The abortion claim was pretty well backed up by numbers; crime declines began earlier in states that legalized earlier, and crime fell by less in states where fewer providers existed. Moreover, the theory behind it makes perfect sense: higher capital investment (be it time or money) in children tends to lead to better life-outcomes; children who would otherwise have been aborted receive lower capital investment, either because their parents aren't ready for them financially, or because their parents don't like them as much.