CAMBRIDGE, Mass. — 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 Dr. 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 Oct. 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,” Dr. 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.”
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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. He endorses the new book “Winning the War on War” (Dutton/Penguin), by the political scientist Joshua S. Goldstein, which argues that the slogan “If you want peace, fight for justice” is precisely the wrong advice.

If you want peace, Dr. Goldstein argues, work for peace. Dr. Pinker agrees.

“It’s psychologically astute, given the massive amount of self-serving biases,” he said. “In any dispute, each side thinks it’s in the right and the other side is demons.”

The moral of his own book might be, If you want peace, understand psychology.