J. Philippe Rushton
Although independent researchers have repeatedly confirmed: (1) The geographical distribution of intelligence, (2), the relationship between intelligence and brain size, (3), the geographical distribution of brain size, and (4), the heritability of intelligence, Diamond, the author of The Rise and Fall of the Third Chimpanzee, is like a composite of the three wise monkeys and does not want to see, hear, or say anything about these topics. Therefore, I will briefly summarize them. Readers seeking a more extensive summary can consult The Bell Curve, and for a complete discussion of how brain size and IQ explain much of human behavior and are in turn explained by human evolution, see my Race, Evolution, and Behavior.
1. The geographical distribution of intelligence. One hundred years of research has established that East Asians and Europeans average higher IQs than do Africans. East Asians, measured in North America and in Pacific Rim countries, typically average IQs in the range of 101 to 111. Caucasoid populations in North America, Europe, and Australasia typically average IQs from 85 to 115 with an overall mean of 100.
African populations living south of the Sahara, in North America, in the Caribbean, and in Britain typically have mean IQs from 70 to 90 (see Lynn, 1997, for a comprehensive review).
Parallel differences are found on relatively culture-free tests such as speed of decision making. All children can perform the task in less than one second, but children with higher IQ scores perform faster than do those with lower scores. Asian children in Hong Kong and Japan average faster than do European children from Britain and Ireland, who in turn average faster than do African children from South Africa.
This same pattern of racial differences is also found in California.
2. The relationship between intelligence and brain size. Diamond neglects to mention any of the remarkable discoveries made during the 1990's "decade of the brain" using magnetic resonance imaging (MRI). Such MRI studies, which construct three-dimensional models of the brain in vivo, show a correlation of about 0.40 between brain size and IQ, as replicable a set of results as can be found in the social and behavioral sciences. The first MRI/IQ studies were published in the late 1980's and early 1990's in leading, refereed, mainstream journals like Intelligence (Willerman et al., 1991) and the American Journal of Psychiatry (Andreasen et al., 1993).
3. The parallel geographical distribution of brain size. Racial differences in brain size have been established recently using wet brain weight at autopsy, volume of empty skulls using filler, and volume estimated from head sizes. Using endocranial volume, for example, Beals et al. (1984, p. 307, Table 5) analyzed about 20,000 skulls from around the world. East Asians averaged 1,415 cm3 (SD = 51), Europeans averaged 1,362 cm3 (SD = 35), and Africans averaged 1,268 cm3 (SD = 85)
. Using external head measures to calculate cranial capacities, Rushton (1992) analyzed a stratified random sample of 6,325 U.S. Army personnel measured in 1988 for fitting helmets and found that Asian Americans averaged 1,416 cm3 (SD = 104 cm3), European Americans 1,380 cm3 (SD = 92), and African Americans 1,359 cm3 (SD = 95). Moreover, a recent MRI study found that people of African and Caribbean background averaged a smaller brain volume than did those of European background (Harvey, Persaud, Ron, Baker & Murray, 1994).
Contrary to purely environmental theories, these racial differences in brain size show up early in life. Data from the U.S. National Collaborative Perinatal Project on 35,000 children found that Asian children average a larger head perimeter at birth than do White children who average a larger head perimeter than do Black children, even though, at age seven, Asian children average smaller body size (and Africans larger body size) than do Europeans.
Further, head perimeter at seven years correlates with IQ at age seven in all three racial groups (see Rushton & Ankney, 1996, for review).
4. The heritability of intelligence. As discussed in The Bell Curve and Race, Evolution, and Behavior, the heritability of intelligence is now well established from numerous adoption, twin, and family studies. Particularly noteworthy are the heritabilities of around 80% found in adult twins reared apart (Bouchard, Lykken, McGue, Segal & Tellegen, 1990). Moderate to substantial genetic influence on IQ has also been found in studies of non-Whites, including African Americans and Japanese. Even the most critical of meta-analyses find IQ about 50% heritable (Devlin, Daniels & Roeder, 1997).
Transracial adoption studies suggest a genetic contribution to the between-group differences. Studies of Korean and Vietnamese children adopted into White American and White Belgian homes show that, although as babies many had been hospitalized for malnutrition, they grew to excel in academic ability with IQs 10 points or more higher than their adoptive national norm
s (Frydman & Lynn, 1989). By 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 had been raised.