Steven Pinker: Men and women are overwhelmingly similar in most of their mental traits. But there are some differences. Among the largest is differences in sexuality. Just what you'd expect, given that the thing that makes men and women different to begin with is their sexual organs. So it's inevitable that the way that the rest of the body uses those organs is going to diverge between the sexes as well.

Helena Cronin: The standard, dominating view, of course, is that natural selection made men's and women's minds as differently as it made our bodies. And that's not at all surprising because we've been faced with different evolutionary challenges over time. And indeed, it would be extremely hard— vanishingly improbable— if we were to turn out the same, not only in our bodies, but in our minds. So we should actually predict that we're going to have different psychologies. And indeed, of course, we do.

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Narrator: For Pinker, psychological sex differences are profound. From your sexual desire to your emotional state, to social and spatial skills. But for some feminists, these scientific observations imply inequality.

Helena Cronin: Science just describes the way the world is. It doesn't talk about the way the world ought to be. Morality is about what we think the way the world ought to be. And so they are two different spheres. The notion that, because Darwinians are saying, for example, men are more promiscuous than women, it's often assumed that Darwinians are sanctioning that. Well, it's not at all. It's describing that.

Steven Pinker: The whole idea that, if you explain behavior, you're getting people off the hook needs to be done away with. People can have all kinds of desires. They can want to sleep around. They can want to strangle their rivals. They can want to do all kinds of nasty and dirty things. The fact that they have those desires, the fact that we may even have an explanation of why they have those desires, doesn't mean they have to act on them.

Narrator: In his own life, Steven Pinker has not acted on the most fundamental evolutionary urge. He is in his second marriage, but has chosen not to have children.

Steven Pinker: Like most modern people, I make many decisions in my own life that don't carry out the supposed dictates of evolution. I don't have children, and most people I know certainly don't have as many children as they could have. This isn't because we've all surmounted human nature. It simply means that, because of contraception and other technologies, we can carry out some of our desires without necessarily bringing about the biological outcomes that, in the past, were the rationale for those desires.

Narrator: Pinker has conjured a vivid image of his personal choice. He says he's told his genes to go jump in the lake.

Richard Dawkins: I've always loved that phrase, "go tell your genes to jump in the lake." The point is that you can be an academic scientist who understands the principle of selfish genes perfectly well and understands that the reason why we're put in the world is because of evolution by natural selection of selfish genes, which means that telling your genes to go jump in the lake is the very last thing you'd expect to do. The whole point is, though, that we have brains which are capable of rising above that. It's not that our brains weren't fashioned by natural selection. Of course they were. But they were fashioned by natural selection to become big enough to do certain Darwinian tasks which incidentally have the consequence of letting us tell them to go jump in the lake.