The know-it-all tone of voice hasn't changed a bit, but the current crop of advice givers are giving advice that is almost the exact opposite of what parents were being told two or three generations ago. Since a large number of parents actually listen to the advice, kids today are being raised very differently from the way their grandparents were raised. They're getting more praise and kisses, fewer smacks and scoldings. Now ask yourself this: Are children today less aggressive than they were two or three generations ago? Are they nicer? Are they happier? The answer is no. Rates of violence, of depression, and of suicide have gone up, not down.

JB: So why haven't the advice-givers noticed that their advice is not having the predicted effects?

HARRIS: Good question. I suppose they'd say, if pressed, either that parents aren't following their recommendations carefully enough, or that changes in the culture as a whole have outweighed the changes in parenting styles. But shouldn't the culture as a whole have become more benevolent if parenting styles have become more benevolent?

Another example of a change that hasn't had the expected effect is the switch to androgynous child-rearing. Middle-class parents are giving their children unisex names and dressing them in unisex clothing. They're giving dolls to their sons and construction sets to their daughters. But the children are as sexist as ever. Grade-school kids still prefer to play with others of their own sex. They still get a kick out of insulting the opposite sex. Boys are still boyish and girls are still girlish, especially in places where there are lots of kids, such as school playgrounds.

JB: Do you think these differences are innate?

HARRIS: Only in part. There's undoubtedly an innate component — the same sex differences are found in every culture. But a lot of the behavioral differences between girls and boys are the result of group socialization. Group socialization is my theory of how kids learn how to behave when they're not at home. How they learn how to behave in public.

JB: How do they learn how to behave?

HARRIS: It's a very complex task — as complex as learning the language. Here are some of the problems kids have to deal with. First, they find out pretty early that behaviors that were acceptable at home, such as displays of emotion, are not acceptable outside the home. Second, the consequences of behaving correctly or incorrectly are also different inside the home and outside. Children don't get patted on the head when they do the right thing in school or on the playground: the usual result of doing the right thing is that nothing at all happens. On the other hand, if they do the wrong thing, the consequences can be much more serious outside the home. At home, children can cry or wet their pants or say something dumb, and nothing terrible is likely to happen, whereas if they do these things at school they might be laughed at or picked on or given an unflattering label that could stick to them for years.

So children know that they have to learn how to behave in public, and they want to avoid making mistakes. The safest way is to observe how others behave, but now there's another problem: which others do you observe? In every society, proper behavior depends on what sort of person you are — whether you're a kid or a grownup, a male or a female, a landowner or a peasant. These are called "social categories," and before children can figure out how to behave, they have to figure out what social categories are available in their society and which one they belong in.

This turns out to be surprisingly easy for them — as easy as learning the language. The fact that people (like most things in the natural world) come in continua rather than convenient clumps doesn't faze them: they don't hesitate to draw the lines, even though they might have trouble deciding where to put a particular individual. I once saw a 6-year-old go up to a 14-year-old and ask him, "Are you a kid or a grownup?"

It's a kind of concept formation and it starts early. By the time they're 2, children have acquired mental categories for grownups and kids, men and women, girls and boys. They know which ones they belong in, and they're already showing a preference for their own social category. Kids are attracted to other children, even at an age when they're wary of strange adults. When they have a choice, most little girls prefer to play with girls, most little boys prefer boys. (If they don't have a choice, they'll choose a child of the other sex over an adult, because the age category usually takes precedence over the gender category.) By kindergarten age, girls and boys are splitting up into sex-segregated groups whenever they have the chance — whenever there are enough kids to form separate groups and whenever there isn't an adult insisting that they play together.