If your favorite “Grand Theft Auto” fan has not yet turned into a raging violent madman, you may be tempted to dismiss the huge pile of evidence linking exposure to media violence to an increase in aggressive and violent behavior. Not so fast.

In a clever study tracking how the brain responds to violent film scenes, scientists at Columbia University found a very specific effect: diminished activity in the region of the brain that controls reactive aggression. In other words, watching gruesomely violent images might not incite violence in someone who lives peacefully in a Buddhist monastery. But in real life--where drivers cut you off, colleagues undermine you, friends tease you and countless other provocations ensue--the loss of control over a reflexively aggressive reaction can have dangerous consequences.

The scientists showed seven men and seven women, mostly in their 20s, two dozen movie clips. Among them: a teenager breaking a bottle over someone’s head in “The Basketball Diaries,” a man getting shot in the head in “The Patriot,” a man slashing another with a sword in “Pulp Fiction,” one man hitting another with a meat cleaver in “Gangs of New York,” and similar edifying footage. The 14 also saw some neutral clips, as well as those of people being frightened (though not meat-cleavered). During the mini-screenings, the volunteers’ brains were scanned by functional MRI, which observes which regions are more or less active during different activities.

The most striking change as the volunteers watched more and more slashings, shootings and stabbings was in what’s called the right lateral orbitofrontal cortex, the researchers report today in the journal PloS One. The right ltOFC is a cluster of neurons nestled just behind the right side of the forehead. A number of earlier studies have linked it to control over reactive aggression. And although the scientists did not see how aggressively their volunteers would react to someone cutting them off in line, they did ask them so engage in some introspection by, for instance, answering whether they agreed that, “given enough provocation, I may hit another person.” The lower the activity in the region controlling reactive aggression, the more likely the volunteers were to answer with the equivalent of “hell yes.”

No wonder the scientists conclude that “even short-term exposure to violent media can result in diminished responsiveness of a network associated with behaviors such as reactive aggression.” Be careful not to jostle a fellow audience member at “American Gangster,” or he might deck you.
More seriously, the work goes some way toward getting beyond the generalities of the effects of media violence--increasing aggression--to pinpointing what kind of aggression is increased. Watching beheadings and the like won’t send you into an unprovoked violent rampage, but it lowers the threshold of provocation needed to do so.