Black–White IQ differences are found worldwide

National IQs have been reported for 192 countries around the world [30,32]. The results show that the average IQ for East Asians centers around 106; for Whites, about 100; for US Blacks about 85, and for sub-Saharan African Blacks about 70 (Fig. 1). The same rank-order of race differences is found for ‘‘culture-fair’’ tests and reaction-time measures. Reaction-time tasks are so easy that all children can do them in less than one second [25,26]. More intelligent children, measured by conventional IQ tests, perform faster on these tasks. East Asians average faster reaction times than Whites who, in turn, have faster reaction times than Blacks.

Mean IQs differ much less within major population groups (that is, races) than between them. Whites have IQs close to 100 whether they live in Europe, Canada, Australia, New Zealand, or South Africa, whereas Blacks in sub-Saharan Africa have IQs closer to 70 regardless of whether they live in East, West, Central, or Southern Africa – or whether the data were collected in the 1920s or the 2000s. This worldwide pattern contradicts the hypothesis that the low IQ of American Blacks is due to the legacy of slavery, segregation, and ‘‘White racism.’’ Many of the African countries showing a mean IQ of 70, such as Nigeria and Ghana, have been independent for half a century, and the Caribbean island of Haiti for two centuries. However, there has been no documented improvement in cultural achievement or in IQ scores.

Currently, the existence of the 15- to 18-point IQ difference (1.1 standard deviations) between Blacks and Whites in the US is not in itself a matter of empirical dispute. Only its explanation is under discussion. For example, Herrnstein and Murray [18] analyzed data from the 12-year National Longi- tudinal Survey of Youth. They found that most 17-year-olds with high scores on the Armed Forces Qualification Test (AFQT), regardless of ethnic background, went on to occupational success by their late 20s and early 30s. Those with low scores were more inclined to welfare dependence. The study also found that the average IQ for African Americans was lower than those for Latino, White, East Asian, and Jewish Americans (85, 89, 103, 106, and 113, respectively). Similarly, a meta-analytic review by Roth et al. [39] confirmed the 1.1 standard deviation Black–White IQ difference for a total sample of 6,246,729 corporate, military, and higher education testees.

Nor can there be doubt that the average African IQ of 70 is reliable and not due to a ‘‘fluke,’’ or to sampling error, or to the prejudice of investigators. Lynn [30] reviewed over two dozen studies from West, Central, East, and Southern Africa and consistently found an average IQ of 70. For example, in Kenya, Robert Sternberg et al. [61] administered the Colored Progressive Matrices to 85 12- to 15- year-olds who scored an IQ equivalent of 70. In Tanzania, Sternberg et al. [60] gave the Wisconsin Card Sorting Test to 358 11- to 13-year-old; they received a score equivalent to the 5th percentile on American norms (that is, IQ = 75). After training on how to solve problems such as those on the test, the children’s scores improved, but only to about the 9th percentile on American norms (IQ < 80).

It is also generally agreed that because test scores provide the best predictors of educational and economic success, average group differences have important societal outcomes. Further, the Black–White IQ difference shows up before 3 years of age on most standardized test batteries, even after matching on maternal education and other variables. Therefore the race differences are not due to poorer educational opportunities since this has not yet begun to exert an effect. (The East Asian IQ advantage appears by five years of age.) Because the same differences are found on relatively culture-free tests, and because the tests show similar patterns of internal item consistency and predictive validity for all groups, many psychometricians have concluded that the tests are valid measures of racial differences.

In Africa too, IQ scores are demonstrably valid. For example, Kendall et al. [28] showed that test scores predicted school grades and job performance equally well for Africans as for non-Africans (i.e., 0.20 to 0.50). Similarly Sternberg et al.’s [61] study of Kenyan 12- to 15-year-old found that IQ scores predicted school grades with a mean r= 0.40. In Rushton et al.’s [53,54] studies of African and non-African university students, scores on one IQ test correlated with scores on another IQ test 3 months earlier (0.60 for Africans; 0.70 for non-Africans) and with end-of-year-exam marks measured 3 months later (0.34 for Africans; 0.28 for non-Africans). The only demonstrated reliable example of bias is the rather obvious one of vocabulary for groups whose first language is not English. Even here, however, language accounts for only about 7 IQ points (out of the 30-point difference).

