A $1.5 million study which began in 2004 and examined 1,2000 children concludes violent video games aren't harmful for children:
Two Harvard researchers have concluded that there's no data to support the notion that violent video games cause the kids who play them to act out violence in real life, contrary to the vast majority of media outlets that would have the public thinking otherwise. The $1.5 million study, which began in 2004, closely examined 1,200 children after bouts with violent games like Grand Theft Auto and not-so-violent titles like The Sims.
Psychologists Lawrence Kutner and Cheryl Olson found that for most kids, playing these games was nothing more than a stress reliever. Sure, some children displayed a playful aggressiveness after hours spent with a violent game, but this was no different than what children experience after seeing a martial arts action movie.
Some researchers, including the Harvard psychologists, even suggest that video games have a positive effect on the brain.