Does media violence really have an effect?
Growing up, my younger brother was obsessed with the Ninja Turtles and the Power Rangers. He watched them constantly, he had all the action figures, costumes and toys. But he also had two sisters, and we liked to dress him up as a girl and force him to play house with us. This must have outweighed any of the supposed influence from the countless hours of "violent" television he watched, because he turned out to be a pretty nice and non-violent guy. Either that or - as I would argue - violence in the media doesn't impact or influence kids as much as some people suggest.
I think there is a tendency to immediately blame the media - TV, movies, video games - for violent incidents involving children and teenagers, and that the violent behavior can be attributed to many other factors that are underestimated or overlooked. I think that gender, social surroundings, and family life have more to do with a child's tendency to act violently than does the media.
With two sisters who liked to paint his nails and have tea parties and a dad who taught him that it was never okay to hit girls, it was unlikely that my brother was going to turn out to be the violent type. On the other hand, the way my cousins (two brothers ages 4 and 5) interact with one another is a completely different story. They are always punching, kicking and jumping on each other. They think it's fun. They like to beat each other up and purposely crash their kid-sized jeeps into each other at full speed. They get away with it more because they're just two boys and that's just "how boys act"; my brother just wasn't exposed to this as much because there were always so many girls around. But I can definitely see a big difference in how they act and how my brother acted at that age. They are much more aggressive and, yes, probably more violent. But the only show they watch is Sponge Bob, which (as far as I know) isn't violent. Their behavior seems to have more to do with who they are surrounded by and who they interact with on a daily basis.
While I think media violence might reinforce kids who already have violent dispositions, I don't think it's fair to say that it CAUSES violence, as this ignores important factors that likely play a larger role.