Most studies done on violence and video games support the conclusion that violent video games can increase aggressive behavior in children and adolescents, especially boys, researchers said on Friday.
An analysis of 20 years of research shows the effects can be both immediate and long-lasting.
"The majority of the studies would suggest there are effects," said Jessica Nicoll of Saint Leo University in Saint Leo, Florida, who worked on the study.
One study showed that children who played a violent game for less than 10 minutes and then took a mood assessment test rated themselves with aggressive traits and aggressive actions shortly after playing.
Teachers of 600 8th and 9th graders, aged 13 to 15, said children who spent more time playing violent video games were more hostile than other children and more likely to argue with authority figures and other students.
The findings, presented at an annual meeting of American Psychological Association, prompted the group to adopt a resolution recommending that all violence be reduced in video games and interactive media marketed to children and youth.
"Additionally, the APA also encourages parents, educators and health care providers to help youth make more informed choices about which games to play," the Association said in a statement.
Video games set a bad example and may be particularly influential because a player takes on the roles of heroes and villains, violent and otherwise, the APA said.
Perpetrators of violence go unpunished 73 percent of the time in all violent scenes, the group said. "Showing violent acts without consequences teach youth that violence is an effective means of resolving conflict," said psychologist Elizabeth Carll, who helps direct the group's Committee on Violence in Video Games and Interactive Media.
Nicoll said in an interview that "only a handful" of the studies she and colleagues examined found no connection between violence and violent video games.
The findings are similar to those seen for violent television shows. Joaquim Ferreira of the University of Coimbra in Portugal and colleagues studied more than 800 youngsters aged from 9 to 14 and found the biggest factor linking television violence and actual aggression was the child's understanding of the violence.
"It is the way you perceive the violence and how you deal with the kids and help them understand reality," Ferreira, who also presented his findings to the APA meeting, said in an interview.
Parents can sit with children and explain cartoons or television shows to them -- something the APA and other groups recommend doing. But this is more difficult to do with video games, Ferreira said.
"You are part of the thing," he said. "You get involved in the violence because you are doing it."