The BBC reports on ten possible explanations. Somehow they didn't even mention more guns equals less crime. Here's my favorite.
There is a controversial theory put forward by economist Steven Levitt that the increased availability of legal abortion after the Supreme Court ruling in 1973 on Roe v Wade meant that fewer children were born to young, poor, single mothers. This, says the theory, stopped unwanted babies in the 1970s and 80s from becoming adolescent criminals in the decades that followed. But some of his peers have questioned whether the evidence really supports the theory.
What's your opinion? Please leave a comment.