Crime had been high in the US since the late 1960s, and really exploded with the crack/cocaine epidemics in the 80s, but there was a precipitous decline after about 1992. I've heard several theories: mass incarceration, decline of lead poisoning, etc. I'm sure there was no one cause, but I'm interested in what most scholars have pointed to as likely.
The book "Freakonomics" points to Roe v Wade. The 'ugly' truth described in the book is basically that the pool of likely criminals was smaller. The correlation posited is that these "unwanted" children, if born, as they had been prior to Roe v Wade, were at a higher risk of being raised poorly, and more likely to turn to crime. That said, I have a feeling that that's still an incomplete picture. Economics also improved, and the correlation between crime and economic situation is well established.