ANYONE LIVING in the United States in the early 1990s and paying even a whisper of attention to the news could be forgiven for having been scared out of his skin. The culprit was crime. Death by gunfire had become commonplace. So too had carjacking and crack dealing, robbery and rape. And the experts warned us that things were about to get even worse. In 1995 the criminologist James Alan Fox wrote a report for the US Attorney General predicting that murders by teenagers would rise by at least 15 per cent over the next decade, and possibly even double.

But instead of going up, crime began to fall. The drop was startling in several respects. It was ubiquitous, with every category of crime falling in every part of the country. It was persistent, with incremental decreases year after year. And it was unanticipated.

The teenage murder rate fell more than 50 per cent within five years. By 2000 the overall murder rate in the United States had dropped to its lowest level in 35 years. So had the rate of just about every other sort of crime, from assault to car theft.

The experts argued that the roaring 1990s economy had helped to turn back crime. It was the proliferation of gun control laws, they said. It was the sort of innovative policing strategies put into place in New York City, where the number of murders fell from 2,245 in 1990 to 596 in 2003.

These theories were logical and encouraging, because they attributed the fall in crime to human initiatives.There was only one problem: they weren’t true. There was another factor that had contributed greatly to the massive crime drop of the 1990s. It had taken shape more than 20 years earlier: the legalisation of abortion in the US.

As far as crime is concerned, it turns out that not all children are born equal. Not even close. Decades of studies have indicated that a child born into an adverse family environment is far more likely than other children to become a criminal. And the millions of women most likely to have an abortion — poor, unmarried, and teenage mothers for whom illegal abortions had previously been too expensive or too hard to get — were often models of adversity. They were the very women whose children, if born, would have been much more likely than average to become criminals. But because of the change in abortion law, these children weren’t being born. This powerful cause would have a drastic, distant effect: years later, just as these unborn children would have entered their criminal primes, the rate of crime began to plummet.

It wasn’t gun control or a strong economy or new police strategies that finally blunted the American crime wave. It was, among other factors, the reality that the pool of potential criminals had shrunk.