Race and intelligence (explanations)

Main article: Race and intelligence

The consensus among intelligence researchers is that IQ differences between individuals of the same race reflect (1) real, (2) functionally/socially significant, and (3) substantially genetic differences in the general intelligence factor. A consensus also exists for the view that average IQ differences between races reflect (1) real and (2) significant differences in the same g factor (Template:AYref, p. 311). However, it is a matter of debate whether IQ differences between races in the U.S. are (3a) entirely environmental or (3b) partly genetic.

Biological differences in brain volume and reaction time, which show low to moderate correlations with IQ, are not by themselves evidence for genetic differences. Even if these differences or the average IQ test score gap indicate a gap in actual intelligence, this may be due to environmental differences in factors such as nutrition during pregnancy or early childhood which may produce such differences without any genetic cause. In general, simple correlations cannot decide the role of genetics. Advanced statistical methods are instead used with hotly debated results.

Because the cause of the IQ gap is ultimately an empirical question, it should be possible to resolve this question in the future. Irrefutable direct evidence is currently lacking and may continue to be so until intelligence is mapped to specific genes.

Most research has been done in the US and a few other developed nations. That research cannot directly be generalized to the world as a whole. Blacks in the US do not constitute a random sample from Africa, and environmental conditions differ among nations. IQ tests done in developing countries are likely to have been affected by conditions associated with poverty that are common in the developing world, such as nutritional deficiencies and the impact of diseases (e.g., HIV, anemia or chronic parasites that may affect IQ test scores).

A recent review summarizing the arguments for a partially-genetic explanation can be found here [1]. A critique can be found here [2].

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 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 Black-White gap (Template:AYref).

The lack of test bias 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."

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 is seemingly discredited by findings promoted by the researcher Arthur Jensen (1998) using elementary cognitive tasks to measure intelligence. 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 (or an advantage to Blacks), 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. The correlation between IQ and reaction time is low (from .20 to .40). A review by Template:AYref that combined several studies with six measures produced a multiple correlation of reaction time to IQ of .67. This correlation is within the range of correlations between different kinds of IQ tests.

Socio-economic factors

Family income and parental education are positively correlated with SAT scores. However, the score gap between races persists at all levels of family income and parental education. For example, in 1995 Black students from the highest income group have lower average SAT scores than White students from the lowest income group.

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 SAT scores that are slightly smaller but still persist for individuals with similar family income and parental education. This stability has suggested alternative explanations:

• Some argue that Blacks are discriminated against such that they must have a higher or at least equal intelligence in order to achieve the same socioeconomic status (SES) as Whites. One should then expect that Black children should have a higher or equal IQ compared to children from Whites with the same SES. That they score lower on SAT tests can thus be interpreted as evidence for strong adverse influence from environmental factors different from SES or from SES factors other than income and parental education, like systematic discrimination discouraging school and achievement motivation and learning or cultural differences in nutrition like duration of breastfeeding.
• It is possible that Black and Hispanic parents achieve higher SES with lower intelligence; perhaps by having (on average) greater amounts of a compensating character, or through affirmative action. However, affirmative action has lts largest effect on young people newly employed with lower income.
• Another alternative explanation is that by comparing the SES of parents to the intelligence of their children, the score gap shown here reflects regression towards different average racial scores from one generation to the next; a partly-genetic origin of intelligence differences would predict this effect.
• SAT scores correlate fairly well with IQ scores but they are not the same and may measure different things.

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 .

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. 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 or Japanese develop spatial reasoning better than the alphabetic writing systems prevalent in Europe and America. The same reasoning has been used to explain why students from Asia-Pacific countries (e.g., Singapore, South Korea) 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. It is not known how many of the East Asians in the U.S. understand logographic writing systems.

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 logographic 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.

Some think that there are no genetic differences and that the Flynn effect will eliminate differences in IQ test scores in the future. They argue that the Flynn effect started sooner and will end sooner for the more affluent parts of society and that Blacks sooner or later will close the gap. They note that the effect is very large over time, one estimate is that the average IQ in the U.S. below 75 before the Flynn effect started.

However, 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).

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 well-studied by researchers doing meta-analyses that combine multiple sources of primary materials.

