Didn't the book Freakonomics have an explanation fro this?
Are you referring to Roe v. Wade and the reduction in the number of late-teen-early-twenties who had been unwanted babies before the Roe v Wade decision?
"To be sure, the legalization of abortion in the United States had myriad consequences. Infanticide fell dramatically. So did shotgun marriages, as well as the number of babies put up for adoption (which has led to the boom in the adoption of foreign babies). Conceptions rose by nearly 30 percent, but births actually fell
by 6 percent, indicating that many women were using abortion as a method of birth control, a crude and drastic sort of insurance policy.
"Perhaps the most dramatic effect of legalized abortion, however, adn one that would take years to reveal itself, was its impact on crime. 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 menter their criminal prime -- the rate of crime began to fall. What this cohort was missing, of course, were the children who stood the greatest chance of becoming criminals. And the crime rate continued to fall as an entire generation came of age minus the children whose mothers had not wanted to bring a child into the world. Legalized abortion led to less unwantedness; unwantedness leads to high crime; legalized abortion, therefore, led to less crime.
"This theory is bound to provoke a variety of reactions, ranging from disbelief to revulsion, and a variety of objections, ranging from the quotidian to the moral. The likliest first objection is the most straightforward one: is the theory true? Perhaps abortion and crime are merely correlated and not causal."
in: S.D. Levitt and S. J. Dubner, (2005) FREAKONOMICS A Rogue Economist Explores the Hidden Side of Everything
. Harper Collins. Where Have All the Criminals Gone? Chapter 4, pp 117-146, quoted from P.139
And there's a whole rest-of-the-chapter in the book that discusses a bunch of other factors and goes into greater depth about how different states and different countries had similar drops in crime rates a similar amount of time after legalization (or decriminalization) of abortion.
Please don't take this as me advocating for abortion - I've put the quotes here for those who may not have access to a copy of the book, and who may wish to read the rest of it for themselves.