I actually wrote a paper in university about whether violence in games and tv causes violent behavior in real life. It turns out that the US has spent tens of millions of dollars since the 1950s on studies trying to prove that violent media does in fact cause real-world violence. After the decades of studies, and the gobs of money that's been spent, still not one shred of conclusive evidence has supported the notion that games cause violent behavior.

Here's a link to a draft of the paper if anyone's interested: http://goo.gl/TJQaU