Arthur Jensen and others have concluded that the 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. Jensen and colleagues reach this conclusion based on an evaluation of some of the following evidence. While accepting that some fraction of the supporting evidence may be false, they argue that the partly-genetic hypothesis is favored over the culture-only hypothesis by a preponderance of the evidence (Template:AYref; Template:AYref). 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 an 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. However, some argue that the higher average IQ of East Asians than Whites is anomalous for an environment-only theory of IQ differences (Template:AYref).

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

Rushton's ordering of the human races (excerpt)
	Blacks	Whites	Orientals¹
IQ²	85	100	106
Cranial capacity (cm³)	1267	1347	1364
Lifespan	Lower	Intermediate	Higher
Skeletal development	Earlier	Intermediate	Later
Gestation time	Shorter	Longer	Longer
2-egg twinning³	16	8	4
Intercourse frequency	Higher	Intermediate	Lower
Law abidingness	Lower	Intermediate	Higher
Cultural achievements	Low	High	High
¹ See Notes ² All figures are group averages ³ Per 1000 births
(See full table)		Source: [3]

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.

File:R&A.gif

Template:AYref, Fig. 4, plotted the cranial capacities and approximate time of evolutionary emergence of the three major human races along with those of some of humans' evolutionary ancestors (above). The relationship is cited as some of the evidence for a partly genetic explanation by Template:AYref.

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 [4]. There have also been many other criticisms of the theory [5][6][7][8][9][10][11]. More recent studies contradicts Rushton's claims. A meta-analysis shows that blacks are not more psychopathic [12], nor do they differ in from whites when testing for the big five personality traits [13], differences in sex hormones between whites and East Asians are best explained by environmental differences [14], and the fundamental prediction of the theory that blacks have a higher frequency of twins is incorrect [15]. 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,[16] both demographic characteristics that are more common among Whites.[17]

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 interpretations

The Black-White IQ gap in the U.S. may be explained by a variety of interpretations. In order to differentiate between alternatives, additional evidence is needed. The "partly-genetic" and "culture-only" interpretations offer contrasting points of view on this evidence.

