Playing violent computer games can make people more aggressive, according to new research.

Studies carried out by Dr Craig Anderson from Iowa State University of Science and Technology, and Dr Karen Dill, from Lenoir-Rhyne College, found that violent video games can be more harmful than watching TV or films because they are interactive and often ask the gamer to identify with the aggressive character.

Both studies concentrated on notably violent games such as Mortal Kombat and Wolfenstein 3D. One of the two studies found that violent video games promoted both aggressive behaviour and delinquency. The relationship between games and aggressive behaviour was stronger in men and in individuals who were already habitually aggressive. The second study also found that 'laboratory exposure' to a graphically violent video game increased aggressive thoughts and behaviour.

The studies say violent video games were a likely factor in the Columbine Massacre last year when two US classmates gunned down teachers and classmates before turning their weapons on themselves. Both boys enjoyed playing so-called 'shoot'em up' video games including Doom, a game licensed by the US military to train soldiers to kill effectively.

The studies conclude: "The present results confirm that parents, educators and society in general should be concerned about the prevalence of violent video games in modern society, especially given recent advances in the realism of video game violence.

"The more realistic the violence, the more the player identifies with the aggressor. The more rewarding the video game, the greater potential for learning aggressive solutions to conflict situations."

The research is published in the American Psychological Association's Journal of Personality and Social Psychology and posted on the internet.