[Researchers] designed a study in which measures of anger levels acted as a proxy for violent behavior. They recruited 135 children, but were forced to kick some out of the study due to bad behavior, leaving them with about 110 boys and 15 girls with a mean age of 14.6 years, all of them familiar with the game of choice, Quake II. The children were given personality profile tests and measured for anger levels, at which point they were set loose for 20 minutes of gaming. Anger levels were measured again following the gaming session.77 of the subjects were not affected by the Quake II session, 22 had their anger levels double after playing, and eight of the subjects started out at high anger levels but registered lower levels after the session. Correlating the results with the teens' personality profiles, the researchers found that those with stable personalities largely fell in the unaffected group, while the remainder had personalities that were "considered less stable."
As Ars reports, the authors of the study suggest that gamers fall into two groups: those with stable personalities who are unaffected by violent games, and those with less stable personalities whose response to violent games depends on their emotional state.