"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.''