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Race, Genetics, and Human Reproductive Strategies

by J. Philippe Rushton Genetic, Social & General Psychology Monographs,Vol. 122 02-01-1996.


The international literature on racial differences is reviewed, novel data are reported, and a distinct pattern is found. People of east Asian ancestry and people of African ancestry average at opposite ends of a continuum, with people of European ancestry averaging intermediately, albeit with much variability within each major race. The racial matrix emerges from measures taken of reproductive behavior, sex hormones, twinning rate, speed of physical maturation, personality, family stability, brain size, intelligence, law abidingness, and social organization. An evolutionary theory of human reproduction is proposed, familiar to biologists as the r-K scale of reproductive strategies. At one end of this scale are r-strategies, which emphasize high reproductive rates; at the other end are K-strategies, which emphasize high levels of parental investment. This scale is generally used to compare the life histories of widely disparate species, but here it is used to describe the immensely smaller variations among human races. It is hypothesized that, again on average, Mongoloid people are more K-selected than Caucasoids, who are more K-selected than Negroids. The r-K scale of reproductive strategies is also mapped on to human evolution. Genetic distances indicate that Africans emerged from the ancestral hominid line about 200,000 years ago, with an African/non-African split about 110,000 years ago, and a Caucasoid/Mongoloid split about 41,000 years ago. Such an ordering fits with and explains how and why the variables cluster.

DISCUSSION OF "RACE" shows little sign of diminishing, despite efforts to debunk the concept. Downgrading the idea of race, however, not only conflicts with people's tendency to classify and build histories according to putative descent but also ignores the work of biologists studying other species (Mayr, 1970). In his 1758 work, Linnaeus classified four subspecies of Homo sapiens: europaeus, afer, asiaticus, and americanus. Most subsequent classifications recognize at least the three major subdivisions considered in this article: Mongoloid, Caucasoid, and Negroid. This classification does not rule out making finer distinctions within these major races.

Those objecting to the idea of race call definitions arbitrary and subjective (Diamond, 1994; Lewontin, Rose, & Kamin, 1984; Yee, Fairchild, Weizmann, & Wyatt, 1993). The main empirical reasons given for negating the race concept are (a) the degree of variance within any one race, (b) the disagreement as to exactly how many races there are, and (c) the blurring of distinctions at category edges because of admixture. For example, with respect to classification, Diamond (1994) surveyed half a dozen geographically variable traits and formed very different races depending on which traits he picked. Classifying people using anti-malaria genes, lactose tolerance, fingerprint whorls, or skin color resulted in the Swedes of Europe being placed in the same groupings as the Xhosa and Fulani of Africa, the Ainu of Japan, or the Italians of Europe.

Many of Diamond's (1994) classifications, however, make no sense because they have little, if any, predictive value beyond the initial classification. In science, the validation of constructs such as race depends on a network of predictive relationships, including item, subject, and sample aggregations. As I show in this article, the construct validity of the three major races--Mongoloid, Caucasoid, and Negroid--has been established at the behavioral level across both time and national boundaries. If race were simply arbitrary, consistent relationships of the type to be presented in this article would not be found.

A race, it should be clear, is what zoologists term a geographic variety or subdivision of a species, characterized by a more or less distinct combination of traits (morphological, behavioral, physiological) that are heritable. Zoologists have identified two or more races in many mammalian species. In humans, the three major races--Mongoloids (commonly "Asians"), Caucasoids (commonly "Whites"), and Negroids (commonly "Blacks")--can be distinguished on the basis of obvious differences in skeletal morphology, hair and facial features, and molecular genetic information. Forensic anthropologists regularly classify skeletons of decomposed bodies by race. For example, narrow nasal passages and a short distance between eye sockets mark a Caucasoid person, distinct cheekbones characterize a Mongoloid person, and nasal openings shaped like an upside down heart typify a Negroid person (Ubelaker & Scammel, 1992). In certain criminal investigations, the race of a perpetrator can be identified from blood, semen, and hair samples. To deny the predictive validity of race at this level is nonsensical.

The currently accepted view of human origins, the "African Eve" theory, posits a beginning in Africa some 200,000 years ago, an exodus through the Middle East with an African/non-African split about 110,000 years ago, and a Caucasoid/Mongoloid split about 41,000 years ago. Evolutionary selection pressures in the hot savanna, where Negroids evolved, differ from pressures in the cold Arctic, where Mongoloids evolved (Stringer & Andrews, 1988). in my book Race, Evolution and Behavior (1995), I proposed that the farther north the populations migrated from Africa, the more they encountered the cognitively demanding problems of gathering and storing food, gaining shelter, making clothes, and raising children successfully during prolonged winters. As the original "out-of-Africa" populations evolved into present-day Caucasoids and Mongoloids, they developed larger brains, slower rates of maturation, and lower levels of sex hormone, and with these changes came reductions in sexual potency, aggressiveness, and impulsivity and increases in family stability, forward planning, self-control, rule-following, and longevity.

The prevailing social science paradigms are giving way to gene-culture co-evolutionary perspectives. During the 1980s, there was an increased acceptance of behavioral genetics and evolutionary theorizing. Discoveries in medical genetics heralded what was to come with gene therapy becoming a possibility for a variety of classic psychological disorders. A renewal of interest in human origins also characterized the 1980s, with Africa identified as the Garden of Eden. Eve was thought to be a long-armed, thick-boned, well-muscled, dark-skinned woman who lived some 200,000 years ago on the East African savanna. She appeared on the front cover of Newsweek (January 11, 1988) and served as the center of a debate on the evolution of modern humanity. However, work on racial differences in behavior, though a necessary concomitant of these revisionist viewpoints, was not included in them and constituted an embarrassment. On the topic of race, a righteous conformity has come to prevail.