Race differences are most pronounced on the more g-loaded components of tests

Charles Spearman [59] coined the term g for the general factor of mental ability (or ‘‘general intelligence’’). A test’s g loading is the best predictor not only of school grades and workplace performance, but also of all the other indicators and correlates of intelligence – including biological variables such as brain size, reaction times, and heritability estimates as calculated from twin studies [25]. Race differences are repeatedly found to be higher on more g-loaded tests. g is the ‘‘active ingredient’’, if you will, of IQ scores, and is embedded to a greater or lesser extent in every intelligence test question. Because Blacks average lower scores on the more g-loaded tests, the Black–White-East Asian IQ differences are unlikely to be the result of any idiosyncratic cultural peculiarities in this or that test. They are more likely to be due to heredity. True, test constructors could, in principle, reduce the Black–White difference to zero (or even reverse it) by including only non-g items, or those negatively loaded on g, but the tests would then have no predictive power.

Studies in Southern Africa have also found that the race differences are mainly on the g factor. Lynn and Owen’s [31] analysis of thousands of high school students and Rushton et al.’s [53] analyses of hundreds of university students consistently found the African-White differences were mainly on g. So too were the differences between Africans, Whites, East Indians, and Coloreds. Importantly, g loadings calculated from the East Indian sample predicted the magnitude of the differences between Africans and Whites, which indicates the remarkable generalizability of the phenomenon. In one study Rushton and Jensen [50] found 77% of the African-White difference was on the g factor.

The gene–environment architecture of IQ is the same in all races

Studies of Black, White, and East Asian twins have shown that the heritability of IQ is about the same in all races (50% or higher). There has been no indication of any special cultural influence – such as extreme deprivation, or being raised as a visible minority – at work in one group and not in the others. If poverty, slavery, and White racism had operated to suppress Blacks’ natural levels of intelligence, it would make the heritability of their IQ scores substantially lower than the heritability for Whites. When tested empirically by comparing several hundred pairs of Black and White twins aged 12- to 18-years on the Basic Test Battery, the Primary Mental Abilities test, and the Cattell Culture Fair Intelligence test, Osborne [36] found heritabilities of about 50% in each group (The heritabilities in the Basic, Primary, and Cattell tests were, respectively: Whites – 0.61, 0.37, and 0.71; Blacks – 0.75, 0.42, and 0.19.).

The genetic basis of Black–White differences is also shown by the differences being more pro- nounced on the more heritable components of tests. For example, Jensen [22] calculated the environmentalities (that is, a measure of the non-genetic component) for 16 tests and found that they were inversely related to the magnitude of the Black–White differences ® = 0.70; P < 0.05). Rushton [43] found a correlation of r = 0.48 ( P < 0.05) between genetic influence on 11 tests estimated from inbreeding depression in cousin marriages in Japan and Black–White differences. Rushton et al. [49] calculated the genetic influence on each of the items of the Raven’s tests using raised-apart-twin similarities and found, in 55 comparisons including several independent samples of African high school and university students, that the European–African differences were consistently larger on the more heritable items.

Another way to test the hypothesis that there is some special ‘‘Factor X ’’ that lowers the IQ of Blacks is to compare the similarity of the correlations between background variables (such as the home environment and the peer group) and out- come measures (such as scholastic achievement and delinquency rates). If a Factor X exists, some of these correlations should be lower for Blacks. A series of studies on some very large samples have tested this hypothesis and disconfirmed it
[41]. For example, Rowe et al. [42] examined test scores for 8528 Whites, 3392 Blacks, 1766 Hispanics, and 906 Asians and found the exact same relation of background variables and outcome variables in each race; there was no evidence of any special factor that acted systematically to lower the IQ scores of Blacks.

Brain size differences

Larger brains are more intelligent because they contain more neurons and synapses and can process information more efficiently. Two dozen studies using magnetic resonance imaging (MRI) have shown that brain size is related to IQ differences (within-race) with a correlation of about 0.40. This is much higher than the 0.20 correlation found using indirect head size measures, though the lat- ter correlation is also reliable and significant. A functional relation between brain size and cognitive ability has been found in four studies finding that the correlation between brain size and IQ holds true within-families as well as between families [4,15,23,24]; although one study failed to do so: [56]. The within-family finding is of special interest because it controls for most of the sources of variance that distinguish families, such as social class, styles of child rearing, and general nutrition, that differ between families. The largest of these studies measured head size at birth and IQ at age 18 at the time of conscription in the Swedish military [4]. Analyses were made of data on 96,189 males who had at least 1 full brother similarly measured.

