One of the things that happen when people split up into groups is that the different groups develop contrasting behaviors and attitudes. You can see this happening in the sex-segregated groups of grade-school kids. It's on the playground where boys act most boyish and girls act most girlish. Boys act tough — they hide their weaknesses and vie with each other for dominance. Girls don't have to hide their weaknesses — they use them as tokens of good faith. You show me yours and I'll show you mine and we'll be friends forever . . . or at least until Wednesday.

Timid girls often remain timid, but timid boys tend to become less timid as they get older — a change that is usually attributed to socialization by the parents. Our culture frowns on timid boys, the story goes, so parents teach their sons not to be timid. But the idea that it's the parents is contradicted by the evidence from behavioral genetics. It's not the parents: it's the peer group! A timid boy has a rough time of it in the boys' peer group. He's going to be picked on until he learns to master his timidity.

JB: So it's "peer pressure"?

HARRIS: In this case, yes. But I don't like to use that term because in most cases it's misleading. Pressure isn't usually necessary, and the impetus comes from the child doing the conforming, not from the group. Tailoring your behavior to that of the other members of your group is something that people of all ages do automatically, usually without even realizing that they're doing it.

JB: One of the criticisms I've heard of your theory is that kids generally associate with other kids who are similar to themselves — for example, good students hang around with other good students. So the kids who belong to the same group were already similar — it's not that they're conforming to the group.

HARRIS: Yes, it's true: kids who belong to the same group were similar to each other to begin with. That's not a valid criticism of my theory — it's one of the premises of my theory. Children identify with a group because they see that it consists of people like themselves. But once they've identified with it, three things happen. They become even more like their groupmates in some ways, less like them in other ways, and the differences between groups get wider.

The first effect is called assimilation. It's how socialization occurs — how children acquire the behaviors and attitudes of their culture. It's how the children of immigrants end up with the language and accent of their peers, not the language or accent of their parents.

The second is differentiation within the group. I think this is where most of the nongenetic variation in personality comes from. The members of a group don't act as a group all the time — sometimes they act as individuals. They vie with each other for dominance. They choose or are chosen for various roles and niches within the group — "group clown," for instance. These roles can be very stable — the dominant members tend to remain on top and those on the bottom tend to remain at the bottom — and I believe they have permanent effects on the personality.

The third thing that happens is called the group contrast effect. When kids split up into two groups — girls versus boys, jocks versus nerds — the differences between groups become exaggerated. The girls become more girlish. The nerds become nerdier. The kids who pride themselves on being weird or bad (these are often kids who were rejected by other groups) become weirder and badder. There's also likely to be hostility between groups, especially at times when group identification is salient, even though individual members of different groups might be friends with each other at other times.

JB: Do you have evidence that these things have an effect on personality?

HARRIS: Not as much as I'd like to have. In fact, my theory of personality development is still largely untested. I said at the beginning of The Nurture Assumption that I had two purposes in writing the book: to dissuade my readers of the notion that a child's personality is shaped or modified by the child's parents, and to present an alternative view of how the child's personality is shaped. But I should have said something about the scientific status of these two purposes. In regard to the first, the evidence is abundant and solid and I'm not saying anything original. I've just put all the evidence together and stated the conclusions more forcefully. The evidence indicates that children are socialized, and their personalities are shaped, by their experiences outside their parents' home. So now the challenge is to specify how this happens, and that's my second purpose. What I've done is to propose a new theory of how children are socialized and how their personalities are shaped. But I don't claim to have proved this theory, because the right kind of research hasn't yet been done. I've had to hunt around for bits and pieces of evidence, some of it anecdotal. I think the theory is promising and I hope it's going to be proven right, but it's early days yet.

JB: How does your theory account for the personality differences between identical twins raised together?

HARRIS: Within-group differentiation. Identical twins raised together usually belong to the same peer group, and a group that contains a pair of identical twins is going to find some way of distinguishing between them. They might be typecast in different ways: one might be regarded as the cautious one, the other as the one who'll do anything on a dare. Or they might differ in social status: questions and suggestions from the other members of the group will be addressed to one twin rather than the other.