Most work on racial differences has focused on Blacks and Whites in the United States, where the achievement of Whites is disproportionately higher than that of Blacks. Ever since Jensen's (1969) monograph in the Harvard Educational Review, a controversy has raged over whether the causes of this disparity involve genetic as well as environmental factors (Eysenck & Kamin, 1981; Loehlin, Lindzey, & Spuhler, 1975). Extensive surveys show that a plurality of experts believe that Jensen was correct in attributing a portion of the racial variation to genetic differences (Snyderman & Rothman, 1987). The debate was widened by data available on (a) Black samples in Africa, the Caribbean, and elsewhere (most Black people live in postcolonial Africa); (b) Asian samples in the Pacific Rim (one third of the world's population); and (c) characteristics in addition to mental ability, showing the same worldwide racial ordering in brain size, personality, speed of maturation, crime rates, family structure, and sexual behavior (see Table 1).

The central theoretical questions are as follows. First, why should Caucasoids average so consistently between Negroids and Mongoloids on so many dimensions? Second, why is there an inverse relationship between brain size and gamete production across the races? It is not simply differences in cognitive ability that require explanation. A network of evidence, such as that shown in Table 1, allows more chance of finding powerful theories than do single dimensions drawn from the set. Nonetheless, it must be emphasized at the outset that there are overlaps in most distributions. Because average differences between races are typically only between 4% and 34%, it is problematic to generalize from a group average to a particular individual.

Maturation, Personality, and Social Organization

In the United States, Black babies have long been known to have a shorter gestation period than White babies. By week 39, 51% of Black children have been born, whereas the figure for White children is 33% (Niswander & Gordon, 1972; Polednak, 1989). Similar results have been observed in Europe, where women of European ancestry have been compared with women of African ancestry (Papiernik, Cohen, Richard, de Oca, & Feingold, 1986). Papiernik et al. (1986) reviewed other observations that, although Black babies are born earlier, they are physiologically more mature than White babies, as measured by pulmonary function and amniotic fluid. I am unaware of data on Asian babies.

Black precocity in physical maturation continues through life. On well-standardized tests, scores indicate that Black babies from Africa, the Caribbean, and the United States mature faster on measures made from birth to 12 months in coordination and head lifting, in muscular strength and turning over, and in locomotion; at 15 to 20 months, Black babies are more advanced in the ability to put on clothing (e.g., Bayley, 1965; Freedman, 1974; but see Warren, 1972, for a critique of the early African data). In contrast, on well-standardized measures, Asian children are more delayed than White children. Asian children typically do not walk until 13 months, compared with 12 months for White children and 11 months for Black children (Freedman, 1979). Regarding dental development, African samples begin the first phase of permanent tooth eruption at age 5.8 years and finish at 7.6 years; Caucasoids begin at 6.1 years and finish at 7.7 years; and Mongoloids begin at 6.1 years and finish at 7.8 years (Rushton, 1995, p. 149, with data from Eveleth & Tanner, 1990).

Behavioral life-cycle traits show a similar set of differences among the three populations. These include age at first intercourse and age at first pregnancy, as well as longevity. For example, at all ages, Blacks have higher mortality rates from numerous causes than Whites in the United States, and the gap has widened over the last 30 years (Polednak, 1989). Asians have lower mortality rates than Whites.

With respect to personality, data show that across ages, across traits, and across methods, Blacks are more uninhibited in temperament than Whites, and Whites are more uninhibited than Asians. For infants and young children, observer ratings are the main method used, whereas for adults, the use of standardized tests is more frequent (e.g., Vernon, 1982). For example, researchers in a study carried out in French-language Quebec examined 825 4- to 6-year-olds from 66 countries. These immigrant children were rated by 50 teachers in preschool French-language-immersion classes. The French-Canadian teachers consistently reported (a) better social adjustment and less hostility/aggression for the Mongoloid children than for the Caucasoid children and (b) more social adjustment and less hostility for the Caucasoid children than for the Negroid children (Tremblay & Baillargeon, 1984).

Rushton(1985) indexed behavioral restraint by low extraversion and high neuroticism scores from the Eysenck Personality Questionnaire, using data collected from 25 countries around the world. Averaging across these samples, Rushton found 8 Mongoloid samples (N = 4,044) to be less extraverted and more neurotic than 38 Caucasoid samples (N = 19,807), who were less extraverted and more neurotic than 4 African samples (N = 1,906).

Social organization depends on following rules. Such behavior can be indexed, for example, by marital functioning, mental durability, and law abidingness. On all of these measures, the rank ordering within the United States is Asian > White > Black (Jaynes & Williams, 1989). The 1.5 million individuals of Asian descent in the United States are very rarely perceived as a "social problem," for they have significantly fewer divorces, out-of-wedlock births, or incidences of child abuse than do Whites, and, in fact, they are very seldom studied. Black family structure, however, has been studied intensively. Since the 1965 Moynihan report documented the high rates of marital dissolution, high frequency of female heads of families, and numerous illegitimate births, the figures cited as evidence for the instability of the Black family have tripled (Jaynes & Williams).