Even before birth, race differences in brain size can be observed. Schultz [57] found that from the 9th week of intrauterine life, 165 Black fetuses averaged a smaller brain case and larger face than did 455 White fetuses. The differences became more prominent over the course of fetal development. From birth to age 7 years, race differences were found for measures of head circumference and IQ from 40,000 children in the US National Collaborative Perinatal Project [6,47]. The results showed that at birth, 4 months, 1 year, and 7 years, the East Asian children averaged larger cranial volumes than White children who averaged larger cranial volumes than Black children (Fig. 2 ). Within each group, children with larger head sizes obtained higher IQ scores (mean r = 0.20). Moreover, since the East Asian children, who averaged the largest crania, were the shortest in stature and the lightest in weight, and the Black children, who averaged the smallest crania, were the tallest in stature and the heaviest in weight, the differences in brain size were not due mere correlates of body size.

Dozens of studies from the 1840s to the 1990s, using four different methods of measuring brain size – MRI, endocranial volume measured from empty skulls, wet brain weight at autopsy, and external head size measurements – all yield similar results. Using MRI, for example, Harvey et al. [17] found that 41 Africans and West Indians in the United Kingdom had a smaller average brain volume than 67 Caucasians. In another British study, Jones et al. [27] found Whites averaged a 30 cm (3) larger cranial volume and smaller ventricles than Afro-Caribbeans.

As far back as 1849, the American anthropologist Samuel George Morton [35] filled over 1000 skulls with packing material and found that Blacks averaged about five cubic inches less cranial capacity than Whites. These results have been confirmed [16,58,65]. The largest study of race differences in endocranial volume was by Beals et al. [2] with measurements of up to 20,000 skulls from around the world. They found that East Asians, Europeans, and Africans averaged cranial volumes of 1415, 1362, and 1268 cm (3), respectively. Also in the 19th century, Paul Broca [5] measured brain weight at autopsy and reported that not only did Whites average heavier brains than Blacks, but also they had more complex convolutions and larger frontal lobes. These results have likewise been replicated with several studies finding an average Black–White difference of about 100 g [3,33,37,66]. In a study of 1261 American adults, Ho et al. [19] found that 811 White Americans averaged 1323 g and 450 Black Americans averaged 1223 g. Since the Blacks and Whites were similar in body size, differences in body size cannot explain away the differences in brain weight.

Cranial volume has also been estimated from external head size measurements (length, width, height). For example, Rushton [44] examined head size measures in 24 international military samples collated by the US National Aeronautics and Space Administration (NASA) and, after adjusting for the effects of body height, weight, and surface area, found cranial capacity for East Asians was 1460 cm 3 and for Europeans, 1446 cm 3. Rushton [45] also calculated average cranial capacities for East Asians, Whites, and Blacks from a stratified random sample of 6325 US army personnel and found an average of 1416, 1380, and 1359 cm (3), respectively. This study allowed precise adjustments for all kinds of body size measures. Yet adjusting for these did not erase the differences in cranial capacity.

Summarizing the world literature on race differences in brain size for the three major methods (autopsies, endocranial volume, and head size measures) as well as head measurements corrected for body size, gives results (in cm 3 ): East Asians = 1364; Whites = 1347; and Blacks = 1267. The overall mean for East Asians was 17 cm 3 more than that for Whites and 97 cm 3 more than for Blacks. Within- race differences due to differences in method of estimation averaged 31 cm 3. Since one cubic inch of brain matter contains millions of brain cells and hundreds of millions of synapses or neural connections, race differences in average brain size may explain their differences in average IQ.

Trans-racial adoption studies

Trans-racial adoption studies provide one of the most powerful methods for studying race differ- ences. They are the human analog to the cross-fostering method commonly used in animal research. Human adoption is clearly a massive environmental intervention.

