Levitt's simplistic model of how abortion would cut crime by reducing "unwanted" births founders when you look at two facts: that, as Levitt admits, legalizing abortion greatly increased the number of unintended pregnancies, and that a lot of those unintended pregnancies go on to get born.
Physicians for Reproductive Health and the Alan Guttmacher Institute provide data that shows Levitt's simplistic model of "unwantedness" is a joke when applied to the real world. According to 1994 data, a full 48% of the 6.3 million pregnancies in the U.S. were unintended. Out of those 3.1 million unintended pregnancies, 47% were aborted, 13% miscarried, and a full 40% were born, or close to 1.25 million unintended births annually -- all with legalized abortion! This makes a hash out of Levitt's neat little model. I don't know what the overall effect is on who gets born, and he doesn't either.