Here is scientific confirmation of what everybody already knew to be true:
Playing violent video games changes brain function and desensitizes chronic players to violence, according to a new study led by University of Michigan researchers. The study, to be published in the Journal of Experimental Social Psychology, was conducted by Brad Bushman, a professor of psychology and communications studies and a faculty associate at the University of Michigan's Institute for Social Research; Bruce Bartholow, an assistant professor of psychology at the University of Missouri-Columbia; and Marc Sestir at the University of North Carolina at Chapel Hill.
Researchers asked 39 male undergraduates how often they played their five favorite video games, and how violent the games were. The researchers also assessed participants' irritability and aggressiveness, asking them how much they identified with statements such as: "I easily fly off the handle with those who don't listen or understand" and "If somebody hits me, I hit back."
The researchers outfitted participants with electrode caps to obtain EEG data, including the average amplitude of a particular type of brainwave, known as P300, which is believed to be an indicator of how people evaluate a stimulus, such as a photograph.
The researchers found participants who routinely played violent video games responded less to violent images, as measured by diminished P300 brainwaves. This was not true of their response to other, equally negative, nonviolent images.
Just as our body reflects what it is fed, so, too, does the brain. Feed the body fat, and it becomes fat (I know this from personal experience). Feed the brain violence, and it becomes violent, or at least more tolerant of violence. It's nice that science frequently just confirms common sense.