Studies of Korean and Vietnamese children adopted into White homes show that although as babies many had been hospitalized for malnutrition, they nonetheless grew to have IQs 10 or more points higher than their adoptive national norms [10,14]
.
By contrast, Black and mixed-race (Black–White) children adopted into White middle-class families have lower average scores than the White siblings with whom they had been raised or than with other White children adopted into similar homes. The Minnesota trans-racial adoption study is the largest and the best-known of these studies and the only one that includes a longitudinal follow-up, with testing of the same children at ages 7 and 17 years [55,67]. The study compared the IQ and academic achievement scores of Black, White, and mixed-race children who were adopted into upper-middle-class White families in Minnesota whose mean IQ was 120 (much higher than the population Black adoptee’s mean I of 100). The biological children of the adopting parents were also tested.

The first testing of 265 children was carried out in 1975 when they were 7 years old and the second in 1986 when the children were 17 years old. Table 1 gives the results. The evidence for the genetic theory grew stronger as the children grew older. At age 17 adopted White children had an average IQ of about 106; mixed-race adoptees, an IQ of 99; and adopted Blacks, an IQ of 89. Although the Black adoptee’s mean IQ of 89 was slightly above the national Black mean of 85, it was not above the Black mean for Minnesota. Further, school grades, class ranks, and aptitude tests all showed this same pattern. Growing up in a White middle- class home produced little or no lasting increase in the IQs of Black children.

Racial admixture studies

Dozens of studies have found that lighter-skinned African Americans have average IQs higher than their darker-skinned counterparts [41]. For example, Lynn [29] examined the National Opinion Research Center (NORC) survey of a representative sample of the adult population. The 442 Blacks were asked whether they would describe themselves as ‘‘very dark,’’ ‘‘dark brown,’’ ‘‘medium brown,’’ ‘‘light brown,’’ or ‘‘very light.’’ The correlation between these self-ratings and a 10-word vocabulary test score was 0.17 (P < 0.01). Rowe [40] examined the National Longitudinal Study of Adolescent Health and found the Black adolescents averaged a lower verbal IQ than the White adolescents. The mixed-race mean fell between those of the other two groups.

The Black American average IQ of 85 (15 points higher than the sub-Saharan African average of 70) is also consistent with the genetic hypothesis because genetic analyses estimate an average of 20–25% White admixture in African Americans [8]. Also, the mixed-race ‘‘Colored’’ population of South Africa has an average IQ of 85, intermediate to the respective African and White means of 70 and 100 [48]. These findings are not due to light- er-skinned Blacks being treated better through ‘‘expectancy effects’’ or ‘‘labeling theory.’’ In the Minnesota trans-racial adoption study, for example, some children were misclassified, with their adoptive parents wrongly believing that the mixed-race children had two Black biological parents. Yet these children averaged the same IQs as those of other mixed-race children correctly be- lieved by their adoptive parents to have had one Black and one White biological parent [55].

Early studies of brain weight data fit with the genetic admixture hypothesis as well. Bean [3] found, as did Pearl [37], that the greater the amount of White admixture (judged independently from skin color), the higher the mean brain weight at autopsy in Black groups. Subsequently, Rushton [47] examined 37 East Asian–European hybrids from the US National Collaborative Perinatal Project and found that they fell intermediate in brain size and IQ to the non-mixed parental groups.

Blacks and Whites regress toward their predicted (and different) means

Basic genetic theory predicts that the IQ of off-spring will regress towards the mean IQ of the population group from which the parents come. This has been amply documented for a number of physical traits in humans and in other species. Regression-to-the-mean is seen when individuals with high IQ scores mate. Their children tend to show lower scores than their parents. The converse happens for low IQ parents; they have children with somewhat higher IQs. This is because the parents pass on some, but not all, of their exceptional genes to their offspring. It is analogous to rolling a pair of dice and having them come up two 6’s or two 1’s. The odds are that on the next roll, you will get some value that is not quite as high (or as low). Genetic theory predicts the precise magnitude of the regression effect. Black children with par- ents of IQ 115 regress to the Black IQ average of 85, while White children with parents of IQ 115 regress to the White IQ average of 100. Regression to a lower average IQ helps to explain the fact that Black children born to high IQ, wealthy, Black parents have test scores 2–4 points lower than do White children born to low IQ, poor White parents.