The race/crime relationship found within the United States, with Asians being most law abiding, Africans least law abiding, and Europeans intermediate, has been found within other multiracial countries, such as Britain, Brazil, and Canada (Rushton, 1990). Moreover, the pattern has been found in China and the Pacific Rim, Europe and the Middle East, and Africa and the Caribbean. The global nature of the pattern is shown in data analyzed from INTERPOL yearbooks, showing that African and Caribbean countries had double the rate of violent crime (murder, rape, and serious assault) than did European countries, which had three times the rate of violent crime than did Asian countries (Rushton, 1990).

Hormones and Reproductive Potency

The average woman produces one egg every 28 days in the middle of the menstrual cycle. Some women, however, have shorter cycles than others, and some produce two eggs in a cycle. Both events translate into greater fecundity because of the opportunities provided for conception. Occasionally, double ovulation results in the birth of dizygotic (two-egg) twins. The races differ in the rate at which they double ovulate. The frequency of dizygotic twins per 1,000 births is less than 4 for Mongoloids, 8 for Caucasoids, and 16 or greater for Negroids (Bulmer, 1970). Subsequent reviews of twinning rates in the United States (Allen, 1988) and Japan (Imaizumi, 1992) have confirmed these data.

Gonadotropin levels differentiate the races in the predicted direction and may underlie the difference in rates of multiple birthing. Testosterone levels may underlie other behavior traits differentiating the races, for they have been found to be 19% higher in a sample of Black U.S. college students than in their White counterparts (Ross et al., 1986). In an older group of U.S. military veterans, Blacks measured 3% higher in testosterone level than Whites (Ellis & Nyborg, 1992). Another study, of testosterone metabolites, showed a 10% to 15% higher incidence in Black Americans than in White Americans and a still lower incidence in the Japanese in Japan (Hixson, 1992).

Rushton and Bogaert (1987) reviewed the literature on frequency of sexual intercourse. For example, Hofmann (1984) examined worldwide premarital coitus rates among young people in high school and found that African adolescents were more sexually active than Europeans, who were more sexually active than Asians. The same pattern has emerged from surveys carried out within the United States, where this pattern also holds for sexual activity after marriage. For example, Rushton and Bogaert (1987) averaged data from a representative cross-cultural review by Ford and Beach (1951) and found that Oceanic and American Indian peoples' self-reported rates of sexual intercourse per week ranged from 1 to 4, U.S. Whites' ranged from 2 to 4, and Africans' ranged from 3 to 10. Subsequent surveys support these data. For married couples in their 20s, the average frequency of intercourse per week for the Japanese and Chinese in Asia is 2.5 (Asayama, 1975; Bo & Wenxiu, 1992, Table 7), whereas for American Whites it is 4, and for American Blacks, 5 (Fisher, 1980).

Racial differences also appear on measures of sexual permissiveness, amount of thinking about sex, and sex guilt. Abramson and Imari-Marquez (1982) observed that each of three generations of Japanese Americans showed more sex guilt than matched Caucasian Americans. In studies carried out in Britain and Japan, using a sex fantasy questionnaire, Iwawaki and Wilson (1983) found that British men reported twice as many fantasies as Japanese men, and British women admitted to four times as much sex fantasy as Japanese women did. By contrast, Blacks reported not only having had intercourse with more casual partners but also with fewer feelings of distaste than did Whites.

Rushton and Bogaert (1987, 1988) examined updated data from the Kinsey Institute for Sex Research (Gebhard & Johnson, 1979) that eliminated sources with known sexual bias, such as prostitutes. Black/White differences were compared on 41 variables. For men and women, college-educated Whites were found to be most sexually restrained, college-educated Blacks least, and non-college-educated Whites intermediate. This pattern was found for early onset of premarital, marital, and extramarital sexual experience; number of sexual partners; and frequency of intercourse. For women, the races were also differentiated on speed of onset and incidence of pregnancy, short duration of the menstrual cycle, and number of orgasms per act of coitus (see Table 2).

Cognitive Abilities

The literature on the global distribution of intelligence test scores was reviewed by Lynn (1991). Mongoloid populations, measured in North America and the Pacific Rim, had average IQs in the 101 to 111 range. Caucasoid populations in North America, Europe, and Australasia had average IQs ranging from 85 to 115, with an overall mean of about 100. Negroid populations living south of the Sahara, in North America, in the Caribbean, and in Britain had average IQs in the 70 to 90 range. Lynn's (1991) estimate of 70 for the IQ of African Blacks has been confirmed in two subsequent studies. In one study, the Wechsler Test was administered to a representative sample of children in Zimbabwe (Zindi, 1994), and in the other study, researchers examined Ethiopian immigrants to Israel (Lynn, 1994). In both studies, the IQs of the Africans were found to be just under 70.

Questions remain about the validity of using tests for racial comparisons. However, because the tests show similar patterns of internal item consistency and predictive validity for all groups, and the same differences are found on relatively culture-free tests, many psychometricians think that the test scores are valid measures of racial differences (Herrnstein & Murray, 1994; Snyderman & Rothman, 1987). Also, novel data about speed of decision making (reaction time) show the same racial pattern as do test scores. Investigations have been done with 9- to 12-year-olds from six countries. In these studies, the children had to decide which of several lights was on or stood out from others, and then they had to move a hand to press a button. All children can perform the tasks in less than one second, but children with higher IQ scores perform these elementary tasks faster than do those with lower scores. Lynn (1991) found that representative Asian schoolchildren from Hong Kong and Japan were faster in reaction time than were similar White children from Britain and Ireland, who, in turn, were faster than were similar Black children from South Africa (see also Lynn & Shigehisa, 1991). Using the same decision-time tasks, as well as those involving retrieval of well-learned facts from long-term memory, researchers also found this same three-way pattern of racial differences in California samples (Jensen, 1993; Jensen & Whang, 1993, 1994).

