A recent publication by Iowa State University researchers titled "The Effects of Video Game Violence on Physiological Desensitization to Real-Life Violence" appears to show a strong empirical link between desensitization to violence and exposure to violent video games. The researchers measured the participants physiological response to real-life violent stimuli. Those exposed to violent video games had a significantly lower response to the violent stimuli than those who had been exposed to non-violent video games.
To avoid confounding variables such as a predisposition toward violent tendencies, the participants were randomly assigned to the violent or non-violent game groups. While those in the violent game group did not show an immediate physiological difference than their non-violent game cohorts, they seemed less able to achieve an expected level of arousal when exposed to real-life violent stimuli. As on researcher stated, "It appears that individuals who play violent video games habituate or 'get used to' all the violence and eventually become physiologically numb to it."
News source: Iowa State University