Christopher J. Ferguson rounds up research on gaming and violence:

As a video game violence researcher and someone who has done scholarship on mass homicides, let me state very emphatically: There is no good evidence that video games or other media contributes, even in a small way, to mass homicides or any other violence among youth. Our research lab recently published new prospective results with teens in the Journal of Youth and Adolescence indicating that exposure to video game violence neither increased aggressive behaviors, nor decreased prosocial behaviors. Whitney Gunter and Kevin Daly recently published a large study of children inComputers in Human Behavior which found video game violence effects to be inconsequential with other factors controlled. And as for the notion of that violent media “desensitizes” users, recent results published by my student Raul Ramos found that exposure to violence on screen had no influence on viewer empathy for victims of real violence. (A study published by Holly Bowen and Julia Spaniol in Applied Cognitive Psychology similarly found no evidence for a desensitization effect for video games.)

Earlier analysis here.