Brain Size

A small but robust relation has been firmly established between cognitive ability measured by both educational attainment and IQ tests and brain size. The correlation between test scores and brain size (estimated from magnetic resonance imaging [MRI], which, in effect, constructs a three-dimensional picture of the brain in vivo), averages at about .40 (Andreasen et al., 1993; Egan et al., 1994; Raz et al., 1993; Wickett, Vernon, & Lee, 1994; Wilierman, Schultz, Rut-ledge, & Bigler, 1991). The MRI measure of brain size, more accurate than previous methods used, results in a substantial increment over correlations of about .20 between head perimeter and measures of intelligence, reported since the turn of the century (Broman, Nichols, Shaughnessy, & Kennedy, 1987; Galton, 1888; Wickett et al., 1994). The head perimeter/IQ relation has been found within samples of Asians as well as Whites (Rushton, 1992b). Jensen and Johnson (1994) found that head size is significantly correlated with IQ within families (i.e., among same-sex full siblings, with age partialed out), thus indicating a functional relation between brain size and IQ.

Although racial differences in brain size were widely believed to exist by researchers in the 19th and early 20th centuries, more recent researchers suggested that differences disappear when corrections are made for body size and other variables (Gould, 1981). Modern studies (described below), however, have confirmed the earlier findings. Three main procedures have been used: weighing wet brains after death, filling empty skulls with lead shot and then measuring the volume of filler, and converting external head sizes into cranial volume. Data from all three sources converge on the conclusion that, after statistical corrections are made for body size, Mongoloids average about 17 cm3 (1 cubic inch) more than Caucasoids, who average about 80 cm3 (5 cubic inches) more than Negroids.

For brain weight at autopsy, Ho, Roessmann, Straumfjord, and Monroe (1980a, 1980b) provided original data for 1,261 adults 25 to 80 years old from Cleveland, Ohio. Ho et al. excluded those brains that were obviously damaged, and they measured all brains using well-standardized procedures. Sex-combined differences were found between 811 Whites (1,323 g, SD = 146) and 450 Blacks (1,223 g, SD = 144). These sex-combined differences remained significant after controlling for age, stature, weight, and body surface area. In the introduction to their article, Ho et al. (1980a) briefly reviewed studies from Japan and Korea, which Rushton(1988) averaged to find a sex-combined brain weight of 1,351 g, higher than that of Caucasoids.

For endocranial volume, an international database of up to 20,000 skulls for 122 ethnic groups was computerized and classified by climate and region by Beals, Smith, and Dodd (1984). A 2.5-cm3 increase in brain volume was found with each degree of latitude. Geographic differences emerged. Table 2 in Beals et al. (1984, p. 306) contains data that show that sex-combined cranial capacity from 26 Asian populations averaged 1,380 cm3 (SD = 83), from 10 European groups = 1,362 cm3 (SD = 35), and from 10 African groups = 1,276 cm3 (SD = 84). When Beals et al. (1984, Table 5) identified continental areas in relation to the presence or absence of winter frost, the geographic differences became even more pronounced (19 Asian groups = 1,415 cm3, SD = 51, 10 European groups = 1,362 cm3, SD = 35; 9 African groups = 1,268 cm3, SD = 85).

As to external head measurements, several studies have been conducted, and evidence has been found (including measurements from a data set compiled by Herskovits, 1930) of a Mongoloid advantage, which is often cited as showing an absence of racial differences. Yet the data actually show (Rushton, 1993) that for 5 male Mongoloid samples, average external head measurement equals 1,451 cm3 (SD = 22); for 9 Caucasoid samples it is 1,421 cm3 (SD = 49); and for 12 Negroid samples it is 1,295 cm3 (SD = 44). In another study, Rushton(1991) calculated cranial capacities for 24 (male only) international military samples collated by the U.S. National Aeronautics and Space Administration. After adjusting for stature, weight, and body surface area, Rushtonfound that cranial capacities of Mongoloids averaged 1,460 cm3 and of Caucasoids 1,446 cm3. For a stratified random sample of 6,325 U.S. Army personnel measured in 1988 for fitting helmets, Rushton (1992a) found that, after adjusting for stature, weight, sex, and rank, 543 Asian Americans averaged 1,416 cm3 (SD = 104), 2,871 European Americans averaged 1,380 cm3 (SD = 92), and 1,387 African Americans averaged 1,359 cm3 (SD = 95). Finally, Rushton(1994) examined 40 samples compiled in 1990 by the International Labour Office in Geneva from tens of thousands of men and women 25 to 45 years old. After adjusting for the effects of stature and sex, 6 east Asian groups = 1,308 cm3 (SD = 37), 18 European groups = 1,297 cm3 (SD = 38), and 4 African groups = 1,241 cm3 (SD = 38).

After converting adult sex-combined brain weight data from grams to the equivalent in cm3 (1 cm3 = 1.036 g) and averaging across all the studies, Rush-ton (1995) found that, in brain size, Mongoloids = 1,364 cm[sup 3], Caucasoids = 1,347 cm3, and Negroids = 1,267 cm3. Differences due to method of estimation within a race are smaller than differences between the races. Overall, Rushton(1995) calculated a world average brain size of 1,326 cm3; Beals et al. (1984) calculated it at 1,349 cm[sup 3].

