Measuring long-time-effects is hard. You probably could convince me with a well-done study that violence in games has a short-term effect on aggression. Like letting people play and provoke them afterwards, measuring the different reactions when playing prior an ego-shooter or Sims.

But long-term-effect? Just a have a look in the study. Using factors like the VGV-general measure fails to convince me that they didn't just discovered that violent people like violent games, or that people of the working-class or lower prefer to play (and are generally more aggressive) and so on.

It would be interesting to test various ways of mitigating violent behavior, like whether or not subjects are reminded before and after playing that "it's just a game." You'd also need to compare playing a video game for x hours to watching a movie for x hours, reading a book for x hours, or being made to wait quietly in a room with nothing to do for x hours. Finally, comparison to other pseudo-violent pastimes like high-contact sports would also be enlightening.

Edit: it should also be considered whether the benefits of video games outweigh any negative behavioral influence they may have.