The murder of Stefan Pakeerah by a youth said to enjoy playing violent video games has focused attention once again on the effects of such entertainment.

Experts are divided on whether bloodthirsty role-playing games like Manhunt, Doom and Mortal Combat can make the people who play them more aggressive.

Psychologists in the United States, like Dr Craig Anderson of Iowa State University, are convinced that they can.

Dr Anderson carried out two studies, each involving just over 200 college students, and says he found that aggressive young men who did badly at college were particularly susceptible to violent games. Levels of aggression were raised after only a few minutes' playing, he said.

He believes that violent video games may be more harmful than violent films because they are interactive, and require the player to identify with the aggressive character.

"We found that students who reported playing more violent video games in junior and high school engaged in more aggressive behaviour," said .

"We also found that amount of time spent playing video games in the past was associated with lower academic grades in college."

But British experts are more sceptical of the claimed link. Dr Guy Cumberbatch of Birmingham University, who recently reviewed all the academic research on the subject on behalf of the Video Standards Council, said Dr Anderson was confusing cause and effect.

"Dr Anderson is one of the more recent academic Mary Whitehouses, who is quite convinced that violence in video games amplifies violence in society," he said.

"But the evidence is really quite questionable. A lot of it is based on college students pressing 'hurt' or 'help' buttons. The effect is very, very small, and to do with how people behave in laboratories rather than in society."

The only firm thing that could be concluded was that aggressive people who already had underlying violent tendencies tended to enjoy playing violent games, said Dr Cumberbatch.

If such people wanted to seek out violent images which would confirm that killing and hurting people was normal in society, they only had to read a newspaper or watch the television news, he said. He also poured scorn on the idea that teenagers could not tell the difference between a game and reality.

Manhunt, the game at the centre of the latest furore, requires players to identify with a scruffy anti-hero released from jail, who has to kill everyone who crosses his path in order to stay free. He gains credit for killing opponents in a more violent way.

The game was rated 18 by the British Board of Film Classification (BBFC) - one of only seven games to receive an adult certificate in Britain last year. Manhunt was banned altogether by the New Zealand Office of Film and Literature Classification last December. It is available without restriction everywhere else in the world.

Sue Clark, the BBFC head of communications, said that it was the victim's parents who were drawing the link between Stefan's death and the video game his killer played, not the police or the prosecutors. No such claim was made in court.

"It is a completely understandable reaction for them to want to find something to blame," Ms Clark said.

"I would probably do the same myself. But there is nothing here for the BBFC to defend. This game has not been officially implicated in this case. There is no good evidence showing a direct link between playing violent video games and carrying out violent actions."

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