re: #1 Destro

Legalized abortions = less crime (sometimes referred to as the Donohue-Levitt hypothesis. Donohue and Levitt suggest that the absence of unwanted aborted children, following legalization in 1973, led to a reduction in crime 18 years later, starting in 1992 and dropping sharply in 1995. These would have been the peak crime-committing years of the unborn children.)

I first read that theory in Freakonomics and it is persuasive. However, since then I have read a competing theory that has excellent data to support its hypothesis.

The banning of lead-based paint in 1978. Lead has been shown to cause not only diminished IQs but has been demonstrated to show an increase in violent behavior.

TL;DR. Less eating of paint chips as a kids = less mugging of grandma on the sidewalk.