I remember reading an article about a study that was published in Japan in (roughly) 2000 that demonstrated a link between videogame playing and violence in children; this link was not what people anticipated though ...
The study demonstrated that the content wasn't that important it was the quantity of videogames a child played that was correlated to how violent they were; if I remember correctly, up to 2 hours a day had little impact, 2 to 4 hours a day had a minor impact, 4 or more hours a day was very noticeable. They also mentioned that studies demonstrated a similar patter related to the ammount of television a child watched.
I came to the conclusion (at that point in time) that children who are not spending enough time interacting with other children and adults probably have very underdeveloped social skills and are probably going to resort to violence out of frustration.
Certainly, I think Manhunt is an inappropriate game for someone under 17 (or so) but I also think that parrents should be careful about assuming that spongebob squarepants is a 'good' choice.