Racial differences in brain size and IQ are revealed early in life. Data collapsed across social class from the National Collaborative Perinatal Project show that, of the sample studied, the 19,000 Black infants had smaller head perimeters at birth, were shorter in stature, were lighter in weight, and had an earlier age of gestation than the 17,000 White infants (Broman et al., 1987). By 7 years of age, catch-up growth favored the Black children in body size but not in head perimeter. Head perimeter at birth correlated with IQ at age 7 years from .10 to .20 for both the Black and the White children.

Additional analyses show that Black/White differences in brain size are correlated with Black/White differences in mental ability. In a sample of adolescents, Jensen (1994) found that the greater the differences between White and Black children on 17 cognitive tests, the higher were the correlations of the test scores with head size, r = .533, p < .05; with unreliability of measurement controlled, r = 0.715, p < .01. In a study of 14,000 4- and 7-year-olds, the White and Black samples differed by about I standard deviation in IQ, and they differed significantly (p < .001) in head size (White > Black), even with age, height, and weight statistically controlled (Jensen & Johnson, 1994). It is noteworthy that there was no difference in average head size between White and Black children who were matched on IQ scores (and on age, height, and weight).

Heritability of Racial Differences

Theories of racial differences based on 100% cultural transmission have formidable problems accounting for the physiological traits such as speed of dental and physical maturation, brain size, gamete production, and testosterone production as well as the data on within-race heritability and the consistency of the racial rankings across time and cultures. Direct evidence for between-group heritabilities also exists. For example, the racial differences in multiple birthing are independently heritable through the race of the mother and not through the race of the father, as found in Mongoloid-Caucasoid crosses in Hawaii and Caucasoid-Negroid crosses in Brazil (Bulmer, 1970).

Because higher heritabilities are stronger indicators of underlying genetic substrates than lower heritabilities (which by definition imply environmental influence), the heritabilities can themselves be used to test theories. If genes are important, then racial differences should be most pronounced on tests with high heritabilities. Jensen (1973, chapter 4) found that Blacks and Whites were indeed most differentiated on genetically influenced tests and least differentiated on environmentally influenced tests. In one study of 543 pairs of siblings, Jensen (1973) found a .67 correlation between the heritability of 13 tests and the magnitude of the Black/White difference. Subsequently, Black/White differences were found to be most pronounced on more g-loaded tests, that is, the general factor common to diverse cognitive tests (Jensen, 1985). The g loadings, the purest measures of cognitive ability, are related to a number of biological variables, including brain-evoked potentials, heritability coefficients determined from twin studies, and the degree to which children's test scores are depressed by inbreeding and raised by out breeding (Jensen, 1987).

Building on Jensen's work, Rushton(1989) carried out a study using as genetic weights the amount of inbreeding depression found on 11 tests from the Wechsler Intelligence Scale for Children. Inbreeding depression occurs when harmful recessive genes combine, an event more likely in offspring of closely related parents. Estimates of inbreeding depression had been calculated from 1,854 cousin marriages in Japan by Schull and Neel (1965) and shown to be related to the g factor by Jensen (1983). As the g loadings (data from Jensen, 1985) and in-breeding depression scores (data from Rushton, 1989) increase, the magnitude of the Black/White difference in scores on the same 11 Wechsler tests becomes larger (see Figure l). The inbreeding prediction was sufficiently strong to overcome generalization from the Japanese in Japan to Blacks and Whites in the United States and so constituted a conservative test of the genetic hypothesis. There really is no explanation for the inbreeding effect and its ability to predict Black/White differences in scores on IQ tests other than a genetic one.

Transracial adoption studies also reveal genetic influence. There have been at least three studies of Korean and Vietnamese children adopted into White American and White Belgian homes (Clark & Hanisee, 1982; Frydman & Lynn, 1989; Winick, Meyer, & Harris, 1975). As babies, many of these children had been hospitalized for malnutrition. Nonetheless, they excelled in academic ability with IQs 10 or more points higher than national norms. In contrast, Weinberg, Scarr, and Waldman (1992) found that at age 17, Black and mixed-race children adopted into White middle-class families performed at a lower level than the White siblings with whom they were raised. Adopted White children bad an average IQ of 106, an aptitude based on national norms at the 59th percentile, and a class rank at the 54th percentile; mixed-race children had an average IQ of 99, an aptitude at the 53rd percentile, and a class rank at the 40th percentile; and Black children had an average IQ of 89, an aptitude at the 42nd percentile, and a class rank at the 36th percentile.

Moderate to high heritabilities are well established for numerous traits from adoption, twin, and family studies. Noteworthy are the 80% heritabilities for IQ test scores found in adult twins reared apart (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990). Inherited genetic influence on mental ability has also been found among non-Whites, including African Americans, Chinese Americans, and the Japanese in Japan. Additional genetic research has built a strong case for heritable factors in personality, psychopathology, violent crime, and other social variables (Plomin, Owen, & McGuffin, 1994). Standard inductive reasoning requires that these high within-group heritabilities be generalized to the differences between groups in the same way that environmental factors are. If poor nutrition has an effect within Whites and Blacks, then it is likely to have an effect between Whites and Blacks. As we have seen, the evidence indicates that genetic effects also operate on the between-group differences.

Life-History Theory

The explanation proposed for the pattern of international evidence summarized in Table 1 lies in primate life-history theory. A life-history is a genetically organized suite of characters that evolved in a coordinated manner so as to allocate energy to survival, growth, and reproduction. One influential life-history theory is that of r-K selection, proposed by E. O. Wilson (MacArthur & Wilson, 1967; Pianka, 1970; Wilson, 1975). At one extreme are r-strategies, emphasizing gamete production, mating behavior, and high reproductive rates, and at the other extreme are K-strategies, emphasizing high levels of parental care, resource acquisition, kin provisioning, and social complexity. As Johanson and Edey (1981, p. 326) succinctly summarized: "More brains, fewer eggs, more 'K'." Table 3 contains a summary of the traits thought to covary with r-K reproductive strategics. Each individual, subspecies, and species has evolved a characteristic life cycle adapted to the particular ecological problems encountered by its ancestors (Wilson, 1975).

