A study of the effects of popular games such as Doom, Mortal Kombat and Grand Theft Auto, which involve brutal killings, high-powered weaponry and street crime, indicates that avid users become desensitised to shocking acts of aggression. Psychologists found that this brain alteration, in turn, appeared to prime regular users of such games to act more violently.

Many studies have concluded that people who play violent games are more aggressive, more likely to commit violent crimes, and less likely to help others. But critics argue that these correlations prove only that violent people gravitate towards violent games, not that games can change behaviour.

However, the new research, carried out by scientists at the University of Missouri-Columbia, goes some way towards demonstrating a causal link between computer games and violence, rather than a simple association. When shown images of real-life violence, people who played violent video games were found to have a diminished brain response. However, the same group had more natural reactions to other emotionally disturbing images, such as those of dead animals or ill children.

The researchers, led by Bruce Bartholow, a psychologist at Missouri-Columbia, found that the particular reduction in response associated with violence was correlated with aggressive behaviour. A type of brain activity called the P300 response, which reflects the emotional impact of an image on the viewer, was measured in 39 experienced gamers.

The participants were shown a variety of real-life images interspersed with violent scenes and other non-violent negative images. In subjects with the most experience of violent games, the P300 response to the violent images was smaller, and delayed. “People who play a lot of violent video games didn’t see them as much different from neutral (images),” Dr Bartholow said.

While such de-sensitivity is well documented and has resulted in the use of video games to prepare soldiers for scenes of war, researchers detected more alarming trends. When the game players were then given the opportunity to “punish” a pretend opponent in another game, those with the greatest reduction in P300 brain responses meted out the most severe punishments.

According to an early report of the study, published on newscientist.com, the website of the scientific journal, even when the team took into account the subjects’ natural hostility, the games experience and P300 response were still strongly correlated with aggressiveness.

Many shocking crimes, mostly committed by teenagers, have been linked to violent video games in recent years. In 1999, two high-school students shot dead 13 people and wounded 23 at Columbine High School in Littleton, Colorado. In 2002, a German teenager murdered 16 people as he walked through Gutenberg school in Erfurt brandishing a shotgun. Both incidents were later linked to violent video games: the American teenagers had enjoyed playing Doom, while the German youth was reported to have spent hours playing particularly brutal computer games.

In 2004, the game Manhunt was blamed by parents of a boy murdered in Britain for contributing to his death. Police found no direct links to the game, although some retailers removed it from their shelves.

Other psychologists said that Dr Bartholow’s findings, due to be published this year in the Journal of Experimental Social Psychology, offered further evidence of a worrying trend. Craig Anderson, of the Department of Psychology at Iowa State University, said: “These brain studies corroborate the many behavioural and cognitive studies showing that violent video games lead to increases in aggression.”

Some critics remain unconvinced by the findings, however. Jonathan Freedman, Professor of Psychology at the University of Toronto, who has prepared several government reports on media and games violence, said that all people “habituate” to any kind of stimulus. “All we are really getting is de-sensitisation to images,” he said. “There’s no way to show that this relates to real-life aggression.”

SO VIOLENT, IT’S ALMOST REAL

Last year’s most violent games, as assessed by the US watchdog Family Media Guide, included:

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