The dominant hereditarian view of intelligence holds that intelligence is mostly fixed by the genes. Richard Nisbett has dismantled the evidence on which the hereditarian theory is based. In his new book, Intelligence and How to Get It, Nesbitt argues that the twin studies on which the hereditarian view is based are deeply flawed. The main problem is that the adoptive homes in which those separated twins often find themselves are uniformly enriched learning environments. Nisbett’s book was reviewed by Jim Holt of the NYT Book Review:
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.
What does Nisbett’s book have to say about race and intelligence? That the differences among the “races” are not genetic. Evidence in point: The “racial” IQ gap has 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.”