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The Newtown, Conn., elementary school massacre has inspired fresh talk about the effect that violent video games like "Call of Duty" and "Grand Theft Auto" are having on children and young men like Adam Lanza, who was known by his friends to be a gamer.

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“It’s impossible to say what caused any given person to commit a horrific act like this,” said Brad Bushman, a professor of communication and psychology at Ohio State University and one of the country’s leading experts on video game violence.

He stressed that multiple factors may have contributed to the incident, including ease of access to guns and Lanza's mental health. “But we can say whether violent video games increase aggression … and they do.”

Bushman’s latest study, published two weeks ago in the Journal of Experimental Social Psychology, showed that people who played a violent video game for three consecutive days showed increases in aggressive behavior and hostile expectations each day they played. While gamers often scoff at such studies, the evidence is overwhelming, Bushman said.

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“It’s like smokers saying tobacco isn’t harmful. The scientific evidence is clear,” he told FoxNews.com. “A single cigarette won’t cause lung cancer, but smoking over weeks or months or years greatly increases the risk.”

Video gaming is a booming business, with sales totaling $2.55 billion in November, according to research firm NPD Data. Ratings from the Entertainment Software Rating Board (ESRB) are widely believed to effectively keep the most violent games out of kids' hands. But many kids simply share access to first-person shooters, logging in to websites rather than buying games at retail.

And the effect of violent games is clear. A second study Bushman recently conducted had gamers using gun-shaped or standard controllers, and aiming at human-shaped targets or bull's-eyes.

“People who played the violent shooting game were much more violent and more likely to aim for the head than people shooting at targets,” Bushman told FoxNews.com. “Video games are excellent training tools,” his paper noted.

While a handful of studies have downplayed the connection between video games and violence, there is a clear, well-established link overall: The American Psychological Association put it succinctly in 2003, nearly a decade ago.

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“Some studies have yielded nonsignificant video game effects, just as some smoking studies failed to find a significant link to lung cancer. But when one combines all relevant empirical studies using meta-analytic techniques, five separate effects emerge with considerable consistency. Violent video games are significantly associated with: increased aggressive behavior, thoughts, and affect; increased physiological arousal; and decreased prosocial (helping) behavior.”

Yet drawing a black and white line from “Call of Duty” to a violent action is nevertheless a challenge. Douglas Gentile, a psychology professor at Iowa State University, recently noted that there is no one simple cause for violent behavior.

“Aggression is multicausal. There are over 100 known risk factors for aggression; media violence is just one of them — not the biggest, but not the smallest.”

Video games seem to be one clear part of that puzzle. The challenge: violence has become an intrinsic part of modern video games, from "Call of Duty 4" -- the immensely popular first-person shooter in the study -- to humorous violence like that in "Duke Nukem Forever."

Nathan Fouts, founder of video game maker Mommy’s Best Games, recently wrote about the topic on popular gaming blog Gamasutra. Although his company is currently developing a game that stacks guns on top of already stacked guns (“I really like guns,” Fouts noted), he tried to imagine a world without video game violence.

“When you think about the whole spectrum of human emotion, violence is just one little sliver on there. You can think of so many other things that we could explore in video games,” he told FoxNews.com.

“Video games can do all those things. It’s such a young art form.”

The problem today is, games simply don’t: The vast majority of today’s best-selling games are violent ones. Eight of the ten most popular games listed on G4TV’s list of the Top 100 are highly violent, including two from the simulation series “Call of Duty” and two games in the “Grand Theft Auto” series.

And the result of those games is crystal clear, Bushman said.

“Violent games increase aggressive thoughts, angry feelings, physiological responses, aggressive behavior …. they decrease feelings of empathy and compassion for our fellow human beings,” Bushman told FoxNews.com.

“The effects are very robust,” he said.