TUESDAY, May 31 (HealthDay News) -- Violent video games trigger aggression among those who play them, according to a new University of Missouri study.

Such players showed more hostility because their brains had become less responsive, or desensitized, to violence -- a response that the researchers linked to increased aggression.

"From a psychological perspective, video games are excellent teaching tools because they reward players for engaging in certain types of behavior. Unfortunately, in many popular video games, the behavior is violence," study co-author Bruce Bartholow, an associate professor of psychology at UM College of Arts and Sciences, said in a university news release.

Although other studies have linked gamers' desensitization to violence with increased aggression, the study is the first to demonstrate cause-and-effect, the researchers said.

The study is published in the current issue of the Journal of Experimental Social Psychology.

In the study, researchers instructed 70 young adults to play either a nonviolent or a violent video game for 25 minutes. Afterwards, their brain responses were measured as they viewed two groups of photos -- one neutral (such as a man riding a bike), the other violent (such as a man sticking a gun in another man's mouth).

The study found that participants assigned to play a violent video game had a reduced response to the violent photos -- an indication their brains had become desensitized to the violence.

Researchers pointed out, however, that participants who had experience with the violent video games prior to the study showed small brain responses to the violent photos, regardless of which type of game they were assigned to play.

"It could be that 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," said Bartholow, adding that there were additional factors to consider.

Researchers also discovered that a reduced brain response to violence predicted the gamers' levels of aggression: The weaker the brain response to violent photos, the more aggressive the participants were.

In this portion of the study, players were assigned a competitive task that allowed them to use a controllable blast of noise against their opponent. How loud the players made the noise for their rival was the measure researchers used to determine how aggressive they were.

The study found those who played one of several popular violent games, such as "Call of Duty," "Hitman," "Killzone" and "Grand Theft Auto," set louder noise blasts for their opponents during the competitive task than those who played nonviolent games.

The researchers concluded that future research should also focus on ways to control the effects of media violence, especially among children whose developing brains could become accustomed to violent behavior.

More information

The American Psychological Association provides more information on the adverse effects of violent video games.