Greg Tito said:
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Bartholow also mentioned that subjects who already play a lot of violent games had less reaction to the violent imagery, which apparently means something more significant than mere familiarity. "Those individuals are already so desensitized to violence from habitually playing violent video games that an additional exposure in the lab has very little effect on their brain responses," he said.
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I'm sorry, this makes no sense to me. If someone watches a lot of CSI, and then is shown a picture of a dead body, of course they are going to have less of a reaction than those who have never seen anything like that. That doesn't mean that these people are "desensitized" to violence, it just means there is a basic familiarity with those kind of images.
Here's another theory; perhaps people who play violent video games aren't 'desensitized' to violence, but just aren't affected by violent
imagery. In other words, if you show them an obviously staged picture of a man with a gun in his mouth, their brains go, "Oh, this is staged. It's just a picture, no need to get worked up about it."
Maybe a better way to test their reactions would be to take them into a coroner's office and show them a real cadaver of someone who was violently murdered, and gauge their reaction to
that. I won't presume to guess what the results would be, but gauging their reactions to the end results of real-life violence, 100% in the flesh, would probably give you more useful data.
Another idea: You could do to your test subjects the same thing EA's marketing department did to Yahtzee; stick them in a room with a source of constant annoyance that can only be removed by open aggression or, say, by solving a moderately complicated puzzle, and see who becomes aggressive and how long it takes them to become aggressive.
EDIT: A personal anecdote; I have a relative who (at least used to) get road rage CONSTANTLY. I almost had to wear ear-plugs every time I rode in the car with them. Every time anyone so much as turned a corner slowly they would explode in obscenities, and I think threats of violence were a common component of these outbursts. I think this tendency actually got this person in some trouble at their job recently.
Anyway, this person never started playing games until Bejeweled and FarmVille came out.
I realize this isn't exactly a statistically significant cross-section of the population, but this little slice of personal experience suggests to me that anger issues and violent behavior, whether they're influenced by media exposure or not, are probably much more significantly influenced by other factors.