Extract from a book review of ‘Freakonomics’, by Levitt & Dubner.

One of his best-known, and in some quarters notorious, findings concerns America’s falling crime-rate during the 1990s. Towards the end of that decade, confounding the expectations of most analysts, the teenage murder rate fell by more than 50% in the space of five years; by 2000, the book notes, the overall murder rate was at its lowest for 35 years. Other kinds of crime fell too. Why? Some gave the credit to economic growth; others to gun control; still others to new methods of policing, or to greater reliance on imprisonment, or to increasing use of the death penalty, or to the ageing of the population.

Mr Levitt goes carefully through these various explanations, checking them against the evidence. He finds that some of them do offer a partial explanation (more jail time, for instance), whereas others do not (greater use of the death penalty, new policing methods). But the most intriguing finding was that one of the most powerful explanations had not even been broached. That explanation was abortion.

The reasoning is simple enough. In January 1973, the Supreme Court made abortion legal throughout the United States, where previously it had been available in only five states. In 1974, roughly 750,000 women had abortions in America; by 1980, the number was 1.6m (one abortion for every 2.3 live births). “What sort of woman was most likely to take advantage of Roe v Wade?” the book asks.

“Very often she was unmarried or in her teens or poor, and sometimes all three…In other words, the very factors that drove millions of American women to have an abortion also seemed to predict that their children, had they been born, would have led unhappy and possibly criminal lives…In the early 1990s, just as the first cohort of children born after Roe v Wade was hitting its late teen years the years during which young men enter their criminal prime – the rate of crime began to fall.”

The theory is the easy part, once you dare to articulate it. Testing it is quite another matter. But the book moves methodically and persuasively through the statistical evidence. It turns out, for instance, that crime started falling earlier in the states that legalised abortion before Roe v Wade; that the states with the highest abortion rates saw the biggest drops in crime (even controlling for other factors); that there was no link between abortion rates and crime before the late 1980s (when unborn criminals, as it were, first began to affect the figures); and that a similar association of crime and abortion has been found in other countries.