Steven Pinker was a 15-year-old anarchist. He didn't think people needed a police force to keep the peace. Governments caused the very problems they were supposed to solve. Besides, it was 1969, said Pinker, who is now a 57-year-old psychologist at Harvard. "If you weren't an anarchist, " he said, "you couldn't get a date. "

At the dinner table, he argued with his parents about human nature. "They said, 'What would happen if there were no police?'" he recalled. "I said: 'What would we do? Would we rob banks? Of course not. Police make no difference. '"

This was in Montreal, "a city that prided itself on civility and low rates of crime, " he said. Then, on October 17, 1969, police officers and firefighters went on strike, and he had a chance to test his first hypothesis about human nature.

"All hell broke loose, " Pinker recalled. "Within a few hours there was looting. There were riots. There was arson. There were two murders. And this was in the morning that they called the strike. "

The '60s changed the lives of many people and, in Pinker's case, left him deeply curious about how humans work. That curiosity turned into a career as a leading expert on language, and then as a leading advocate of evolutionary psychology. In a series of best-selling books, he has argued that our mental faculties - from emotions to decision-making to visual cognition - were forged by natural selection.

He has also become a withering critic of those who would deny the deep marks of evolution on our minds - social engineers who believe they can remake children as they wish, modernist architects who believe they can rebuild cities as utopias. Even in the 21st century, Pinker argues, we ignore our evolved brains at our own peril.

Given this track record, Pinker's newest book, published in October, struck some critics as a jackknife turn. In The Better Angels Of Our Nature (Viking), he investigates one of the most primal aspects of life: violence. Over the course of 802 pages, he argues that violence has fallen drastically over thousands of years - whether one considers homicide rates, war casualties as a percentage of national populations, or other measures.

This may seem at odds with evolutionary psychology, which is often seen as an argument for hard-wired Stone Age behaviour, but Pinker sees that view as a misunderstanding of the science. Our evolved brains, he argues, are capable of a wide range of responses to their environment. Under the right conditions, they can allow us to live in greater and greater peace.

The Better Angels Of Our Nature is full of the flourishes that Pinker's readers have come to expect. He offers gruesomely delightful details about cutting off noses and torturing heretics. Like his other popular books, starting with The Language Instinct (1994), it is a far cry from his first published works in the late 1970s - esoteric reports from his graduate work at Harvard, with titles like "The Representation and Manipulation of Three-Dimensional Space in Mental Images".

Better Angels has impressed many experts on historical trends of violence. "Steven Pinker's great achievement is to weave these trends into a much larger pattern of reduced violence, greater empathy and, indeed, a comprehensive civilising process, " said Nils Petter Gleditsch, a research professor at the Peace Research Institute Oslo in Norway.

Reviews for the new book have been largely enthusiastic, though not unmixed. In The New Yorker, Elizabeth Kolbert called it "confounding, " "exasperating" and "fishy". "Hate and madness and cruelty haven't disappeared, " she concluded, "and they aren't going to. "

Pinker's response was equally scornful. "No honest reviewer would imply that this is the message of the book, " he wrote on his website. Though violence has indisputably declined, he says, it could rise again. But by understanding the causes of the decline, humanity can work to promote peace.