Violent video games have a much more damaging effect on children than parents would like to believe, leading them to perform poorly in school, argue with teachers, condone aggression and get into physical fights with their peers, according to a series of new studies.

The four studies, published in the Journal of Adolescence, serve as a warning to parents and educators as video games become a greater and greater part of children's leisure activities.

"There is a growing group of ultraviolent games, and the research is becoming clearer that they're not good for kids," said Douglas Gentile, an assistant professor of psychology at Iowa State University, and lead researcher of one of the studies. Such games feature large numbers of people being shot or attacked, and graphic depictions of violence.

The study found that teenagers who have non-aggressive personalities but play a lot of violent video games are almost 10 times as likely to get into a physical fight than teens who don't play the games. What's more shocking, Prof. Gentile found, is that teens who play a lot of violent games are more likely to get into a fight than those who are aggressive but don't play them.

Prof. Gentile asked more than 600 Minnesota students in Grades 8 and 9 about the video games they played, their school grades and whether they had been in fights.

The study concludes that children who play more violent video games see the world as a more hostile place: They get into more arguments with teachers, are more likely to be involved in fights and get poor grades. On average, they spend nine hours a week playing the games, the research says. Boys play more than girls.

Another researcher found greater exposure to violent video games causes lower levels of empathy and stronger pro-violence attitudes.

Jeanne Funk, a professor of psychology at the University of Toledo, asked pupils in Grades 4 and 5 about their use of media and exposure and attitude toward real-life violence, and then took a measure of their empathy. She said her study adds another aspect to the risks of playing violent video games.

"This is maybe a first step in thinking that it's possible that playing video games could be associated with desensitization to violence," she said yesterday.

The use of violent and graphic video games has caused intense public debate.

Last year, two Tennessee teenagers were sentenced to an indefinite term for reckless homicide, endangerment and assault after imitating a video game. William Buckner and his stepbrother, Joshua, told investigators they took rifles from a locked room in their home and shot at random at passing vehicles, inspired by the video game Grand Theft Auto. The popular game involves murder and prostitution.

Andrew, a 15-year-old who declined to have his last name used, said he doesn't play ultra-violent games. He sticks to sports and less graphic shooting games. His parents watch what he buys. Asked if other students mimic video games, he said: "Depends on the person. If a person is very impressionable, then that's what they'll do."

Another study in the journal surveyed Grade 8 students in Germany, and says that those who like violent video games are more likely to condone physical aggression. The German researchers asked students which games they liked, and how they would act in various real-life scenarios.

The final study shows that even brief exposure to a violent video game (Doom, in this case) can lead people to associate themselves with aggressive traits and actions. Half the students in the study played Doom for 10 minutes, while the other half played a non-violent puzzle game. They were then given a series of tests, designed to measure how aggressive they felt, and how they'd react to other people.

Researchers say parents need to pay attention to the ratings on video games and how much time their children spend playing them.

"There are lots of risk factors for violent and aggressive behaviours: poverty, drugs, gang membership. . . . The difference between media violence and all those other ones is it's the one that's easily controlled," Prof. Gentile said. "We can just turn it off. We can say 'No, you can't play that game. No, you can't watch that show.' "