I get really annoyed by these studies, not because I think there wrong but because they seem to be missing the point.
*Disclaimer*
IM NO EXPERT, but isn't this true for most forms of media? Im talking about the general sense of violence leading to more violance, thats what I was always taught. Chances are people are going to be more aggressive after watching Conan the Barbarian then say if they watched Toy Story because thats what art does, gets you feeling stuff. Games even more so as there a experience dictated by the player, interactive media/art = it puts you in the mind set of the game, at least, a good game does. The same could be said for sports to, the competitive nature of these pass times breed a us vs them point of veiw leading to higher levels of aggression. Lets not joke about, theres a much higher of chance of people involved in sports being aggressive then people involved in mathematics. This is of course disregarding the fact that alot of violent people get into these pass times because their just naturally more violent.
HOWEVER, I wouldn't say the problem was people experiencing these pass times but their lack of balance. If your always in a violent mind set, you'll naturally become more violent, the same as any other emotion. If I had a kid and they enjoyed playing CoD then thats fine, as long as they tried other stuff to. I seriously don't think violent medias the problem, its the fact people arn't trying other stuff. After all the sames true for things like sex, doing it every now and then is seen as healthy while doing it every moment of the day is seen as a little unhealty, or legendary depending on who you ask... or maybe im just babbling and its pointless to try and catergorise people as their all different... yeah lets just go with that coop out shall we.