HARRIS: I'm giving you proof that, in the ways parents are usually thought to have influence, they have little or no influence on the outcomes that have been measured so far. But I can't prove that they have zero influence.

For one thing, there's that 5 percent I mentioned. A typical behavioral genetic study shows that about 5 percent of the variation in adult personality can be attributed to the environment shared by siblings who grew up in the same home. But one of the weaknesses of the behavioral genetic method is that it can't distinguish between the shared home environment and the environment that siblings share outside the home. Siblings who grow up in the same home also live in the same neighborhood and go to the same schools. If they're twins they probably belong to the same peer group. I attribute the 5 percent to experiences shared by siblings outside the home, and, with the methods currently available, there's no way to prove that I'm wrong. On the other hand, I have no way to rule out the possibility that some or all of that 5 percent is due to shared experiences at home.

Second, researchers haven't looked at all the possible ways that parents could conceivably influence their kids. Perhaps there are subtle effects that their measuring instruments have missed. I must say, though, that they've been looking for them for an awfully long time.

Third, the data we have don't cover the entire range of families. They cover a wide range, but the same kinds of families that slip through the net of the census takers are also likely to missed by researchers. I can't rule out the possibility that a home environment could be bad enough to inflict permanent damage on a child's personality or mental health.

And fourth, it's possible that parents influence their children in ways that are completely unsystematic and unpredictable. What the behavioral genetic results show is that children raised in the same family don't turn out alike, except to the degree that they share genes. But why should they turn out alike when everybody knows that parents don't treat their children alike? Maybe parents treat their children differently in a completely random fashion — eeny, meeny, miney, mo — and maybe these random differences have important effects.

JB: Why random?

HARRIS: Random in the sense that the parents aren't just reacting to pre-existing differences in the children themselves — differences the children were born with. We know that parents do react to genetic differences between their children — for example, that a child with a troublesome disposition will be treated more harshly than one who was born agreeable. The problem is that these differences in parental behavior can't explain what we're trying to explain: the differences in personality, not caused by differences in genes, between two people who grew up in the same household. These unexplained differences turn up in adoptive siblings, in ordinary biological siblings, and in identical twins reared together, and we need an explanation that will work for all three kinds of sibling pairs. We need an explanation for the personality differences between identical twins reared in the same household, and we can't blame them on the parents' response to genetic differences between the twins, because there aren't any genetic differences between the twins. The vague but popular idea that it must be an "interaction" between heredity and environment won't work either. If there isn't any genetic difference, an interaction between heredity and environment can't produce a difference. It has to be a difference in environment, which puts us back to where we started.

JB: Maybe the parents are reacting to differences between the twins that aren't genetic. Identical twins aren't necessarily exactly alike when they're born. One can be larger or healthier than the other.