In one study, Jensen [22] tested the regression predictions using data from siblings (900 White sibling pairs and 500 Black sibling pairs). These provide an even better comparison than parent–offspring comparisons because siblings share very similar environments. Black and White children matched for IQ had siblings who had regressed approximately halfway to their respective racial means rather than to the mean of both races combined. For example, when Black children and White children were matched for IQs of 120, the siblings of Black children averaged close to 100, whereas the siblings of White children averaged close to 110. A reverse effect was also found for children matched at the lower end of the IQ scale. When Black children and White children were matched for IQs of 70, the siblings of the Black children averaged about 78, whereas those of the Whites averaged about 85. Throughout the range of IQ from 50 to 150 the results were exactly as predicted by genetic theory, not by culture-only theory.

The riddle of the three-way pattern of race differences

On a wide variety of attributes, East Asians and Blacks fall at the two ends of a continuum [46,64]. Whites are in the middle. In addition to brain size and IQ, this shows up on a suite of 60 other life-history variables. These include speed of maturation and longevity, personality and temperament, family stability and crime, and sexual behavior and fertility.

Table 2 lists some of these differences. One striking example: Around the world the rate of dizygotic (i.e. two-egg) twinning is less than four per 1000 births among East Asians, eight among Whites, and 16 or greater among Blacks [7,46]. The tendency to produce dizygotic twins is heritable and mediated by sex hormones. Another example: Black babies sit, crawl, walk, and put on their clothes earlier than Whites or East Asians. These racial group differences have been found to be heritable. For example, children of mixed Black–Japanese ancestry average a faster rate of skeletal development than do children of mixed Japanese–White ancestry, who average a faster rate of skeletal development than do children with two Japanese parents [12]. For walking: East Asians, 13 months; Whites, 12 months; Blacks, 11 months. Blacks also have an earlier age of sexual maturity than do Whites, who in turn have an earlier age than do East Asians, whether measured by age of first menstruation, first sexual experience, or first pregnancy [46]
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Race differences and human origins research

The current consensus view of human origins, the ‘‘out-of-Africa’’ theory, posits that Homo sapiens arose in Africa about 150,000 years ago and then expanded northward beyond Africa about 100,000 years ago, with a European–East Asian split about 41,000 years ago. Evolutionary selection pressures were different in the hot savanna, where Africans lived, than in the cold northern regions Europeans experienced, or the even colder Arctic regions where East Asians evolved. Thus, the further north the ancestral populations migrated out of Africa, the more they encountered the cognitively-demanding problems of gathering and storing food, gaining shelter, making clothes, and raising chil- dren successfully during prolonged winters. As these populations evolved into present-day East Asians and Europeans, the ecological pressures selected for larger brains, slower rates of maturation, and lower levels of sex hormone, and all the other life-history characteristics.

Culture-only theory hypotheses fail to account for the race–IQ differences

Contrary to many hopes and some claims, the narrowing of the gap between Black–White social conditions has not led to any change in the magnitude of the Black–White IQ difference in over 100 years. Massive society-wide interventions such as ending segregation, the subsequent nationwide program of school busing to achieve racial balance, and the Head Start programs have failed to reduce the differences. Head Start programs did produce modest gains in school retention and graduation rates among Whites – but not Blacks. Other large scale, often well-publicized, countywide amelioration projects (such as the $2 billion program in affluent Montgomery County, Maryland, as well as the Kansas City, Missouri, school district, under judicial supervision since 1985), have not reduced the Black– White achievement gap (despite low student–teacher ratios and computers in every classroom).

Whenever closely examined, culture-only theories have proven incapable of explaining race differences in IQ. Adjusting for socioeconomic status only reduces the Black–White IQ difference by about one-third. Nor does the evidence support other culture-only hypotheses such as test bias, test anxiety, or the consequences of being a minority in a White society. Culture-only theories have proven especially inadequate in explaining East Asian IQs because, despite lower socioeconomic conditions, they average slightly higher in IQ and educational achievement than do Whites. One culture-only hypothesis currently promoted is based on the secular increase in test scores. It is known as the Flynn Effect after James Flynn’s [13] demonstration that the average IQ in several countries has systematically increased by about 3 points a decade over the last 50 years. He extrapolated these findings to imply that the 15-point IQ difference between Blacks and Whites will gradually disappear over time. However, analyses show that the Flynn Effect is not on the g factor, the principal source of the Black–White difference [68]. Moreover, there has been no narrowing of the Black–White IQ difference over the last 100 years [52]