Partly genetic	Culture only
Black-White-East Asian differences in IQ, reaction time, brain size and other physiological variables (such as skull structure, and degree of convolution of the brain) in the United States and a few other developed countries (e.g. UK, Japan). Larger brain size and higher IQ of East Asians than Whites is a challenge for the culture-only theory. Genetics are known to influence brain structure (Template:AYref) and some aspects of cognition (Template:AYref). Brain size correlates with intelligence (Template:AYref), and that correlation is mediated entirely by genetic factors (Template:AYref).	The gaps may be explained by many other factors except genetics (see above). Differences among some White groups are as large as the difference between Whites and Blacks in the US. Many racial groups show great variation when tested at different times and in different places, indicating large environmental influences. The IQ scores and larger brain size of East Asians have numerous possible explanations, e.g. many East Asian nations have a very high consumption of fish. One study has shown larger head size at birth and higher IQ scores at 4 years of age when the mothers took fish oil supplement during pregnancy and lactation.
Black-White-East Asian differences in culture-fair and reaction-time IQ test scores exist world-wide despite international differences in social, cultural, and economic conditions. Template:AYref, among others, argues that higher IQ scores among East Asians (living in East and South Asia) than Whites (living in North American and Europe) is a challenge for culture-only theories because standards of living in Asia are lower than or equal to those in North America or Europe. For example, average IQ scores are higher in the People's Republic of China (Template:AYref) than for African Americans even though per capita GDP (PPP) is lower in China ($5,000 as of 2003) than per capita African American income ($15,583 as of 2003) (Template:AYref). {Template:A(Y)ref discusses his observation among California children that very low IQ Blacks are qualitatively normal in social and motor skills, but perform no better than Whites with equally low IQ on cognitive tasks except those that require rote memorization, where "mentally retarded" Blacks do significantly better than Whites. He speculates that 12.5% of cases of IQ <70 are due to organic defects in Blacks, compared to 50% in Whites, giving the impression that low IQ Whites are more handicapped than low IQ Blacks.	The only nationwide IQ tests have been done in a few developed countries, and the few studies in other nations have been severely criticized, see IQ and the Wealth of Nations. One argument of many against the reliability of the IQ scores in developing nations is that some such countries a majority of the population would be classified by the IQ scores as mentally retarded. For example, Equatorial Guinea is one of the few African nations that actually have a study and is classified as having the lowest average IQ in the world, 59, based on study of 48 persons 10-14 years old. A large proportion of the population should also be classified as moderately (<16%) and severely (<2%) mentally retarded. In the U.S., the moderately mentally retarded require moderate supervision and the severely mentally retarded often have other physical disabilities and may thus require constant supervision, be unable to provide for themselves, be unable to speak long sentences, and, in many cases, be unable to do things like getting dressed without help.
The Black-White IQ gap in the US has remained constant at approximately one standard deviation since it was first measured in the early 1900s despite social and economic change during that time, including the civil rights movement and Brown v. Board of Education. Recent changes in skill test score gaps do not indicate changes in g.	Other studies show that the gap in the US is narrowing. For example, one large recent study found much smaller differences than earlier studies in math and reading skills in young children and found that all of the remaining differences could be explained by a few environmental factors. Moreover, the extent of concrete social and economic change is debateable; for example, Jonathan Kozol, in his 2005 book Shame of the Nation, found that public schools are more racially segregated today than they were in 1969.
The IQ gaps remains approximately constant as individuals age. Some environmenal theories would predict a growing or shrinking IQ gap during development.	Many environmental factors, like nutrition during pregnancy and lactation, can have a life long effect. Jensen (1998) reports some evidence of decreasing IQ among African American boys at adolescence.
Correlations between an IQ subtest's heritability or inbreeding depression and the magnitude of the Black-White-East Asian score gap for that subtest (Template:AYref). Environmental theories would predict the opposite.	What subtest are most heritable and how to measure this is debated.
Correlations between an IQ subtest's g-loading, and the magnitude of the Black-White-East Asian score gap for that subtest (Template:AYref; Template:AYref pp. 47-8; Template:AYref pp. 138-9). For example, the Black-White gap is greater on backward digits span (a test where subjects repeat digits in the reverse order that they are given, and the more g-loaded test) than forward digits span (a test where subjects repeat digits in the same order that they are given, and the less g-loaded test). As predicted by Spearman's hypothesis, the B-W gap is largest on the most g-loaded tests. Narrowing of the B-W gap has been seen mostly on less g-loaded tests, such as literacy tests. A culture-only hypothesis cannot explain why the B-W gap is greater on one version of the test than another.	g-loading and the method of correlated vectors, the statistical method used in many older studies, has been criticized heavily in recent research as discussed previously.
Rising heritability of IQ with age, and decreasing shared-family effects (e.g., socioeconomic factors) on IQ after adolescence. An environmental cause of the IQ gap would necessarily be a shared family effect. Lack of shared family effects on adult IQ is a challenge to culture only theories.	High within-group heritability does not logically exclude the all environmental interpretation.
