How are you to objectively measure the effect of video games on level of violence?
Here are some articles about a study that tries to create an objective measure.
eurekalert: Press Release
So the way the "study" creates this objective measure is by weighing the number of publications produced by both sides of an argument. I wonder if they have ever heard of the field called publication bias.
Here is another study that finds a link between video games and mental health issues