"No one will like it," said Steve Levitt, a University of Chicago economist. "I don't think it is our job as economists or scientists to withhold truth because some people are not going to like it."

Women whose children would have been most likely to commit crimes as young adults instead chose to have terminations after abortion was legalized in the early 1970s, says the report by Mr. Levitt and John Donohue, a law professor at Stanford university. "Abortion provides a way for the would-be mothers of those kids who are going to lead really tough lives to avoid bringing them into the world," said Mr. Levitt.

The paper, which has not been submitted for publication in any academic journal, says that states with high abortion rates in the 1970s had big drops in crime in the 1990s that each 10% rise in terminations led to a 1% drop in crime years later.

Serious violent crimes by 12 to 17-year-olds dropped by 40% between 1993 and 1996, says the justice department's latest crime victimization survey and the number of youths arrested for murder fell 39% between 1993 and 1997, says the FBI.

But David O'Steen, executive director of the national right to life committee in Washington, said: "You mean killing babies in the 1970s led people in the 1990s to do less shoplifting? I can't believe that any significant percent of the population would argue that we should kill unborn babies to affect whatever they say is being affected."

The fall in crime this decade started about 20 years after abortion was legalized across the states in 1993. Previous theories for the decline included increased prison sentences, more effective policing policies, a slower crack cocaine trade and a strong economy. Inclusion of abortion in the debate comes at a time when terminations are at a new low, about 1.2m a year, down from the 1990 peak of 1.4m. The latest poll on abortion shows that 42% describe themselves as pro-life, up from 36% three years ago, and those who are pro-choice fell eight points over the same period to 48%. During the 1990s doctors and abortion clinics have been the targets for at least 19 bombings, 100 arson attacks, seven murders and 621 death threats or stalkings. Ten months ago Dr. Barnett Slepian was shot through the head by a sniper at his home in Buffalo, upstate New York.

Women aged under 25, who are separated and have never married, are poor or from minorities, are about twice as likely to have an abortion as others of child-bearing age, said the Alan Guttmacher institute, a New York-based research organization. One in four pregnancies ends in abortion now, compared to 20 years ago, when the figure was one in three.

Cory Richards, the institute's vice-president of public policy said of the Levitt-Donohue paper: "This is not an argument for abortion per se. This is an argument for women not being forced to have children they don't want. This is making the point that it's not only bad for the women, but for children and society."