Studies suggesting that IQ heritability and gene-environment interactions within races are the same for Blacks and Whites. That is, no race-specific statistical factors, such as an effect of White racism, have been identified in such analyses. The IQ gap exists even among middle- and upper-class Black and White families where within-race heritabilities are high and shared family effects are near zero.	Many factors that can affect IQ differ between Blacks and whites, for example duration of breastfeeding or poverty. Many older studies have only studied middle class families but SES has recently been shown to be relatively more important in poorer families.
The finding that when Black and White children are matched for IQ, their siblings tend to have IQs that regress towards different means (85 for Blacks and 100 for Whites). For example, among Black and White children matched with an IQ of 120, the siblings of the Black children have an average IQ of 100 whereas the siblings of the White children have an average IQ of 110. This is a stronger test of the party-genetic hypothesis than regression from parents to offspring because siblings share a similar environment (Template:AYref). This is a novel prediction of the partly genetic hypothesis.	Regression towards the mean only shows that mean IQ scores are different which is not a new finding. That is not evidence that the cause of this difference is genetic.
American Blacks have a lower average IQ than Hispanic and Native American groups, which are more socio-economically deprived. For example, the Inuit, who live in the Arctic, have higher average IQs than North American Blacks (Template:AYref; Template:AYref) despite being extremely poor (Template:AYref; Template:AYref).	That Blacks are less socioeconomically deprived than Hispanics or Native Americans in the US is controversial. The Inuit cannot be directly compared to the US population, for example they have substantially different nutrition from eating large amounts of fish.
Theories holding that Blacks score lower than Whites because of test bias would predict that Asians would also score lower than Whites. However, the reverse is true.
The three-way differences in the IQ and SAT scores of children persists even after controlling for parental income or education, which seems to counter arguments that the gap is due to socioeconomic conditions. In addition, some researchers have argued from studies in siblings that IQ affects socioeconomic status, rather than the other way around (Template:AYref). Studies which simultaneouly control for dozens of social and economic conditions are uninformative because they assume that such differences are the cause rather than the consequence of IQ differences (Template:AYref; Template:AYref). SAT scores correlate with IQ as well as scores from one IQ test correlate with another (Template:AYref.	Adjustments for socioeconomic conditions almost completely eliminate differences in IQ scores between black and white children. The remaining difference is statistically insignificant. SAT scores are not the same as IQ scores.
Ashkenazi Jews have often been persecuted and discriminated against, but they still display the highest average IQ of any ethnic group, as well as SAT scores higher than those of non-Jewish Caucasians. Template:AYref, pp. 67-68 argues that this counters arguments that depressed IQ scores of African Americans are due to discrimination or prejudice.	Persecution of and discrimination against Jews was strongest in the past, while Blacks are still being discriminated against in various ways today.
The three-way difference in average IQ can be measured in very young children and before the start of schooling. For example, a one standard deviation gap is observed in Black and White 3-year olds matched for gender, birth order, and maternal education (Template:AYref). Template:AYref found that by age 6 the average IQ of East Asian children is 107, 103 for White children and 89 for Black children. Template:A(Y)ref found that the same trichotomy in brain size and IQ held at 4 months, 1 year, and 7 years of age.	Environmental factors can affect very young children, for example nutrition by the mother during pregnancy and breastfeeding. More breastfeeding gives IQ gains and the duration of this is known to differ between White and Black mothers. Two studies in Chile shows that nutritional status affects IQ, scholastic achievement, and brain volume.
Three-way differences in reaction times have been demonstrated (Template:AYref; Template:AYref; Template:AYref; Template:AYref), and it is difficult to explain differences in reaction time through lack of motivation or cultural differences on the part of the subjects. Reaction times correlate with g (Template:AYref).	Differences in reaction time or brain volume may be caused by environmental factors. As noted, there have been large changes in cranial vault size and shape during the last century in the US for both Black and Whites, far beyond what can be explained genetically. In addition, reaction time and brain size have only a low correlation with IQ test scores and are not shown to have any real-world significance.
A relationship between times of evolutionary emergence and brain sizes for East Asians, Whites, and Blacks. (Template:AYref)
Racial differences in biological characteristics such as myopia that correlate with g. Asians and Jews are more myopic than Whites, who are more myopic than Blacks. Jews are about twice as myopic as Gentiles. Myopia is likely pleiotropic with g (i.e., myopia and g are caused by the same genes). Arthur Jensen argues that this supports the partially-genetic explanation. (Template:AYref, p. 487-489)
Average Black-White-East Asian differences in IQ (both positive and negative) remain following transracial adoption. Three studies of East Asian children adopted by White familes reported average IQ scores in the adopted East Asian children that are as high or higher than Whites, despite the fact that some of the children in the studies had suffered some forms of preadoptive deprivation or malnutrition and associated developmental delays (Template:AYref; Template:AYref; Template:AYref). Template:A(Y)ref found Black-White-East Asian differences in cognitive and psychological variables among adolescents adopted by white families. They also found that Black-White mixed-race children fell in between the White and Black averages. See also the Minnesota Transracial Adoption Study. The higher average IQ of adopted East Asian children is a challenge to the culture-only theory.	Existing adoption studies lack behavior genetic controls needed to distinguish between genetic and environmental effects. Several other adoption studies finds no IQ difference between Whites and East Asians.
