Shortly after moviegoers had settled in to watch a midnight premier of The Dark Night Rises on Friday morning, a heavily armed gunman entered the Aurora, Colo., theater through an emergency exit and opened fire. In just a few minutes the assailant shot more than 60 children and adults—killing at least a dozen—before police arrested him outside the theater.

This massacre was the only latest in a string of eerily similar incidents in recent years involving the mass murder of civilians, spectators and bystanders by an individual with a firearm and a frightening lack of regard over its use. In April 2007 32 people were shot to death and 17 injured on the campus of Virginia Polytechnic Institute and State University in Blacksburg, Va., by a former student. In January 2011, former U.S. Rep. Gabrielle Giffords was shot in the head by an assailant who killed six people during an attack outside a Tucson supermarket. Just a few months ago a gunman killed seven people at Oikos University in Oakland, Calif.

In each of these and other similar cases, the lone assailant who was either captured or found dead at the scene of the crime matched a particular profile—a disgruntled loner with grievances against societal institutions and who displayed an abhorrent inability or unwillingness to exercise control over violent impulses. Following the attempted assassination of Giffords, Scientific American spoke with Marco Iacoboni, a University of California, Los Angeles, professor of psychiatry and biobehavioral sciences and director of the school's Transcranial Magnetic Stimulation Laboratory, about why some individuals act on their violent thoughts whereas others do not. Although details about the life of University of Colorado neuroscience Ph.D. student James Holmes, arrested for the Aurora shootings, are still being uncovered, several of Iacoboni's observations about accused Giffords gunman Jared Lee Loughner seem apt to shed some light on the violence that recent took place Friday morning.

Scientific American: What turns anger into action?

Iacoboni: Mostly cognitive control, or to use a less technical term, self-control. Self-control is key to a well-functioning life, because our brain makes us easily [susceptible] to all sorts of influences. Watching a movie showing violent acts predisposes us to act violently. Even just listening to violent rhetoric makes us more inclined to be violent. Ironically, the same mirror neurons that make us empathic make us also very vulnerable to all sorts influences.

What has neuroscience uncovered about the capacity of the person who shot Giffords, the person responsible for the Virginia Tech massacre in 2007, and many others (yet still a small percentage of people) to behave so violently?

Their cognitive-control mechanisms are deranged. Mind you, these individuals are not out-of-control, enraged people. They just use their cognitive-control mechanisms in the service of a disturbed goal. There are probably a multitude of factors at play here. The subject is exposed to influences that lead him or her to violent acts—including, unfortunately, not only the violent political rhetoric but also the media coverage of similar acts, as we are doing here. A variety of issues, especially mental health problems that lead to social isolation, lead the subject to a mental state that alters his or her ability to exercise cognitive control in a healthy manner. The cognitive control capacities of the subject get somewhat redirected—we don't quite understand how—toward goals and activities that are violent in a very specific way. Not the violent outburst of somebody who has "lost it" in a bar, punching people right and left. The violence is channeled in a very specific plan, with a very specific target—generally fed by the media through some sort of rhetoric, political or otherwise—with very specific tools, in the Giffords case, a 9-millimeter Glock.

What are the signs that a person is disturbed enough to take action?

The signs are quite visible, although difficult to interpret without a context—and unfortunately they unfold very quickly, and people can rarely witness them before the action is taken. The action itself is a sign, a desperate form of communication from a disturbed individual. Unfortunately, nobody was chatting with the guy when he left his final messages on Internet before getting into action. But I bet that if somebody was communicating with him before the act and saw those signs and read those messages on MySpace or whichever social network he was using, that person could have done something, could have engaged him in a sort of conversation that might have redirected his deranged plans. Indeed, by connecting with the subject, that person might have redirected some of the activity of mirror neurons toward a truly empathic behavior, rather than in the service of the deranged imitative violence leading to action.

Image of an AR-15 assault rifle muzzle courtesy of Guy Sagi, via The suspected gunman who killed 12 people during a screening of The Dark Knight Rises was carrying an AR-15 assault rifle when arrested by police.