In Freakonomics, Levitt offered the hypothesis that current lowered crime rates seem to date from the coming of age of kids who were not born as a result of Roe vs Wade. If he's wrong, it is certainly a remarkable coincidence.
I do recommend Freakonomics and it's follow up Super Freakonomics if anyone is interested.
If you liked reading Freakonomics, you might also like Malcom Gladwell's books.
Anyway, the hypothesis of lower crime, due to less young adults from unwanted pregnancies, is avoiding the other angle that some women utilize pregnancy to finagle a male into marriage, and if abortion allows a woman to move on after one relationship (with preganancy) does not end in marriage, then that woman is free to continue hunting for a marriageable male, via the bait of pregnancy. In effect, abortion may allow for a less than ethical manner to continue the hunt for a husband, in my opinion.