The Freakonomics explanation (I just finished reading that chapter), says legalized abortion was a major contributor to decreasing crime rates in the 90's. However there were other contributing factors too. Author notes that it wasn't the "innovative policing strategies" of NYC that helped decrease crime rate, but the fact that they hired so many more police. There's a correlation there too. There's also a lot of discussion about crack cocaine and how that increased crime at its peak, but then related crime subsided.