BackFire
Blood. It's nature's lube
Plus it's completely dependent on each individual kid.
When I was a kid, MK and Doom were both at the height of their popularity, all my friends at the time played them, myself included, and, none of them were aggressive in any way. In fact, the most aggressive people were those who didn't play any video games, the kids who skated and all that shit.
"studies" like this have no validity for the most part, because as pr1983 shows, we don't know the details of what they are referencing when it comes to "aggressive" behavior. The kid was playing the game, trying to complete an objective, failed, got mad and threw the controller down on the ground and said "damn it". Then the doctor takes this as "aggressive behavior caused by violent video games", when in reality it's simply caused from the same frusturation that could be caused in ANY video game, violent or not.
The ONLY kids who are actually affected by video games are those who are too young to realize that what they're seeing is not real, and think that they should mimic what they see on the game. In this case, it's naive and idiotic to blame the game for this, since it would be entirely the parents fault for allowing a child to play such a game. Also, kids who have prior mental problems are more apt to be affected by the game, but again, this is a fault of the parents, and no one else.