Stupid Study Game

I have decided to start an occasional series on silly studies, I leave it as an exercise to the reader to figure out the flaw. Generally, it will be a correlational study leading to a causal solution, but varieties exist:

Inaugural study: Video games and violence

After looking at more than 600 Grade 8 and Grade 9 students, we found that playing a lot of violent video games was a serious risk factor linked with children's anti-social and aggressive behaviour -- even after controlling for the amount the children play, their gender and whether they have naturally hostile personalities. Surprisingly, even the kids who are not naturally aggressive are almost 10 times more likely, if they play a lot of violent video games, to get into physical fights than kids who do not.

Can you spot the flaw?