"Harris and Klebold enjoyed playing the bloody, shoot-'em-up video game Doom, a game licensed by the U.S. military to train soldiers to effectively kill."

So reads the introduction to a new report on the relationship between video games and violent behavior, which cites high school gunmen Eric Harris and Dylan Klebold, who went on a killing rampage last year.

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The study, published in the April issue of the Journal of Personality and Social Psychology, was released on the Web on Sunday, just days after the one-year anniversary of the Columbine High School shootings.

Following the shooting, Klebold and Harris -- who shot and killed 12 students and one teacher, wounded 23 others, and finally killed themselves -- were found to have been video game enthusiasts.

Researchers Craig Anderson and Karen Dill concluded in two related studies, which involved notably violent games including Mortal Kombat and Wolfenstein 3D, that playing violent video games made players, especially men, act more aggressively.

"In the short term, playing a violent video game appears to affect aggression by priming aggressive thoughts," they reported. "Longer-term effects are likely to be longer-lasting as well, as the player learns and practices new aggression-related scripts that become more and more accessible for use when real-life conflict situations arise."

One of the two studies found that violent video games promoted both aggressive behavior and delinquency. The relationship between games and aggressive behavior was stronger in men and in individuals who were already characteristically aggressive. The second study also found that "laboratory exposure" to a graphically violent video game increased aggressive thoughts and behavior.

"In both studies, men had a more hostile view of the world than did women," the report said.

The report said results from both studies are consistent with the "General Affective Aggression Model," which Anderson and Dill created prior to the study for predicting aggressive behavior. The model expects aggressive behavior after violent game play in both the short term -- subjects getting more aggressive right there in the lab -- and the long term, i.e. juvenile delinquency.

The research was supported by the Psychology Department at the University of Missouri–Columbia.

The study doesn't put all the blame on video games, but points to them as a likely factor in the Columbine killings.

"Although it is impossible to know exactly what caused (Eric Harris and Dylan Klebold to attack their own classmates and teachers, a number of factors probably were involved. One possible contributing factor is violent video games."

The report noted that Harris' website included a customized version of the violent video game Doom. The personalized game featured two shooters with extra weapons and unlimited ammunition and opponents unable to fight back, according to the findings of the Simon Wiesenthal Center, which examined the shootings as part of its campaign to raise awareness of hate-related violence.

"For a class project, Harris and Klebold made a videotape that was similar to their customized version of Doom... . An investigator associated with the Wiesenthal Center said Harris and Klebold were 'playing out their game in God mode.'"

Anderson and Dill concluded the active, learning nature of video games make them potentially more dangerous than TV and movie violence. "As scientists, we should add new research to the currently small and imperfect literature on video game violence effects and clarify for society exactly what these risks entail."