I know i'm in the minority here, but I think violent video games and movies do cause violence, BUT, I believe this is the case with CHILDREN playing them, not adults.
It's hard for me to put it into words but I'll have a go. Children's brains are still busy wiring themselves and it is a bad thing for them to be exposed to violence of any kind whether it be in video games, movies or domestic violence. Mainly because children have very active imaginations and the line between fantasy and reality is sometimes blurred (eg, My son used to think we had a dinosaur visit us and it would talk to him).
If they're exposed to violence at a young age they are desensitised to it. And I think it especially worse these days with the awesome graphics in the games, you know blood, brains, limbs flying everywhere, that kind of thing.
When my son was younger I rarely let him play violent games, I could notice the difference in his behavior after playing Tekken (and others, but that's the only one that springs to mind at the moment).
He's 13 now and I let him play anything he wants.
I work in a game store and I'm horrified when a 5 year old hands me a rated 15+ game full of gore (we don't have R rating over here because our classification system sucks arse big time). I have to confirm with the parent that it's 15+ and I can't sell it to a 5 year old without parental consent and they look at me like I'm an idiot.