In Australia, we are limited by an MA15+ system, which is ineffective as far as keeping under 15's away from violent, drug referenced or 'advlt' themed games - because parents etc don't take responsibility for what their kids get up to, on the whole. What it does do is limit the choice that advlts can have...which means we are treated as children, because the basic issue is that when it comes to video games the lawmakers generally consider gamers to be in the teens, which isn't borne out by the facts - with the average age of gamers being 28+.
Consequently, we are trying to run a canditate against the attorney general of my state (who is the relcacitrant politician holding up an effective national ratings system), and a few of us are taking screenshots of graphical or distasteful nature and emailing them to him, his staff and his church. I've also pointed out to him that under his laws, his 'advlt themed tastes' would not be acceptable in games under the ratings system that we have, and I've also emailed that to his 'happy man's' social group, just to let them know what he really thinks of them.
All that aside, there are links between video game violence and real world violence - but it's not what most people would think of. Games don't make people go out and murder other people, but what they can do is desensitise them to combat or violent type situations...this is supported by the fact that most western law enforcement and military agencies use some form of video training for their people - whether this ranges from very basic 'target identification' systems all the way through to full on 3D combat environments prior to deployments is immaterial. The fact that it is used is enough to validate the hypothesis.
The basic problem is that most people get the wrong end of the horse when they consider the problem - mass incident shooters don't become mass incident shooters because of video games, its just that most younger people play video games and therefore when the background of a mass incident shooter is examined they generally turn up video games. This also provides a good scapegoat rather than blaming the failure of parents/teachers/guardians/friends/neighbours to recognise or act on the indicators; or the mental health system; or the gun laws of the place concerned.
With a guzillion people in the world today, the incidence of mass incident shootings/murders is going to be higher than it was 50 years ago, that's just the nature of things.