Our minds work the way they do because we have inherited our ancestors' genes, and the brain is an organ designed for computation. Steven Pinker explains his determinist vision to Jerome Burne, while Steven Rose argues that it's all a bit more complicated than that.

At first glance you might place him as a survivor from a Seventies rock band - good features, designer suit and a Roger Daltrey-style tumble of dark curls, now streaked with grey. But while the rock dinosaurs have long given up even the pretense of revolution, Steven Pinker has just written a manifesto whose aim is to storm some of the social sciences's most cherished bastions - that our minds are largely shaped by our culture, that parents mould their children's personality, that social factors decide our behaviour, that psychological problems are rooted in childhood. "I suppose I do line them up and mow them down" he says, imitating someone with a machine gun.

Not that he's remotely rabid. He's a psycholinguist at the Massachusetts Institute of Technology, but in another incarnation he might have been a successful analyst or chat-show host. He leans forward and nods encouragingly as you talk, as if to say "I understand what you're saying and it's really interesting".

When it comes to ideas though, he's very clear and utterly ruthless. His book How the Mind Works (Allen Lane pounds 25) pulls together a broad sweep of recent research to present an explanation that many find very threatening, or (as Steven Rose argues below) badly misconceived.

Stripped to its barest essentials, his thesis comes in two parts. The first says that our mental life - the way we think, our feelings, our goals, our desires - is shaped by the pressures of evolution, just as much as our hands or our hearts. Our minds work the way they do because those of our ancestors whose minds also worked like that passed their genes on.

The second part is that there is nothing special or magical about the brain. It is an organ designed to do a job, just like the stomach or the lungs.

"Everyone accepts that the job of the heart is to push blood round the body," says Pinker, "and knowing that enables you to understand why it's engineered the way it is. But people still balk at accepting that the brain is an organ designed for computation. Now, that doesn't mean it's a computer. What it does mean, though, is that its job is to work out strategies that boost the chance of survival. These range from navigating around a 3-D world using 2-D information from the retina - an awesome feat by the way - to selecting the mate who gives you the best chance of passing on your genes."

One charge made by his critics is that evolutionary explanations are merely dressed-up "Just So" stories - genes predisposing to homosexuality stay in the population because those with them help to bring up relatives' children. "Good evolutionary explanations make very definite predictions, says Pinker. "I write about one that explains sickness during pregnancy.

"It predicts a number of features of morning sickness, such as when it should occur, how long it should last, what effect it should have and so on. All of which turn out to be true. There's also an evolutionary- based theory of short-term memory that successfully predicts data retrieval systems on a computer."

Not surprisingly, given the misuse of theories that explain human nature in terms of their genes, Pinker is frequently accused of promoting a version of genetic determinism, portraying us as the puppets of our genes. "I don't believe genes determine behaviour" he says. "What they do determine is the repertoire of thoughts and feelings that individuals then translate into behaviour, depending on their life history and situation.

"Genes are what evolution works with. If you accept evolution, then adaption must have shaped our brains along with the rest of Nature." All told, a very reasonable revolutionary.

l Steven Pinker and Steven Rose will debate this issue on Wednesday at the Institute of Education, Gordon Square, London WC1. Call 0171-636- 1577 for tickets.