HARRIS: No. I believe that children learn separately how to behave in each of their environments and with each of the important people in their lives. The learning device that humans come equipped with doesn't operate on the principle that what worked in one context will work equally well in another. The baby who learns that his mother will pick him up and feed him when he cries can't assume that his cries will have the same effect on his father or his sister or the kids at the day-care center. It would be foolish of him to make that assumption, and he doesn't. The human mind is very good at making fine distinctions and at storing things in separate bins.

JB: What kind of evidence do you have for that?

HARRIS: There's quite a lot of evidence. Researchers have looked to see whether children who are dominated by older siblings at home are more likely to be dominated by their peers at school, and the answer is no, they aren't. Similarly, children who fight all the time with their siblings are not more likely to have stormy relationships with their peers. A baby who behaves in a somber, subdued fashion with his depressed mother will behave normally with a caregiver who is not depressed. A baby who has learned to kick her left foot in order to jiggle a mobile hanging over her crib will stare up at the mobile cluelessly if the crib is moved to another room. A child who is a troublemaker at home may be well-behaved in school, or vice versa.

JB: On the other hand, there are children who are troublemakers wherever they go.

HARRIS: Yes, that's true. I'm not saying that an individual's behavior in one situation is uncorrelated with that individual's behavior in a different situation: I'm saying that what the individual learned in one situation doesn't carry over to a different situation. Learning isn't the only thing that determines behavior: behavior is influenced by genetic factors as well, and our genome goes with us wherever we go. What we learned at home we can leave at home, but what we were born with we always have with us. The timid child tends to behave in a timid fashion in every environment. It was recently demonstrated that this consistency of behavior is due almost entirely to the genetic component of timidity.

The other reason why there is sometimes a correlation between behavior in different environments is that there is a good deal of similarity in the environments themselves. Many of the behaviors children learn at home — speaking English, saying please and thank you, not taking things that don't belong to them — work equally well outside the home. Most of the attitudes and values that people think they got from their parents — "Be honest," "Trust in God," "Work hard," whatever — are the values of the society as a whole, or of the subculture they grew up in. You might have learned these things at home to begin with, but the reason you took them with you, the reason you kept them, is that they agreed with what you encountered outside the home.

JB: Fair enough. But you were telling me what was wrong with some kinds of birth order studies.

HARRIS: Right. We were talking about what happens when you have people make personality judgments of their siblings, or of themselves relative to their siblings. What you get when you use these methods is a picture of how people behave with, or how they feel about, the members of the family they grew up in.

I don't doubt that birth order influences how siblings think and feel about each other and about their parents, and how they behave in the family setting. What I doubt is that people drag these effects along with them wherever they go. The kid who's bossed around by his older sister at home might find that he's the largest and strongest kid in his nursery school classroom. It wouldn't make sense for him to behave the same way with his classmates as he does with his sister, and he doesn't behave the same way. This is true even at nursery-school age.