I was reading this study that tried to link increased aggression in kids with playing violent video games. Aggression is not always a bad thing, my guess is that they were focusing on violent aggression. They basically asked the kids how often they play video games, and then asked the kids to rate themselves on their aggression.
Rating themselves on what criteria? There are other factors that should be considered as variables in this situation. How about income level of the family? Where they live? Their parents involvement in their lives? Personally, there are too many variables to be considered here to decide that video games are the source of aggression.