JB: But doesn't the same sort of thing happen at home?

HARRIS: Yes. People find their own niches within their group and also within their family, and in both cases it affects the way they behave in that context. But the behaviors children acquire in the family don't have lasting effects, whereas the behaviors they acquire outside the home do.

JB: Why do you think that is?

HARRIS: It seems to be a built-in bias. It starts very early — by nursery-school age. Kids start dropping the accent they acquired at home and picking up the accent of their peers at an age when they're still spending more time with their parents than with their peers. It's not simply that they adapt readily to their two different language environments — it's that they favor one over the other, right from the beginning. They bring the language or accent of the nursery school home with them, they don't bring the language or accent of the home to school (unless, of course, it's being used there, too).

Simon Baron-Cohen made an interesting observation about accents in his review of my book in the journal Nature. He said that my theory had helped make sense of a study he did years ago, involving children with autism and their non-autistic siblings. These were the children of immigrants — one or both of the parents spoke a language other than English. Baron-Cohen found that the non-autistic children rapidly acquired the accent of their peers, but the autistic children generally retained the accent of one of the parents (the mother, in most cases). Children with autism have something wrong with the part of the brain responsible for social development. Studying these children has helped us appreciate aspects of normal development that might otherwise have gone unnoticed.

JB: Why do you think normal children are biased toward what they learn outside the home?

HARRIS: I think it's an evolved adaptation. Humans were designed by evolution to become members of a group, and to strive to become valued members of their group, because that's what it took to make a go of it during most of our evolutionary history. As Robert Trivers has pointed out, in the long run it would be counterproductive for children to allow themselves to be molded by their parents, because parents have their own agenda and it doesn't necessarily coincide with the child's. Anyway, if nature wanted to turn children into little replicas of their parents, there's a much easier way to do it. It's called heredity.

JB: What does your theory say about the transmission of culture?

HARRIS: That the usual view of cultural transmission — that the culture is passed down from the parents to the child — is inadequate and misleading.

Let me show you how it really works, using language as an example of a social behavior that is part of a culture. I like to use language because it's free of the genetic complications that plague other sorts of social behavior. If a person behaves in a cold or affectionate or aggressive manner, her behavior could be partly genetic, but we know that she didn't inherit her language or accent from her parents.

In the usual situation, the parents speak the same language as their neighbors. Let's say we're talking about an American family and their language is English. The child learns English at home and when she gets to nursery school she finds that everyone there speaks English too. No problem. She may be tentative about using it at first — she has to make sure it's going to work — but there's no need for her to acquire a new language or accent because her peers are using the same language and accent. She simply goes on speaking the way she learned to speak at home.

That's the usual situation, and it's the one that psychologists and anthropologists have in mind when they construct their theories. But if the child's parents are immigrants who speak English poorly or not at all, the child who grows up in a neighborhood where everyone speaks English will nonetheless become an English speaker, even if English was not the first language she learned and even if she goes on speaking her parents' language at home. She'll learn English from her peers, and she'll speak it the same way they do — without the foreign accent of her parents — and quite soon it will begin to supplant her parents' language. It will become her primary language, the language she'll think in as an adult.

JB: But sometimes the children of immigrants do end up with an accent.

HARRIS: That happens either because they were too old when their parents made the move — puberty generally puts an end to the ability to learn a new language without an accent — or because they grew up in a neighborhood where there were a lot of immigrants from the same country. A child who grows up in a Mexican-American neighborhood, for instance, will learn to speak English but she may always speak it with a Mexican accent, because that's how everyone in the neighborhood speaks it. That's how her peers speak it.