No studies of Black-White genetic admixture have been performed with the multi-locus DNA sequencing required to make reliable conclusions.	IQ have very low positive to low negative correlation with Whiteness of skin, degree of European blood groups, or self-reported degree of European ancestry among Blacks.
No studies that make use of proper behavior genetic techniques have been able to identify environmental factors to explain the IQ gap.	A study which showed near-disappearance of the black-white gap among children of black and white servicemen raised by German mothers after World War II. Some, like the American Psychological Association, consider this study be strong evidence against the genetic explanation.
Average IQ scores gaps within the U.S. and internationally have been stable since they were first measured in the early and mid 20th century (Template:AYref; see above), despite the Flynn effect. One statistical analysis suggests that the Flynn effect is qualitatively different than the Black-White IQ gap (Template:AYref).	One estimate is that the average IQ in the U.S. was below 75 before the Flynn effect started and it seems likely that the effect started earlier and may end sooner for Whites. Some studies show that the IQ gaps is decreasing in the US and even if they are not this may change in the future if the Flynn effect ends first for Whites. Lynn and Vanhanen only have IQ scores from 3 developing nations before 1950, in two of these only 1 study, make any claims of knowing average continental IQ strange indeed. The Wicherts study refers to "measurement invariance", is not a statement about the role of genetics in the B-W gap, and is a relatively minor statement that not mentioned in the abstract.
Template:AYref argues that culture-only explanations cannot explain why East Asians score higher on tests of spatial reasoning than verbal reasoning.	Dichotomy in intelligence is entirely compatible with an all environmental explanation but may not be with the g theory. East Asians may have higher spatial ability for example due to their knowledge of iconographic languages.
A similar dichotomy in spatial/nonspatial intelligence test scores is present in both East Asians and several Native American and Inuit populations (Template:AYref; Template:AYref; Template:AYref; Template:AYref; Template:AYref Template:AYref).
Template:AYref, pp. 827-828, argues that culture-only explanations cannot explain the pattern of IQ development in East Asian children where IQ development intially slower than Caucasians is followed by IQ development that later catches up to Caucasians.
Partly-genetic theory is predicated on a model that the IQ gap has (the genetic) part of its origin in human evolution. Thus, it predicts that the Black-White-East Asian differences in average IQ, reaction-time, and brain size should be accompanied by a similar pattern of differences in other inherited traits. Proponents cite three-way average differences such as personality, maturation, and reproductive traits as support of this prediction; Template:AYref, p. 273, cite a "matrix of 60 life-history traits". Research on racial differences in twining and testis size was the subject of a review by author and scientist Jared Diamond (1986) in the journal Nature, in which he investigated correlations between possible racial variations in testicular size and hormone levels and found one small study suggesting that dead Danish men on autopsy have larger testicles than dead Chinese men. Some studies also suggested lower hormone levels and frequency of twins among Asians than Africans. Template:A(Y)ref has additionally pointed to the Black-White-East Asian gradient in average hip size, arguing that nothing can explain this phenomenon except the need to give birth to children with different brain sizes.	Differences may have environmental causes and may be unrelated to one another. Many of these claims of differences have been shown to be false, for example references to scientific literature with respects to racial differences in sexual characteristics turned out to be references to a nonscientific semipornographic book and to an article in the Penthouse Forum.. Regarding Jared's study, he notes that smaller testicle size among Koreans was not associated with a lower frequency of sexual intercourse, which directly contradicts Rushton rK-theory. There is only an insignificant difference in frequency of twins between Whites and Blacks in the U.S (34.8 vs 34.7), also contradicting the theory. Higher frequency of twins in certain African populations can be explained by large scale consumption of Yam which in rats produces such results.

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.

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 Maori 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%.

A selection of survey results

Question	Responses
What heritability would you estimate for IQ differences within the White population?	Average estimate of 60 (± 17) percent.
What heritability would you estimate for IQ differences within the Black population?	Average estimate of 57 (± 18) percent.
Are intelligence tests biased against Blacks?	On a scale of 1 (not at all or insignificantly) to 4 (extremely), mean response of 2 (somewhat).
What is the source of the average Black-White difference in IQ?	Both genetic and environmental (45%, or 52% of those responding).

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

Group	Entirely Environment	Entirely Genetic	Both	Data Are Insufficient
Journalists	34%	1%	27%	38%
Editors	47%	2%	23%	28%
IQ Experts	17%	1%	53%	28%

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. [18]

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 Janurary 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

References

Main article: Race and intelligence (References)

External links

Partially-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

Culture-only view

• Heredity, Environment, and Race Differences in IQ Richard E. Nisbett