March 26, 2006

The best measures of IQ also differentiate blacks and whites most strongly

I am one of those awful people who mention IQ in public from time to time so I thought a few quick words of background on it might not go astray:

IQ tests exist because 'g' (general intelligence) exists. It just is a fact that people who are good at solving one sort of problem tend to be good at solving lots of other different problems. Some problems, however, are particularly good at detecting people who are generally good at solving problems. Researchers speak of such "good-predictor" problems as ones that "load highly on 'g'". And, pesky though it may be, the problems that load most highly on 'g' (i.e. the ones that are the purest measure of general intelligence) are also the ones that differentiate blacks and whites most strongly. As Charles Murray explains:

"As long ago as 1927, Charles Spearman, the pioneer psychometrician who discovered 'g', proposed a hypothesis to explain the pattern: the size of the black-white difference would be "most marked in just those [subtests] which are known to be saturated with g." In other words, Spearman conjectured that the black-white difference would be greatest on tests that were the purest measures of intelligence, as opposed to tests of knowledge or memory.

A concrete example illustrates how Spearman's hypothesis works. Two items in the Wechsler and Stanford-Binet IQ tests are known as "forward digit span" and "backward digit span." In the forward version, the subject repeats a random sequence of one-digit numbers given by the examiner, starting with two digits and adding another with each iteration. The subject's score is the number of digits that he can repeat without error on two consecutive trials. Digits-backward works exactly the same way except that the digits must be repeated in the opposite order.

Digits-backward is much more 'g'-loaded than digits-forward. Try it yourself and you will see why. Digits-forward is a straightforward matter of short-term memory. Digits-backward makes your brain work much harder. The black-white difference in digits-backward is about twice as large as the difference in digits-forward. It is a clean example of an effect that resists cultural explanation. It cannot be explained by differential educational attainment, income, or any other socioeconomic factor. Parenting style is irrelevant. Reluctance to "act white" is irrelevant. Motivation is irrelevant. There is no way that any of these variables could systematically encourage black performance in digits-forward while depressing it in digits-backward in the same test at the same time with the same examiner in the same setting".




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