University of Michigan psychologist Richard Nisbett has an excellent piece on IQ and race.
Nearly all the evidence suggesting a genetic basis for the I.Q. differential is indirect. There is, for example, the evidence that brain size is correlated with intelligence, and that blacks have smaller brains than whites. But the brain size difference between men and women is substantially greater than that between blacks and whites, yet men and women score the same, on average, on I.Q. tests. Likewise, a group of people in a community in Ecuador have a genetic anomaly that produces extremely small head sizes — and hence brain sizes. Yet their intelligence is as high as that of their unaffected relatives.
Why rely on such misleading and indirect findings when we have much more direct evidence about the basis for the I.Q. gap? About 25 percent of the genes in the American black population are European, meaning that the genes of any individual can range from 100 percent African to mostly European. If European intelligence genes are superior, then blacks who have relatively more European genes ought to have higher I.Q.’s than those who have more African genes. But it turns out that skin color and “negroidness” of features — both measures of the degree of a black person’s European ancestry — are only weakly associated with I.Q. (even though we might well expect a moderately high association due to the social advantages of such features).