''Unwantedness leads to high crime; abortion leads to less unwantedness; abortion leads to less crime.''

Levitt is a full professor in the University of Chicago's economics department, the most legendary program in the country. (He received tenure after only two years.) He is an editor of The Journal of Political Economy, a leading journal in the field. And the American Economic Association recently awarded him its John Bates Clark Medal, given biennially to the country's best economist under 40.

He is a prolific and diverse writer. But his paper linking a rise in abortion to a drop in crime has made more noise than the rest combined. Levitt and his co-author, John Donohue of Stanford Law School, argued that as much as 50 percent of the huge drop in crime since the early 1990's can be traced to Roe v. Wade. Their thinking goes like this: the women most likely to seek an abortion -- poor, single, black or teenage mothers -- were the very women whose children, if born, have been shown most likely to become criminals. But since those children weren't born, crime began to decrease during the years they would have entered their criminal prime. In conversation, Levitt reduces the theory to a tidy syllogism: ''Unwantedness leads to high crime; abortion leads to less unwantedness; abortion leads to less crime.''

In the abortion paper, published in 2001, he and Donohue warned that their findings should not be seen ''as either an endorsement of abortion or a call for intervention by the state in the fertility decisions of women.'' They suggested that crime might just as easily be curbed by ''providing better environments for those children at greatest risk for future crime.''

Still, the very topic managed to offend nearly everyone. Conservatives were enraged that abortion could be construed as a crime-fighting tool. Liberals were aghast that poor and black women were singled out. Economists grumbled that Levitt's methodology was not sound. A syllogism, after all, can be a magic trick: All cats die; Socrates died; therefore Socrates was a cat.

Profiling one of the leading figures in personal and social economics.