Of course, some maintain that ‘‘races’’ do not exist at the genetic level. This argument is contradicted by Tang et al.’s [62] study of 3636 individuals who donated a DNA sample and identified themselves as being White, East Asian, African–American, or Hispanic. The study found that the self-identifications clustered almost perfectly according to 326 measured DNA markers. Only five individuals had DNA that matched a racial/ethnic group different than the box they had checked to
classify themselves. That is an error rate of only 0.14%.

Coroners in crime labs regularly identify race from a skeleton or even just the skull. They can determine race from blood, hair, teeth, or semen as well. To deny the biological reality of race is unscientific and unrealistic. Genetic studies confirm that race is real. A genetic hypothesis predicts that for those Black individuals who possess more White genes, their physical, behavioral, and other characteristics will approach those of Whites. These procedures have become routine for evaluating admixture in genetic studies of dis- ease [38]. They can be recommended for studies of IQ.

Wanted: More race realism, less moralistic fallacy

The ‘‘naturalistic fallacy,’’ identified by philosopher David Hume (1711–1776), occurs when reasoning jumps from statements about what is to a prescription about what ought to be. An example of the naturalistic fallacy would be approving of all wars if scientific evidence showed warfare was part of human nature. The converse of the naturalistic fallacy is the ‘‘moralistic fallacy’’ – jumping from prescriptions about what ought to be to statements about what is. An example of the moralistic fallacy: Claiming that, because warfare is wrong, it cannot be part of human nature. The term ‘‘moralistic fallacy’’ was coined by Harvard University microbiologist Bernard Davis [11] in response to demands for ethical guidelines to control the study of what could allegedly be- come ‘‘dangerous knowledge’’... such as the genetic basis of IQ. For well over a generation, the study of the genetic and racial aspects of IQ has given rise to the best examples we have of the moralistic fallacy in action. Happily, under the sheer weight of evidence, there are now signs that this anti- intellectual and unscientific prohibition is breaking down, at least in the academic world.

Despite repeated claims to the contrary, there has been no narrowing of the 15- to 18-point aver- age IQ difference between Blacks and Whites (1.1 standard deviations). The differences are as large today as when first measured nearly 100 years ago. Racial group differences, and the associated gaps in living standards, education levels etc., are rooted in factors that are largely heritable, not cultural. For example, Lynn and Vanhanen [32] found that national IQ scores correlate 0.68 with per capita income and rate of economic development. They further show that national IQs cause a number of other social phenomena, such as adult literacy (0.64), enrolment in tertiary education (0.75), life expectancy (0.77), and democratization (0.57). Templer and Arikawa [63] found that per capita income and IQ were related to skin color in international comparisons.

Subsequently, Templer [64] found that national IQs and skin predict infant mortality, fertility, and even HIV/AIDS rates. IQ differences are attributable more to differences in brain size than to social, economic, or political factors. There is little or no value in denying reality. Improving opportunities and removing arbitrary barriers is a worthy ethical goal. Equal opportunity is laudable. But we must realize that it will result in equitable, though unequal outcomes.

Expanding on the application of his ‘‘default hypothesis’’ that Black–White differences are based on aggregated individual differences, themselves based on both genetic and environmental contributions, Jensen [26] proposed ‘‘two laws of individual differences’’– (1) individual differences in learning and performance increase as task complexity increases, and (2) individual differences in performance increase with practice and experience (unless there is a low ceiling on proficiency). Consequently, the more we remove environmental barriers and improve everybody’s intellectual performance, the greater will be the relative influence of genetic factors (because the environmental variance is being removed). However, this means that equal opportunity will result in unequal outcomes, within-families, between-families, and between population groups. The fact that we have learned to live with the first, and to a lesser degree the second, offers some hope we can learn to do so for the third.

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J. Philippe Rushton
Department of Psychology, The University of Western Ontario, London, Ontario Canada

Arthur R. Jensen
The University of California at Berkeley, Berkeley, CA 94720-1670, USA

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