Cutting IQ Down to Size?

Richard Nisbett engages in wishful thinking (“All Brains are the Same Color,” Observer, February, 2009). We published an editorial about the James Watson affair in November last year (J. P. Rushton and A. R. Jensen. “James Watson’s Most Inconvenient Truth: Race Realism and the Moralistic Fallacy.” Medical Hypotheses, 71, 629-640; 2008) that concisely summarized our previous work, concluding that race differences were at least 50% heritable. We based our conclusion on 10 categories of research evidence. It was this evidence that Watson had at his disposal when making his claims about low African IQ test scores.
One of those lines of evidence is that the races differ in brain size. Larger brains contain more neurons and process information more efficiently. Magnetic resonance imaging (MRI) shows that brain size correlates with IQ within race about 0.40. One study followed 50,000 children from birth to age 7. The East Asian Americans in the sample averaged a larger head circumference at birth, 4 months, 1 year, and 7 years than White children who averaged a larger head circumference than Black children. By age 7, East Asian American children average an IQ of 110, Whites averaged 102, and Blacks averaged 90. These findings on race differences in brain size are highly reliable and have been confirmed using four independent procedures — MRI, endocranial volume from empty skulls, wet brain weight at autopsy, and external head size. Despite Nisbett’s half-suggestion to the contrary (wisely short of a full claim), Black, White, and East Asian twins show equal heritabilities. And IQ differences between all three are more pronounced on the more heritable items. The totality of the evidence can no longer be explained away in terms of old ideas based on pure environmentalism.

J. Philippe Rushton
The University of Western Ontario
Arthur R. Jensen
University of California, Berkeley

Author’s Response
I have responded to all of the Rushton and Jensen claims about the causes of Black/White differences in IQ in my book Intelligence and How to Get It: Why Schools and Cultures Count. With regard specifically to the finding that Blacks and Whites differ in brain size: a) It is not always found that Blacks have smaller brains than Whites; b) brain size differences between Blacks and Whites, when they are found, may have environmental causes; c) It is far from clear that the relation between brain size and IQ is causal — siblings in a given family who have larger brain sizes are not more intelligent than those with smaller brain sizes; d) Men and women differ more in brain size than anything claimed by Rushton yet have approximately equal IQ; e) There are people in a community in Ecuador with head sizes several standard deviations below average yet these people do better in school than normal individuals; f) The direction of brain size over the last few thousand years is down; and g) Einstein had a smaller brain than the average found by Rushton for Blacks!

Richard E. Nisbett
University of Michigan