Copy and pasted from another site:
Perhaps the most dramatic effect of legalized abortion, however, and 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 men enter 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.
If you think youre going to counter their hypothesis with crime drop explanations like innovative police strategies, increased reliance on prisons, changes in crack and other drug markets, aging of the population, tougher gun control laws, strong economy, increased police, all other reasons (capitol punishment, gun buybacks, etc.), be prepared to have your butt handed to you. These guys take each of these reasons and show statistically how irrelevant they are.