Psychologist Steven Pinker calls Noam Chomsky 'a pen and paper theoretician', while Chomsky criticises Pinker's 'terrible reasoning'. Now Pinker is further testing Chomsky's controversial language theory on children. Harriet Swain reports

When Steven Pinker, eager to investigate the workings of language at first hand, decided to use children in his research, his adviser issued a warning: "You never know what's going on in their minds."

Undeterred, Pinker has dedicated himself to finding out just what is going on in all our minds - not only in terms of language but in relation to everything else too.

So his bestselling book The Language Instinct, which explored how a child's linguistic ability develops, was followed a couple of years later by How the Mind Works, which starts with a chapter on "standard equipment" and ends with "the meaning of life", negotiating examples from LA Law, Gary Larson cartoons and Indiana Jones along the way as "rewards for the reader".

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These two books, and the academic rows that followed them, have propelled Pinker into the big league of international scientists. He carries his academic credentials - professor of psychology at the Massachusetts Institute of Technology - alongside the style of a 1970s rock star, with his mane of curls framing a chiselled jaw and a penchant for designer suits and coloured ties.

A recent debate in London between Pinker and Richard Dawkins about science and the soul attracted more than 2,300 people, with 300 more queuing at the door. On the same trip he appeared on Newsnight, sparred with Melvyn Bragg and Jonathan Miller on Radio Four and wrote pieces for a couple of British broadsheets. Happening to arrive around Valentine's Day, he was more than happy to answer editors' requests for a cerebral view on love and romance.

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Fame, he says, "is kind of fun". Occasionally he is recognised in the street but people do not always know who he is. Sometimes, he says with a smile and a tug of that hair, they think he is the conductor Simon Rattle.

The only time celebrity has worried him is when he wrote an article suggesting women who murdered their new-born babies might not be evil but unconsciously obeying primitive instincts. This view, which he knew would be controversial, was targeted for attack by the American Christian rightwing.

"I got flooded by emails - some of them threatening," he says. Painstakingly, he replied to every communication - "I didn't want them to think I believed in infanticide" - and the fuss eventually died down.

Pinker is endlessly patient when there is something to be explained, spooning out phrases, heaped with goodies, in a soft voice. He explains the mind as "a system of organs of computation, designed by natural selection to solve the kinds of problems our ancestors faced."

The mind is not the brain but "what the brain does" and what the brain does is process information. To do this, it is organised into "mental organs", each designed by a genetic programme and shaped by natural selection to perform a specific function. Unravelling it all demands a kind of "reverse engineering", identifying what each "mental organ" is designed to do and then working out how it has developed to do it.

While Pinker suggests that "our minds lack the equipment to solve the major problems of philosophy", his theory offers an answer for almost everything else. Children in cities fear snakes because when humans were hunter-gatherers they needed to fear them. People like gossip because knowing what other people do not know gives strategic advantages in the games of life. They appreciate art because then they "gain status through cultural machismo".

And language - the gift humans alone possess - is just one more of the facilities we have to deal with the world, evolved over thousands of generations to enable better communication and increase our chances of surviving long enough to have children and pass our genes on to the next generation.

This belief, that language is a product rather than an accidental by-product of evolution, is one way in which Pinker's work diverges from American linguistics professor Noam Chomsky's theory of "universal grammar". In the 1950s Chomsky came up with the then controversial theory that all children are born with a sort of mental template for grammar that enables them to create sentences they have never heard before. But when Pinker suggested that the basis for this linguistic ability might be genetic, Chomsky disagreed, saying there was no scientific evidence for such speculation. Pinker has conducted experiments with children to explore how language works. He describes Chomsky as a "pen and paper theoretician".

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"Chomsky looked more at the output - what is the logic of language in an adult?" says Pinker. "I am interested in the nitty-gritty of language - how you account for real child language, for example, the way a child will say 'he breaked it'."

Pinker has taught words to toddlers and observed how they use them in sentences, measuring the speed at which they convert a verb into the past tense. In particular, he has focused on the way children use regular and irregular verbs and inflexion, seeing it as a way of encapsulating the two main processes of language: combining the memory of words heard with the rules of mental grammar to make new sentences. This will form the basis of his next book, Words and Rules:the Ingredients of Language.

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Pinker's own childhood was spent in Montreal, Canada. His grandparents were Jewish-Polish immigrants - "it was a culture where there was a lot of arguing" - his father a travelling salesman.

He studied social science at McGill and later Harvard University, arriving in the early 1970s, in time for discussions about what human nature was about but too late for the hey-day of student radicalism.

Then he discovered experimental psychology, which combined abstract theorising with the practical testing of such theories in the lab. His thesis was on visual imagery and spatial cognition and half his research was in that area until he decided to concentrate on language. This was not through any particular aptitude as a linguist, although he speaks some French and a smattering of Spanish and Hebrew.

Rather, he discovered the visual field was already crowded. "Also, when I was 17, I read about Chomsky and it seemed tremendously exciting. I'm interested in big ideas."

But he is anxious to point out that he never studied with Chomsky, although he did attend one of his courses. Nor does he see much of him these days, despite working in the same institution for the past 17 years.

As director of the McDonnell-Pew Centre for Cognitive Neuroscience at MIT, where he moved after Harvard and Stanford, he still teaches a class of 300 students.

Explaining his ideas to students helps him with his books for the general reader. These books - four so far, with another on the way - take application. He works at them seven days a week, often until 3am. But he enjoys the popular works because he can write less defensively than for a scholarly audience. Academics tend to write everything in such a way that they cannot be nailed by a quotation, he says. The problem is that by writing general books he risks people coming back at him in academic debate later.

And people do come back at Pinker.

Biologist Steven Rose clashed with him last year, accusing him of oversimplifying the nature of living things. Barbara Herrnstein Smith criticised How the Mind Works in the Times Literary Supplement, saying Pinker "evidently fails to grasp how some of the most elementary and ubiquitous instruments of human socialisation operate", while in The É«ºÐÖ±²¥S, psychologist Stuart Sutherland said the book carried at times "a touch of mania". Some say he relies too much on the ideas of others, in particular those of Chomsky, while Chomsky himself said The Language Instinct exhibits "terrible reasoning and confusions about evolution".

Stephen Jay Gould, too, has become an arch enemy, attacking Pinker's brand of evolutionary psychology in The New York Review of Books as "pure guesswork in the cocktail party mould". Pinker replied he was being "discourteous" and "uninformed".

Pinker says much of the hostile reaction to bringing evolution into psychology is because people think of adaptation in terms of what we ought to do. For example, when he uses evolution to explain tendencies among males to compete aggressively for dominance, people get angry. "But there is a confusion between is and ought," he says. The implication is that if we know why men behave like this, we can perhaps try to change their behaviour.

Another common confusion is to interpret theories of the "selfish gene", driven to replicate itself, as producing selfish people. But sometimes, he says, the most selfish thing a gene can do is to produce a selfless person.

Ironically, for someone who espouses evolutionary theory, Pinker, 44, and his second wife, Ilavenil Subbiah, do not have children. Pinker seems to revel in the situation. "Well into my procreating years I am, so far, voluntarily childless, having squandered my biological resources reading and writing, doing research, helping out friends and students and jogging in circles, ignoring the solemn imperative to spread my genes," he writes in How the Mind Works, adding succinctly, "and if my genes don't like it, they can go jump in the lake."

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For now, his contact with infants is through his work, as subjects of his language research. Again comes the slight smile as Pinker recalls another warning from a colleague. Apparently, the man, a new father, found his pet theory in tatters as soon as his own child spoke.