According to a recent study conducted at Australia’s Swinburne University of Technology, stable children, or children that had not shown signs of aggression and anxiety before playing a game, were unaffected by the violence portrayed in violent video games.
The study tested 120 kids from ages 11 to 15 before and after they played a 20 minute round of Quake II. Signs of aggression among other things were likely to become increased in children who had already shown aggression beforehand. Children who were not instable in some way beforehand seemed to show little change afterwards.
So parents, if your kid is in some way unstable, please do not let them play mature rated games. Help keep our society safe!