Species are, of course, only relatively r and K. Thus rabbits are K-strategists compared with fish but r-strategists compared with primates. Primates are all relatively K-strategists, and humans may be the most K of all. The life phases and gestation times of primates display a natural scale of prolongation ranging from lemur, to macaque, to gibbon, to chimp, to early humans, to modem humans with a consistent trend toward K (Lovejoy, 1981; Schultz, 1960). Note the proportionality of the indicated phases in Figure 2. With each step in the natural scale, populations devote a greater proportion of their reproductive energy to subadult care, with increased investment in the survival of offspring. The postreproductive phase of life is restricted to humans.

Dental development (which I related to racial differences earlier in this article) is a maturation variable that accurately reflects primate life-histories. Smith (1989) correlated the age at eruption of first molar with life-history factors. First molars are the earliest permanent teeth to erupt in primates and are stable in many aspects of their growth. Smith found that, across 21 primate species, age at eruption of first molar correlated .89, .85, .93, .82, .86, and .85 with the body weight, length of gestation, age at weaning, birth interval, sexual maturity, and life span. The highest correlation was .98 with brain size. She interpreted her data in terms of the r-K life-history model.

Brain size is the key factor acting as the biological constant determining the rank order of many cross-species variables, including the number of individuals comprising the group cohesively maintained through time (Dunbar, 1992), speed of maturation, degree of infant dependency, and longevity (Harvey & Krebs, 1990; Holman, 1993). The hominid brain has tripled in size over the last 4 million years. Australopithecenes' brain averaged about 500 cm3, the size of a chimpanzee's. Homo habilis' brain averaged about 800 cm3, Homo erectus' brain about 1,000 cm3, and modern Homo sapiens' brain about 1,350 cm3. If the en-cephalization quotient, the expected brain ratio given a certain body size, is plotted over the same evolutionary time frame, the increase is proportionately less, although still substantial. On the most recent calculations, the figures go from 2.4 to 5.8 (McHenry, 1992).

Metabolically the brain is an expensive organ. Representing only 2% of body mass, the brain uses about 5% of the body's basal metabolic rate in rats, cats, and dogs, about 10% in rhesus monkeys and other primates, and about 20% in human beings (Armstrong, 1990). Across primates, large brains are also expensive in life-history tradeoffs, requiring a more stable environment, a longer gestation, a slower rate of maturation, a higher offspring survival rate, a lower reproductive output, and a longer life (Harvey & Krebs, 1990). Unless large brains substantially contributed to fitness, therefore, they would not have evolved.

A comparison of the pattern of racial differences summarized in Table I with the attributes listed in Table 3 suggests that Mongoloids are more K-selected than Caucasoids, who in turn are more K-selected than Negroids.

Out of Africa

Knowledge about racial differences in reproductive strategies may help in choosing between alternative theories of racial origins. Africa, as Darwin surmised, is "the cradle of mankind," with Australopithecus, Homo habilis, and Homo erectus all making their first appearance there. However, two very different theories are currently competing to explain how racial differences evolved during the final stages of hominid evolution. These are the single-origin and the multiregion-origin theories (see Figure 3).

Both models assume that, between 1 million and 2 million years ago, Homo erectus emerged out of Africa to populate Eurasia. The models are divided on whether the descendants of these erectus populations (the Neanderthals in Europe, Beijing Man in China, and Java Man in Indonesia) gave rise to modern ancestors, or whether the erectus groups were evolutionary dead ends supplanted by a wave of anatomically modern people arising in Africa less than 200,000 years ago.

The single-origin, or "African Eve," theory proposes that fully modern human beings emerged recently, about 200,000 years ago, from a primeval African population. After a dispersal event in the Middle East about 100,000 years ago, they migrated into all corners of the world. In the process, specific racial features developed, and existing Neanderthal and Homo erectus populations were replaced. A strong version of this theory holds that no genetic mixture took place between the modern and the older populations, and that after the African/non-African split about 100,000 years ago, a Caucasoid/Mongoloid split occurred about 40,000 years ago (Nei & Roychoudhury, 1993; Stringer & Andrews, 1988).

The multiregion-origin theory holds that, over a 1-million-year period, modem races evolved in parallel in Africa, Europe, and Asia through intermediate stages from Homo erectus. Thus, Europeans evolved from Neanderthals, Chinese from Beijing Man, and Australian Aborigines from Java Man. Unique morphological features are seen to persist from the archaic populations to modem ones, including (a) the prominent noses of modern Europeans and those of Neanderthals (200,000 to 35,000 years ago), (b) the flat faces and shovel shaped incisor teeth of modern Chinese and those of Beijing Man and the Zhoukoudian fossils (500,000 to 200,000 years ago), and (c) the continuous brow ridge of modern aboriginal Australians and those of Java Man and the Ngandong fossils (700,000 to 100,000 years ago). Necessary to this view, much gene transfer must have occurred among the various groups to keep them evolving in concert (indicated by arrows in Figure 3).

Although it is not crucial for the r-K thesis which of the two (or other) approaches turns out to be correct, the single-origin model provides a more parsimonious explanation for why Caucasoids average so consistently between Mongoloids and Negroids. The racial-geographic succession fits with and explains how and why the variables cluster. No consistent pattern of character appearance is expected from multiregional models based on long periods of separation with unknown amounts of gene flow. Because of the closeness of the separation times, the single-origin model also explains why heritabilities are predictive across races.

