HARRIS: Sure. And there's no question that parents act differently toward a healthy child and a sick one, or to a larger child and a smaller one. But if these differences in parental behavior had long-term effects, we would have known about them a long time ago, because they would have turned up in birth order studies. There's no question that parents treat older children differently from younger ones, and firstborns differently from laterborns. These are systematic differences in parental behavior, not random ones, so if they had important effects it would be easy to detect them.

JB: You mean, because parents tend to favor firstborns?

HARRIS: No, I mean parents are more demanding with firstborns. Firstborns are given more responsibility; more is expected of them; they tend to be punished more harshly for mistakes. Parents are more tolerant of laterborns.

JB: But doesn't the firstborn get more attention?

HARRIS: Only till the second child arrives. Once another baby is born and the parents are taking care of both children at the same time, it's the younger one who gets the lion's share of attention and affection. This is true the world around: parents pay more attention and are more indulgent towards the youngest and smallest of their offspring. The firstborn may end up with the title or the farm, but it's the laterborn who gets the kisses.

Of course, this is going to vary from one family to another — an adorable 3-year -old might get more affection than a whiny or unattractive baby. But the trend is clear. In the studies I cited in my book, at least half of the parents questioned admitted that they gave more affection to one child than the other, and more than 80 percent of these parents favored the younger child.

You'd expect favoritism of this magnitude to lead to sizable birth order effects, if differences in parental treatment had important effects on children's personalities. You'd expect birth order to account for a noticeable amount of the unexplained variation in personality. But it doesn't. If birth order effects on personality exist at all, they must be very small and fragile, because big, well-done studies more often than not fail to find them.

However, there's an interesting exception to that rule. Significant birth order effects usually do turn up when personality is judged in a family context — for example, when parents are asked to judge the personalities of their children, or people are asked to compare themselves to their siblings.

JB: What's wrong with that method?

HARRIS: Nothing, if you want to know how people behave in the presence of their parents and their siblings. The trouble is, it doesn't tell you how they function in the world they inhabit as adults, which is what we would like to explain.

JB: But isn't there a carryover from one to the other?

HARRIS: That's what most people believe: that what you learn at home in the early years forms a template for your future. That your early relationships set a pattern you are constrained to follow, at least to some extent, for the rest of your life.

JB: And you don't think it's true?