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Steven Pinker: You can't understand human nature without thinking about how humans evolved. The fact that we're products of natural selection explains some of our deepest strivings, why we love our children, why we love our spouses, why we enjoy sex, why we strive to stay alive. All of these are exactly what you'd expect as the outcome of a Darwinian process.

Narrator: At the heart of Pinker's new work is an assault on a cherished conviction that has dominated intellectual life, that when you were born, your mind was a blank slate and your character formed by the culture you live in.

Steven Pinker: The blank slate refers to the idea that the mind has no inherent structure, that babies are born without any talents or abilities and that the entire mind and personality is a product of the environment, of socialization by parents and of absorption of values from the culture.

Man: The blank slate is immensely pervasive in the social sciences that still react in an emotional, almost hysterical, way to the idea that genes have anything to do with those aspects of human behavior which are of sort of social interest. Yes, it's very pervasive.

Narrator: Why has the idea that your human nature is sculpted from nurture rather than nature carried such weight?

Steven Pinker: Well, there are some politically and morally attractive features of the doctrine of the blank slate. If all of us are born with nothing that means that all of us are, by definition, equal. And there's no room, a priori for some people, or one sex, or one race to be smarter, or faster, or healthier, or more conscientious than any other. If we're all blank slates then by definition we're all equal.

Narrator: And the blank slate was not merely a laudable, liberal aim. It was also a reaction to the dark heart of a century.

Steven Pinker: Part of the appeal of the blank slate is that the opposite view, the idea that there's a biological constitution to humans, was horrendously perverted in the late 19th and early decades the 20th century. In eugenics, the desire to reshape humans by biological means, in social Darwinism, the idea that the upper classes are dominant nations deserve their success, and were carrying out the wisdom of nature. And that the downtrodden and the poor were being eliminated as part of the process of survival of the fittest.

And most horrifically of all in Nazism, the idea that there were superior and inferior races. That the superior races were being contaminated by inter-breeding. And that it was part of the process of evolution that the inferior races be eliminated. And it's quite legitimate to refute and to denounce the pseudo scientific beliefs about biology and race that led to those horrors.

Narrator: But the 20th century was blighted by more than Nazism. And in the other epic atrocities Steven Pinker believes the blank slate was an ideological driving force.

Steven Pinker: There were two sets of holocausts in the 20th century. And the other one was carried out by the Marxist regimes of Stalin, and Mao, and Pol Pot, and others. , Remarkably their theory of biology and human nature was the exact opposite of the social Darwinism behind Nazism.

Narrator: China's cultural revolution killed millions in its attempt to remold the people of the Republic. And Chairman Mao asserted boldly that on a blank sheet of paper can be written the most beautiful words.

In Cambodia the Khmer Rouge captured the spirit of the blank slate in the slogan, "only the newborn baby is spotless."

Steven Pinker: If the mind is indefinitely malleable then that opens the door to a totalitarian regime to do the molding. If people are being molded anyway, may as well do it systematically. Instead of allowing it to happen haphazardly. And so it justifies totalitarian social engineering. So ironically Nazism and Marxism shared the idea that humanity could be reshaped. In the case of Nazis by biological means. In the case of the Marxists by social means.