Why Did The Crime Rate Drop In USA?

I'm reading a weird and wonderful book at the moment that someone lent me. Freakonomics, by Levitt and Dubner, is a really fascinating read about some of the stranger correlations in the world and the reasons why things work the way they do; from sumo wrestling* matches being fixed to why drug dealers live with their parents.

For example, the youth crime rate in America was predicted to rise and rise. But suddenly and miraclulously it dropped. Everything was suggested from a better economy to public morality rising. But the authors found an ingenious and rather satisfactory cause: abortion being legalised. The majority of children who were mugging and committing other crimes just weren't being born anymore.

And it's only £4.99

*I got my Japan link in.