A multiregional model was once proposed to explain racial differences. Coon (1962) postulated a separate but parallel evolution for several subspecies of Homo erectus occurring simultaneously in various regions of the world over about I million years. He proposed that each of these subspecies crossed the critical threshold to sapient status at different times. To account for observed differences in cranial capacities (see also Coon, 1982), he suggested that African populations "lagged behind" the other races. His theory has been rejected by other multiregionalists, who now hypothesize much gene flow between the subspecies to keep them evolving in parallel (Frayer, Wolpoff, Thorne, Smith, & Pope, 1993). In fact, both the behavioral-genetic and molecular-genetic data suggest that substantially more relatedness exists among human populations than is likely from either Coon's (1962) model or from the modern alternatives. The generalizability of the heritabilities (e.g., Figure 1) shows that the variegated cognitive structures of the populations are extremely similar.

Challenges and Rejoinders

Some critics have charged that the data I have presented on racial group differences (Table 1) were misleadingly selected and, by implication, that if a more representative sampling of the literature had been carried out, the null hypothesis would have been supported (e.g., Cain & Vanderwolf, 1990; Fairchild, 1991). However, if the racial differences were truly randomly distributed around a mean of zero difference, then these critics should have been able to point to just as much evidence occurring in the opposite pattern. This they have been unable to do.

The principle of aggregation, a major methodological point, must be kept firmly in mind when discussing racial differences. This principle states that the sum of a set of multiple measurements is a more stable and unbiased estimator than any single measurement from the set. One reason for this principle is that there is always error associated with measurement, and combining several measurements allows the errors to average out, thereby providing a more accurate picture of relationships in. the population (Rushton, Brainerd, & Pressley, 1983). Critics can always deconstruct a data set to identify particular elements not conforming to the general pattern and then conclude that the general pattern does not exist. This logical fallacy is akin to finding that some women are taller than some men and so denying that men are, on average, taller than women.

I have applied the aggregation technique to several published data sets purporting to show racial rank orders contrary to those depicted by me (Rushton, 1995). With respect to brain size, Zuckerman and Brody (1988) showed that one sample of Black Americans had a larger cranial capacity than one sample of Nordic Swedes; Cain and Vanderwolf (1990) showed that one 1986 Negroid series had a larger cranial capacity than one 1923 Caucasoid series; and Groves (1991) showed that one sample of African Xhosa had the second largest cranial capacity of 61 different populations. However, when these data sets were aggregated, I found each time that the Mongoloid-Caucasoid-Negroid average ordering held. For example, using the cranial capacity data given by Groves (1991), the sex-combined averages for Mongoloids, Caucasoids, and Negroids are, respectively, 1406, 1385, and 1331 cm3.

For crime figures, it can be shown that on some self-report measures the racial differences become minimal or even nonexistent. But when the frequency of offending or more serious offending is taken into consideration, the expected racial differences re-emerge (e.g., Wilson & Herrnstein, 1985). Crime differences are also shown to vary enormously from offense to offense, from geographic area to geographic area, and from decade to decade (Roberts & Gabor, 1990), but again, when the figures are aggregated, the typical racial differences emerge. Critics are unable to explain why aggregation results in predictable patterns.

Another error that critics make is to focus on highly salient minor points and so obscure the larger picture. Thus, concerning reproductive behavior, Weizmann, Wiener, Wiesenthal, and Ziegler, (1990, p. 8; 1991) ridiculed references to the ethnographic record (e.g., French Army Surgeon, 1898/1972), calling it "anthroporn" because it contained "a recipe for do-it-yourself penis enlargement employing an eggplant and hot peppers." They thereby sidestepped my global review of sexual behavior and AIDS.

Although extreme environmentalists used to suggest that within-race heritabilities might be set at zero (e.g., Kamin, 1974), this position is no longer credible. Instead, it is now argued that because genetic by environment interactions are so ubiquitous, it is impossible to disentangle causality and apportion variance (e.g., Lerner, 1992). Bouchard (1984) replied to this general point by referring to the Minnesota study of monozygotic twins reared apart. Bouchard asked: If context and interaction effects are so important, how can it be that siblings raised apart grow to be significantly similar to each other, with their degree of similarity being predicted by the number of genes they share? The presence of genetically based stabilizing systems that drive development into common channels is clearly implicated.

One critique of my application of r-K theory to human populations is that I get wrong the climatic conditions most likely to produce K-selection (Anderson, 1991; Weizmann et al., 1990, 1991). Some have followed Barash (1982) and assumed that K-selection is greatest in the tropics, where Negroids evolved, and r-selection greatest in temperate and Arctic conditions. This premise, however, is incorrect. Predictability is the ecological necessity for K-selection, and this can occur in either a stable environment or a predictably variable one like the Arctic (Rushton & Ankney, 1993). What has apparently been misunderstood is that sub-tropical savannas, because of sudden droughts and devastating viral, bacterial, and parasitic diseases, are especially less predictable for long-lived species than are temperate and Arctic environments. Although the Arctic climate varies greatly over 1 year, it is highly predictable, though harsh, over time (Calvin, 1991).

