THE COGNITIVE CURVE:

Get Smart : a review of INTELLIGENCE AND HOW TO GET IT: Why Schools and Culture Count By Richard E. Nisbett (Jim Holt, 3/29/09, NY Times Book Review)

[Nisbett] grants that I.Q. tests — which gauge both “fluid” intelligence (abstract reasoning skills) and “crystallized” intelligence (knowledge) — measure something real. They also measure something important: even within the same family, higher-I.Q. children go on to make more money than their less-bright siblings.

However, Nisbett bridles at the hereditarian claim that I.Q. is 75 to 85 percent heritable; the real figure, he thinks, is less than 50 percent. Estimates come from comparing the I.Q.’s of blood relatives — identical twins, fraternal twins, siblings — growing up in different adoptive families. But there is a snare here. As Nisbett observes, “adoptive families, like Tolstoy’s happy families, are all alike.” Not only are they more affluent than average, they also tend to give children lots of cognitive stimulation. Thus data from them yield erroneously high estimates of I.Q. heritability. (Think: if we all grew up in exactly the same environment, I.Q. differences would appear to be 100 percent genetic.) This underscores an important point: there is no fixed value for heritability. The notion makes sense only relative to a population. Heritability of I.Q. is higher for upper-class families than for lower-class families, because lower-class families provide a wider range of cognitive environments, from terrible to pretty good.

Even if genes play some role in determining I.Q. differences within a population, which Nisbett grants, that implies nothing about average differences between populations. The classic example is corn seed planted on two plots of land, one with rich soil and the other with poor soil. Within each plot, differences in the height of the corn plants are completely genetic. Yet the average difference between the two plots is entirely environmental.

Could the same logic explain the disparity in average I.Q. between Americans of European and of African descent? Nisbett thinks so. The racial I.Q. gap, he argues, is “purely environmental.” For one thing, it’s been shrinking: over the last 30 years, the measured I.Q. difference between black and white 12-year-olds has dropped from 15 points to 9.5 points. Among his more direct evidence, Nisbett cites impressive studies in population genetics. African-Americans have on average about 20 percent European genes, largely as a legacy of slavery. But the proportion of European genes ranges widely among individuals, from near zero to more than 80 percent. If the racial gap is mostly genetic, then blacks with more European genes ought to have higher I.Q.’s on average. In fact, they don’t.

Nisbett is similarly skeptical that genetics could account for the intellectual prowess of Ashkenazi Jews, whose average I.Q. measures somewhere between 110 and 115. As for the alleged I.Q. superiority of East Asians over American whites, that turns out to be an artifact of sloppy comparisons; when I.Q. tests are properly normed, Americans actually score slightly higher than East Asians.

If I.Q. differences are indeed largely environmental, what might help eliminate group disparities? The most dramatic results come from adoption. When poor children are adopted by upper-middle-class families, they show an I.Q. gain of 12 to 16 points. Upper-class parents talk to their children more than working-class parents do. And there are subtler differences. In poorer black families, for example, children are rarely asked “known-answer questions” — that is, questions where the parents already know the right answer. (“What color is the elephant, Billy?”) Consequently, as Nisbett observes, the children are nonplussed by such questions at school. (“If the teacher doesn’t know this, then I sure don’t.”)

The challenge is to find educational programs that are as effective as adoption in raising I.Q.