QUESTION for Pinker: Steve Pinker talks about the 45% personality variation which is not under genetic control or family influence. I have a question about identical twins. In both of your books, you selected convergent examples of identical twin behavior and did not talk about the divergent behavior, which is so interesting, in identical twins. When one interviews identical twins that are divergent, what one is struck by is the thoughtful way in which they have thought about their differences and come to observe them compared to the extraordinarily boring way in which the identical twins converge. It's almost as bad as memes, as in the time when wearing your baseball hat backwards was a similar piece of behavior many people did. They're like that. Divergent twins seem to have fought their way along different pathlines, and if they end up with a different inner environment, which leaves them freer. Can you say a word about divergent twins?

PINKER: Yes, I talk not only about the extraordinary similarities in quirks of behavior, such as sneezing in elevators; that I mentioned just to illustrate that the mind has a great deal more genetic specificity than we would have naively predicted. But I also talk about more profound similarities and differences between identical twins. The similarities are not just in the quirks; they are in fundamental dimensions of personality, such as whether you're conscientious or sloppy, whether you're anxious or relaxed, and whether you're antagonistic or friendly. Those traits also show a high, though nowhere near perfect, correlation between identical twins.

I also discuss hypotheses about why identical twins, though highly similar, are not identical in personality. One possibility is sibling interaction, in which each twin strives to differentiate herself or himself from the other twin. I also talk about chance factors that occur in an individual's lifetime: perhaps there is some effect of being chased by a dog, or receiving an act of kindness. Also, there are surely many unpredictable factors in the growth of the brain, since the gene can't specify every connection. I think it's an exciting project for psychology to test these hypotheses, and many personality psychologists are engaged in it. We know that one putative factor, namely growing up with a given set of parents, has a surprisingly small effect on long-term personality. In general, this research focuses our attention on the factors other than the genes that make us what we are.

QUESTION for Pinker: I partly agree with Pinker that procreation is important for loving your partner. But I would argue that procreation actually can much better explain why partners cheat each other, trying to find a higher chance for procreation, but it doesn't necessarily explain why. So: why would a partner stay with their spouse, as opposed to cheating and trying to find higher chances for procreation. Also, you didn't comment on what Rose said about homosexual love.

PINKER: I actually do have an extensive discussion of love as opposed to lust and sexual desire. I think the long-term commitment that you see in a husband and a wife, or in two close friends, and in homosexual lovers-although I don't talk much about homosexuality in the book-comes from a different dynamic. It's analogous to symbiosis in the natural world. You start off with a commonality of interest, that is, what is good for me is good for someone else. In the case of heterosexual marriage that trigger can be the shared genetic interest in the children, but in the case of close friends it could be things like having common interests, having common enemies, having common tastes, and so on. Once what's good for you is good for someone else, that gives you a stake in their well-being, and so you're apt to value them. If you value them, that makes you more valuable to them, and they're likely to value you, and you can get a positive feedback loop where the coalition of two people with common interests can develop into a long-term attachment. We experience this as the emotion of long-term companionate love. I think that's what keeps married couples together, and what keeps close friends together. It's a different emotion than sexual desire, and it's a different emotion than the head-over-heels infatuation that often gets a couple together to begin with. So love is a set of emotions, and I discuss them separately in the book.

ROSE: Steve has provided a neat cost benefit analysis of the merits of love, and it's precisely the point that I was making before about metaphors which he was so uneasy about. Here's a metaphor and a mode of thinking that he's taken over lock stock and barrel from a particular set of economic theories, and applied with enormous energy and ingenuity by evolutionary psychology. I happen to think it's a very impoverished way of trying to describe much more complex phenomena.