Many researchers hold that environmental explanations are sufficient to explain racial difference. In the 1950s, a toilet-training variant of Freud's theory held that African children, not trained to control their bowels until a considerably later age than European children, developed an extraverted culture with values of sensual self-expression and a relaxed heterosexual attitude to sex. At the other end of the scale were Asians, who were toilet trained at a very early age and thereby became puritanically self-disciplined. From the 1960s through the 1980s, social learning theory dominated. This approach emphasized the importance of role models and incentives through the family, the mass media, and the educational system.

Most recently, an "environmental" r-K theory has been espoused (see Figure 4). Belsky, Steinberg, and Draper (1991, p. 647) succinctly described two diverging pathways: One is characterized, in childhood, by a stressful rearing environment and the development of insecure attachments to parents and subsequent behavior problems; in adolescence, by early pubertal development and precocious sexuality; and in adulthood, by unstable pair bonds and limited investment in child rearing; the other is characterized by a stable and secure childhood and longer lasting marital bonds in adulthood.

Several longitudinal studies have confirmed this expected pattern of covariation (see Chisholm, 1993). These environmental variables add important perspective to my genetic polymorphism viewpoint. Some theorists have gone further, however, and insisted that the racial pattern can be explained entirely from a life-history perspective "without necessitating any underlying genetic variability" (Mealey, 1990, p. 387). However, there is no environmental factor known to cause an inverse relation between brain size and gamete production or to produce covariation across so multifarious a set of variables. Postulating some genetic variance is indispensable to explaining the consistency of the racial ordering. A mixed 50% evolutionary and 50% environmental model fits the data better than either the 100% environmental or the 100% genetic alternatives.


The r-K theory of racial group differences may help to explain other individual and group differences, including those of social class, law-abidingness, health, and longevity. One advantage of an evolutionary perspective is the focus it brings to underlying physiology. A person's position on the r-K dimension might be set by a hormonal switch mechanism. Reproductive strategies need to be coherent and harmonized, not with some traits going to one pole and other traits going to the opposite pole. Because hormones go everywhere in the body, they are uniquely able to exert more or less simultaneous effects and coordinate widespread development and functioning.

One simple switch mechanism to account for a person's position on the r-K dimension is level of testosterone. A model based on one proposed by Nyborg (1994) is shown in Figure 5. At the beginning of the inverted U-shaped curve, the men with the most testosterone (T5) would be farthest from the zenith of K, with intermediately androgenized men (T3) closer and men with the least testosterone (T1) closest. With increasing degrees of estrogenization (E1 to E5), women move away from optimum. Such a model can accommodate both genetic and environmental effects. The initial setting is genetically based with environmental factors then modifying and fine-tuning the system. In this model, Mongoloids are T2/E2, Caucasoids are T3/E3, and Negroids are T4/E4.

Finally, r-K theory may help to explain the "fertility paradox." Fisher (1958) asked why civilizations have declined. He showed that ruling groups fail to reproduce themselves because of low fertility, and he hypothesized a trade-off between the capacity for economic success and fertility. According to r-K theory, this trade-off may be even more profound than Fisher realized, being related to a whole complex of characteristics partly genetic in origin. When there are abundant resources, selection pressures are off, and natural selection favors r-genotypes so that that segment of the population expands. Eventually, a saturation point is reached and, following Malthus, the population crashes. With selection pressures back on, K-genotypes are again favored. These cycles occur with rodents (Krebs, Gaines, Keller, Myers, & Tamarin, 1973), and a direct parallel is suggested with human beings. Thus, the r-K dimension may apply not only to demographic trends but, ultimately, to the very sweep of history.

In conclusion, it is time to end the relative neglect of theorizing about racial differences in behavior. International data show a distinct pattern. Asians and Africans average at opposite ends of a continuum that ranges over 60 anatomical and social variables, including brain size and testosterone. with Europeans intermediate. The pattern can be explained adequately only from a gene-based evolutionary perspective. If all people were treated the same, most racial differences would not disappear. This does not mean that environmental factors are unimportant for individual development. But, to deny or obfuscate the reality of a genetic basis for racial differences, as so many critics of the race concept have done, does not change reality.

This research was supported by grants from The Pioneer Fund and draws on my book Race, Evolution and Behavior (1995). I am grateful to C. D. Ankney, D. N. Jackson, R. Lynn, and A. R. Jensen for valuable comments.

TABLE 1 Relative Ranking of Races on Diverse Variables

TABLE 2 Analysis of Kinsey Data on Race and Socioeconomic Status Differences in Sexual Behavior

TABLE 3 Some Life-History Differences Between r and K Strategists

[Note: see original text for graphs and diagrams] GRAPHS: FIGURE 1. Regression of Black/White differences on g loadings (Panel A) and on inbreeding depression scores (Panel B). The numbers indicate subtests from the Wechsler Intelligence Scale for Children-Revised: 1 coding, 2 arithmetic, 3 picture completion, 4 mazes, 5 picture arrangement, 6 similarities, 7 comprehension, 8 object assembly, 9 vocabulary, 10 information, 11 block design. From Race, Evolution and Behavior (p. 188), by J.P. Rushton, 1995, New Brunswick, NJ: Transaction. Copyright 1995 by Transaction Publishers. Reprinted by permission.

GRAPH: FIGURE 2. Progressive prolongation of life phases and gestation in primates.

DIAGRAM: FIGURE 3. Alternative models for the evolution of the human races: Multiregional and single origin.

DIAGRAM: FIGURE 4. Developmental pathways of divergent reproductive strategies. From Belsky, Steinberg, & Draper (1991, p. 651, Figure 1). Copyright 1991 by the Society for Research in Child Development. Reprinted with permission.

DIAGRAM: FIGURE 5. Sex hormone model for coordinating development across body, brain, and behavioral traits.


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