According to research conducted by the Centre for European Economic Research (ZEW), Baylor University and the University of Texas at Arlington, playing violent video games lowers the likelihood of committing violent crimes. The theory is simple: Violent people are drawn to violent games; they spend many hours playing them instead of, say, stealing some jerk's BMW or hitting an old lady with a bat.
While the study conceded the (debatable) point that violent games can cause aggression, the researchers concludes that the net benefit to society of keeping violent gamers cooped up in the living room playing Call of Duty instead of robbing trains and stealing stadiums outweighs the negative of making a few borderline psychos go over the violent edge.
"The effects of playing video games include both the promotion of aggressive behavior and a time use effect restricting the time for practicing aggressive behavior," says Benjamin Engelstätter, researcher at ZEW. "Our findings for the United States show that the time use effect on players is stronger than the aggression-promoting effect. Therefore, possible regulations of violent content in video games should be carefully designed. They could lead to a reduction in long-term aggressive tendencies. However, in the short-term, they would probably lead to a rise in crime rates as a number of gamers would spend less time playing video games that might have lost their appeal due to the regulations."
While it is true that crime rates have gone down over the last 20 years while video game playing has increased (see chart)...
... it's also true that correlation is not causation, and broad societal trends like declining crime rates have much more complex causes than the growing popularity of video games. For that matter, individual human action have more complex causes too, meaning attempts to understand the real-life effect of violent games (or any games) on a person is probably impossible. There's just too much input in the system to determine what causes things as complicated as crimes. If I were a betting man, though, I'd put my money on crime being caused by a combination of genetics and early childhood experiences. Call me crazy.
Also: As always, I feel I must remind anti-gaming people of this: Games are just pretend.