Violent crime rates have reached a 40-year low, and many have tried to explain it, but there haven't been any universally acceptable answers. Decreasing lead exposure, higher incarceration rates, less crack cocaine use, closed-circuit cameras, increased video game use, and better detective methods leading to increased risk of getting caught are among the explanations.

Since violent crime, indeed all crime, is mainly the province of young men, how about this for an explanation: pornography. Porn has become widespread over the last several decades, but even more so with the internet. Virtually any young man who wants to view pornography can.
http://mangans.blogspot.com/2011/06/porn-as-explanation-for-falling-crime.html