2/2/131
29. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&list_uids=14992214&dopt=Citation
30. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1488860&dopt=Citation
31. http://www.mugu.com/cgi-bin/Upstream/miller-r-personality
32. http://www.mugu.com/cgi-bin/Upstream/Library/Miller/env-vary.html
33. http://www.ingentaconnect.com/search/expand?pub=infobike://els/10905138/2003/00000024/00000005/art00040&unc=
34. http://www.crispian.demon.co.uk/RUSHRV.htm
35. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=15638207&dopt=Citation
36. http://www.getcited.org/pub/103361483
37. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9626146&dopt=Citation
38. http://www.cdc.gov/nchs/fastats/pdf/nvsr52_10t50.pdf
39. http://www.azcentral.com/families/articles/0228fam_twins.html
40. http://links.jstor.org/sici?sici=0014-7354(200005%2F06)32%3A3%3C132%3AUOISIT%3E2.0.CO%3B2-Z
41. Gregory Cochran, Jason Hardy, Henry Harpending, "Natural History of Ashkenazi Intelligence," Journal of Biosocial Science (June 2005).
42. Template:AYref, Template:AYref.
43. Template:AYref
44. Template:AYref, Template:AYref, Template:AYref, Template:AYref, Template:AYref, Template:AYref: "If enough markers are used... individuals can be partitioned into genetic clusters that match major geographic subdivisions of the globe".
45. Template:AYref
46. Template:AYref.
47. Neil Risch argues: "One could make the same arguments about sex and age! . . you can undermine any definitional system. . . In a recent study. . . we actually had a higher discordance rate between self-reported sex and markers on the X chromosome between genetic structure versus self-description, 99.9% concordance. . . So you could argue that sex is also a problematic category. And there are differences between sex and gender; self-identification may not be correlated with biology perfectly. And there is sexism. And you can talk about age the same way. A person's chronological age does not correspond perfectly with his biological age for a variety of reasons, both inherited and non-inherited. Perhaps just using someone's actual birth year is not a very good way of measuring age. Does that mean we should throw it out? . . . Any category you come up with is going to be imperfect, but that doesn't preclude you from using it or the fact that it has utility" (Template:AYref).
48. Template:AYref cites Fisher RA. Statistical Methods and Scientific Inference. Edinburgh: Oliver and Boyd. 1956.
49. ^ How "Caucasoids" Got Such Big Crania and Why They Shrank, Lieberman 2001
50. ^ Cite error: The named reference cooper was invoked but never defined (see the help page).
51. Template:AYref, p. 5
52. Editorial: Census, race and science. Nat Genet 2000, 24:97-98.
53. Template:AYref
54. IQ studies by year from IQ and the Wealth of Nations by Lynn & Vanhanen
55. Jensen, Arthur R. (2000) A Fuzzy Boundary of Racial Classification Attenuates IQ Difference, Psycoloquy: 11,#22 Intelligence G Factor (34)
56. See note 3 above.
57. Murray, C. (2005) "The Inequality Taboo". Commentary Magazine, September 2005.
58. Template:AYref
59. Nature Reviews Neuroscience
60. http://www.loni.ucla.edu/~thompson/PDF/nrn0604-GrayThompson.pdf
61. Arthur Hu's Index of Diversity, Scholastic Aptitude Test.
62. "Myth: The black/white IQ gap is largely genetically caused.
63. Nature Reviews Neuroscience
64. D.M. Ivanovic et al., "Nutritional status, brain development and scholastic achievement of Chilean high-school graduates from high and low intellectual quotient and socio-economic status," British Journal of Nutrition 87, no. 1 (January 2002): 81–92; D.M. Ivanovic et al., "Head size and intelligence, learning, nutritional status and brain development. Head, IQ, learning, nutrition and brain," Neuropsychologia 42, no. 8 (2004): 1118–31.
65. Marinus H. van IJzendoorn, Femmie Juffer, and Caroline W. Klein Poelhuis, "Adoption and Cognitive Development: A Meta-Analytic Comparison of Adopted and Nonadopted Children’s IQ and School Performance," Psychological Bulletin 131, no. 2 (2005): 301–316.
66. William T. Dickens, "Behavioral Genetics and School Readiness," The Future of Children 15, no. 1 (Spring 2005): 55–69; "Intelligence: Knowns and Unknowns," Report of a Task Force established by the Board of Scientific Affairs of the APA.
67. Race and IQ: Molecular Genetics as Deus ex Machina, Richard S. Cooper

    The technical errors contained in Rowe’s (2005) article include both misuse of broad scientific concepts and incorrect or biased misinterpretation of specific scientific data. The author’s broad argument assumes that a quantity definable as “intelligence” exists (in contradistinction to the view that multiple types of cognitive functioning can be identified that are valued and manifested differently, conditional on the setting and the observer), that intelligence can be measured with “IQ tests,” that demographic groups known as “continental races” divide humans into discrete categories on the basis of important concordant variation in genetically determined traits, that molecular genetics can (or will) make it possible to define the architecture of complex traits in terms of “genes for X or Y” (i.e., “genes for intelligence”), and that significant variation in polymorphisms in those genes overlap with the traditional demographic categories, such as those promulgated by the U.S. government.

68. Abstracts from the Journal of American Indian Education 1987; Rhett Diessner and Jacqueline L. Walker, "A Cognitive Pattern of the Yakima Indian Students," Journal of American Indian Education 25, no. 2 (January 1986); "Smart Fraction Theory II: Why Asians Lag," La Griffe du Lion 6, no. 2 (May 2004).
69. Zack Cernovsky, "On the similarities of American blacks and whites: A reply to J.P. Rushton," Journal of Black Studies 25 (July 1995): 672.
70. Table 50 National Vital Statistics Reports Vol. 52, No. 10, December 17, 2003. CDC.
71. "What's in a yam? Clues to fertility, a student discovers," Yale Medicin Summer 99
72. William T. Dickens and James R. Flynn, "The IQ Paradox: Still Resolved," Psychological Review 109, no. 4 (2002).
73. Richard Herrnstein and Charles Murray, "Race, genes and I.Q.—an apologia: the case for conservative multiculturalism," The New Republic 211, no. 11 (October 1994): 27.
74. http://www.lrainc.com/swtaboo/taboos/apa_01.html

References

External links

Partly-genetic view

• Thirty Years of Research on Race Differences in Cognitive Ability J. Philippe Rushton & Arthur R. Jensen
• Black-White-East Asian IQ differences at least 50% genetic, scientists conclude in major law journal

Primarily-environmental view

• Heredity, Environment, and Race Differences in IQ Richard E. Nisbett
• Race and IQ: Molecular Genetics as Deus ex Machina Richard S. Cooper

Template:Race and intelligence

• Race and intelligence controversy