It's a story line that has become all too familiar: A young man with a gun snaps, for whatever reason, and sets out to take as many lives as he can. What emerges about the killers in the aftermath also tends to follow a similar pattern – they often showed signs of mental illness, kept to themselves, and in many instances played violent video games.

Several friends of Navy Yard shooter Aaron Alexis said he often played such games for hours.

But it's very difficult to claim that playing violent games triggers aggression and violence, said Patrick Markey, associate professor of psychology at Villanova University.

"There is really no evidence to suggest that such violent media is linked to these shootings, or really any real world violence," said Markey. "What we find in our laboratory studies are very small effects affecting our thoughts, our cognitions, but not so much affecting our actual real behaviors."



Many such studies are conducted on people who do not suffer from serious mental problems.

"It is irresponsible for researchers in my field to take research done on ... normal adolescent males and so forth, and try to generalize the findings to these actual horrific acts," said Markey. "We're kind of going beyond the data we collected in the laboratory when we try to predict it."

Markey said the bigger cause of such violence as was seen Monday, is the shooter himself. Alexis suffered from mental health issues, he reportedly heard voices, and suffered from paranoia.

"Looking for whether or not video games contributed to that (act of violence), really seems to be looking at perhaps the smallest effect here," said Markey.

There is also no evidence, said Markey, that playing violent video games where players are physically acting out the role of a killer, like shooting a fake gun, leads to aggression.

"There are theoretical reasons why violent video games should have a bigger effect than violent television, or movies. But empirically there is absolutely no difference," said Markey. Looking at "all of the studies done in these areas, we actually find the average effect of video games is actually lower than the average effect of movies, television, and so forth."

"At the very least, it is unfair to say that violent video games have a bigger effect on an individual than other violent media," said Markey.

As to why the perpetrators of mass shootings so frequently turn out to have played violent video games, Markey said that is because most men of a certain age play such games.

"The profile of most of these shooters are they are males, between 20 to 30 years old, and the average age of video gamers is about 30 years old," said Markey. "It'd be more unusual to find one of these shooters that didn't play video games."

"Finding that they play violent video games is not particularly surprising, considering if you take a random sample of any males, most of them are going to report playing violent video games," said Markey.