I believe there have been social studies showing the positive impact to crime by increasing access to abortion. I don’t have citations or the time to look them up now, but I am pretty sure it is referenced in Freakonomics and possibly one of Gladwell’s books.
There's no way to tell for sure, because it's such a broad statistic. Other people have opined that the removal of lead gasoline was also a reason of the drop in crime rates. No one really knows what is causing it.
