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JB: What do developmental psychologists have to say about the power of the genes?

HARRIS: That's a curious thing, actually. Nowadays most of them are quite willing to admit that children are born with predispositions to develop in a certain way, and they're even willing to admit that these predispositions have a genetic basis. But it doesn't seem to have dawned on them that children get their genes from their parents. They still haven't acknowledged the fact that whatever genetic predispositions the children have, there's a good chance the parents have them too. A child who was born timid has a better-than-average chance of being reared by a timid parent. A child who was born aggressive has a better-than-average chance of being reared by an aggressive parent. And the parents' genes are going to influence how they rear their kids. Timid parents are likely to be wimpier in their child-rearing methods; aggressive parents will be quicker to punish; affable parents will dole out more affection and praise. So everything's correlated: the parents' characteristics, the children's characteristics, and the parents' style of child-rearing. We haven't a hope of untangling this mess unless we have some way of factoring out genetic effects.

JB: How - by studying twins or adopted children?

HARRIS: That's one way — using the methods of behavioral genetics. And when the behavioral geneticists used these methods, they found something very surprising and puzzling, and it didn't have to do with genes, it had to do with the environment. What they found, in study after study, was that the environment shared by two kids reared in the same home could account for no more than 5 percent of the variance in personality characteristics. Heredity accounts for about half, so the other half must be due to the environment. But it isn't the home environment — at least, it isn't the environment shared by siblings who grow up in the same home.

The reason this is surprising is that the environment shared by siblings includes most of the things that are generally thought to have important effects on a child's development. The parents may have a happy or unhappy marriage or no marriage at all; the mother may stay home or go to work; the parents may spend their free time reading books or watching TV or going to a gambling casino. All these things are part of the shared environment: all the children in the family experience them in common, and if they're twins they experience them at the same age. But once we control for heredity by looking at twins or adopted children, we find that the shared environment has little or no effect.

JB: But most kids aren't twins and aren't adopted.

HARRIS: The problem is that if you don't use these methods, any environmental factor that you look at is likely to reflect genetic variation too. The parents who spend their time reading books are likely to differ in personality from the ones who opt for the gambling casino, and personality is partly genetic. So if their kids turn out differently, you can't tell if it's because of the environment the parents provided or the genes they provided.

One thing we can do, though, is to look at environmental factors that are less likely to reflect genetic variations in personality. For example, the home environment of the only child is very different from that of the child with siblings. All the affection, all the criticism, all the hopes and dreams that would normally be divided among two or three kids gets piled on this one poor kid. And yet researchers have been unable to find any consistent differences between only children and children with siblings. They've spent a lot of time looking for them, and published studies often do report minor differences — it's hard to get something published if there are no significant effects — but it's a different difference in each study. There's no overall tendency for the only child to be more neurotic or selfish, or less friendly or popular, than the child with siblings.

JB: Aren't you giving me proof that parents have zero influence?