In the search to understand why crime rates across the United States have been dropping through the 1990s, a new study broaches the provocative possibility that the advent of legalized abortion may be a chief reason.

The research, which has been making waves lately at a round of academic seminars, suggests that, as women gained the right to terminate pregnancies, they gave birth less often to unwanted, economically deprived children who grow up into the kind of young adults most prone to commit crimes.

By analyzing abortion rates and the resulting drop in crime 20 years later, the study by a pair of researchers from Stanford Law School and the University of Chicago concludes that abortions account for about half the drop-off in murders, other violent crimes and property crimes since 1991.

"In some senses, this is a conjecture," said one of the authors, Steven D. Levitt, a University of Chicago economist who is a fellow of the American Bar Foundation, a nonprofit legal research institute. "We're talking about social phenomenon [and] 20-year time lags. This hypothesis can never be proven to the degree of certainty that a scientist might demand."

Nevertheless, Levitt said, he and co-author John J. Donohue III of Stanford were struck by the close link between abortion and criminal patterns. For example, in the five states that made abortion legal three years before the Supreme Court's landmark 1973 ruling, Roe v. Wade, crime rates started falling slightly earlier than in other parts of the country.

Moreover, states that had especially high rates of abortions in the first years after Roe have tended to have particularly large recent declines in crime, even when factoring out other considerations, such as changes in prison sentences and the size of police forces, states' unemployment patterns, and how much crime had been occurring there before crime began tapering off.

The researchers further demonstrate that most of the recent reduction in crime is occurring because fewer offenses are being committed by people younger than 25, essentially the age cohort that was born after women gained access to legal abortions.

And they cite previous research showing that the women most likely to have abortions are teenagers, unmarried, and African American -- groups of women whose children are at greatest risk of committing crimes once they become young adults.

As unorthodox as their thesis is, Levitt said, in a country in which one-fourth of all pregnancies end in abortion, it should not be surprising that this would produce "massive social effects." He said the study was intended to help shed light on the causes of crime -- or the lack of it -- and was not meant to add to the highly polarized debate that continues to swirl around the issue of abortion.

But if their ideas are correct, he added, crime might be expected to ebb gradually for about two more decades as the full impact of women's access to abortion on birth rates has been phased in.