It has different effects on different people but I believe it can affect people but it greatly effects people who were already mentally disturbed to begin with, which are usually the ones who commit crimes. Of course people will say "lolz, gamez don't make peoplez violent!" because they don't want to see their precious games disappear. If it doesn't make people violent, it doesn't desentize people after witnessing so many violent scenes. When they see violence in real life, they aren't as effected by it because they've seen so many graphic scenes. So it does change people to some degree, some more than others. It all depends.
I know some will say violent crime has dissipated which I disagree as there's still plenty of violent gun related crimes in Chicago and Detroit, but one has to be careful where they research. I've seen studies saying that mercury can help your brain function better, when mercury is known for causing brain damage. So we have to be careful what we look up.