Updated

Violent video games really may make kids more violent, new research suggests.

Previous such studies had a big flaw — they couldn't prove whether the games had made the kids violent, or if already-violent kids were drawn to violent games.

But data from three long-term studies in the U.S. and Japan shows that otherwise peaceable kids first played violent games — and then months later became more aggressive in school, more likely to get into fights.

In a paper published in the medical journal Pediatrics, the researchers found the same effect in all three samples, despite the sharply different relative levels of violence in American and Japanese societies.

Younger kids were in fact more affected. American children 9 through 12, and Japanese kids 12 through 15, became more violent than did the teenagers in a larger pool of Japanese students aged 13 to 18.

"Playing violent video games is a significant risk factor for later physically aggressive behavior," concludes the study. "The research strongly suggests reducing the exposure of youth to this risk factor."

In a related study in the same issue of Pediatrics, a long-term study of kids aged 10 to 15 found a high correlation between exposure to violence on the Internet and self-reported criminally violent behavior, including assault, robbery, sexual assault, stabbings and shootings.

• Click here to read more about the video-game study in the Washington Post.

• Click here to read the abstract of the video-game study in Pediatrics.

• Click here to read the abstract of the generalized Internet-violence study in Pediatrics.

• Click here to visit FOXNews.com's Video Gaming Center.