The author considers whether differences in genetic endowment may account for racial and ethnic differences in school readiness. While acknowledging an important role for genes in explaining differences within races, he nevertheless argues that environment explains most of the gap between blacks and whites, leaving little role for genetics.

Based on a wide range of direct and indirect evidence, particularly work by Klaus Eyferth and James Flynn, the author concludes that the black-white gap is not substantially genetic in orgin. In studies in 1959 and 1961, Eyferth first pointed to the near-disappearance of the black-white gap among children of black and white servicemen raised by German mothers after World War II. In the author's view, Flynn's exhaustive 1980 analysis of Eyferth's work provides close to definitive evidence that the black disadvantage is not genetic to any important degree.

But even studies showing an important role for genes in explaining within-group differences, he says, do not rule out the possibility of improving the school performance of disadvantaged children through interventions aimed at improving their school readiness. Such interventions, he argues, should stand or fall on their own costs and benefits. And behavioral genetics offers some lessons in designing and evaluating interventions. Because normal differences in preschool resources or parenting practices in working- and middle-class families have only limited effects on school readiness, interventions can have large effects only if they significantly change the allocation of resources or the nature of parenting practices.

The effects of most interventions on cognitive ability resemble the effect of exercise on physical conditioning: they are profound but short-lived. But if interventions make even small permanent changes in behavior that support improved cognitive ability, they can set off multiplier processes, with improved ability leading to more stimulating environments and still further improvements in ability. The best interventions, argues the author, would saturate a social group and reinforce individual multiplier effects by social multipliers and feedback effects. The aim of preschool programs, for example, should be to get students to continue to seek out the cognitive stimulation the program provides even after it ends.