http://www.physorg.com/news73230545.html
"Scientists define desensitization to violence as "a reduction in emotion-related physiological reactivity to real violence." Research indicated people who played a violent video game experienced skin response measurements significantly lower than those who had played a non-violent video game. The participants in the violent video game group also had lower heart rates while viewing real-life violence as compared with the non-violent video game group. "The results demonstrate that playing violent video games, even for just 20 minutes, can cause people to become less physiologically aroused by real violence," said Carnagey. "It appears that individuals who play violent video games habituate or 'get used to' all the violence and eventually become physiologically numb to it." [Brief]
The study is detailed in the current issue of the Journal of Experimental Social Psychology
