Road Rage Science: Former NFL Player's Death Reveals Why We Lose It

"I would be surprised if the person who killed Smith didn't have an anger problem," University of Chicago psychiatry professor Dr. Emil Coccaro told Live Science, referring to the driver who rear-ended Smith's car. [Hypersex to Hoarding: 7 New Psychological Disorders]

After the collision, the two men pulled over and exchanged words, during which time the other driver pulled out a handgun and shot both Smith and his wife, injuring her and killing Smith at the scene, according to a release by the New Orleans Police Department. Police are investigating the possibility that road rage, among other potential causes, was a factor in the shooting, according to the Washington Post.

Coccaro and other psychologists consider road rage a manifestation of a psychiatric disorder called intermittent explosive disorder (IED), which is essentially a problem of "recurrent, problematic, aggressive, impulsive outbursts," Coccaro said. Though scientists can't say what proportion of road rage is related to IED, road rage is one type of outburst that people with IED have, he said. 

More specifically, according to the current (fifth) edition of the Diagnostic and Statistical Manual of Mental Disorders, IED can involve either frequent, low-intensity outbursts or less frequent but more destructive ones (three or more episodes within the course of a year that to lead to destruction of property or bodily harm). Most people with IED, however, experience both kinds of outbursts, Coccaro said.

What's the line between an angry outburst while driving and a psychiatric disorder? In addition to the two types of outbursts, IED is associated with aggression that is disproportionate to the situation; it isn't premeditated; and it's not related to substance abuse or another mental disorder, Coccaro said. Additionally, the outbursts "have to get [the person with IED] into trouble. They have to cause them distress, or they have to get them into difficulty with others, either in their relationships or at work. Typically, it's with relationships," Coccaro said. [Understanding the 10 Most Destructive Human Behaviors]

People with IED often "have trouble understanding what's happening in their interactions with other people, particularly when those interactions are ambivalent," Coccaro said. They are more likely to misinterpret people's actions as threats and respond with aggression. Interpersonal interactions tend to trigger outbursts in people with IED, he said.

Biological factors in IED

As for why some people have "shorter fuses" than others, that boils down to biology, Coccaro said. People with IED tend to have higher levels of proteins that are linked to inflammation, according to a 2013 study led by Coccaro and published in the journal JAMA Psychiatry. It's unknown if inflammation leads to aggression, though animal research suggests that it might, Coccaro said.  

Brain abnormalities also factor into aggression, he said. In people with aggression problems like IED, "the frontal areas of the brain function less well, and they're the brakes of the system," said Coccaro. The accelerator in this aggression system would be the lower areas of the brain, such as the amygdala, he added. "That tends to be hyper-responsive to threat stimuli."

Research has shown that people with IED tend to have less gray matter in the cortical limbic regions — their "brakes" are less well-endowed, Coccaro said.

Coccaro noted that the kinds of correlations he has observed among aggression and other factors also hold for people who do not have IED. In other words, the same correlations, say between gray matter levels and aggression, are seen when people have less intense outbursts that wouldn't be considered IED. Rather, IED is aggression at a level deemed clinically significant. "When you get up to high levels of aggression, it becomes its own disorder," Coccaro said.

Sometimes an accident can result in an abnormally aggressive driver. For instance, people who have had a traumatic brain injury (TBI) are about four times as likely to engage in driver aggression as those who have not experienced such an injury, according to a study of Ontario adult drivers published last year in the journal Accident Analysis and Prevention. According to the same study, TBI survivors were also more likely to have been involved in vehicle collisions. [The Odds of Dying]

"When we look at the large proportion of adult drivers with a history of traumatic brain injury" — about 17 percent in the Ontario study — "it's very possible that these individuals may account for a great proportion of [or] burden of all traffic-safety problems," said study author Gabriela Ilie, now a faculty member at Dalhousie University, in Nova Scotia.

Driving armed

External factors also play a role. Motorists who drive with guns in the car are significantly more likely to engage in aggressive driving behavior (such as making obscene gestures or aggressively following another vehicle), according to two studies led by the Harvard Injury Control Research Center of the university's School of Public Health.

"We don't know whether having a gun in the car changes people's behavior, or if they tended to be more aggressive to start with," David Hemenway, director of the center and a co-author on both studies, told Live Science. 

"What we would like is that people driving around with guns were the most calm and least aggressive motorists," he said. "The opposite seems to be the case."

"Add to that the fact that violent behavior with a gun is much more likely to prove lethal than violent behavior without a gun, [and] what we can say is that Will Smith stood a much better chance of being alive today if the rageful encounter on the road did not involve a gun," added center co-director and study author Dr. Matthew Miller.

Helping the perpetrators

To clear the roads of such harmful tantrums, Coccaro suggested paying some heed to the actual perpetrators.

"People get very concerned about victims of violence. They have no concern for the perpetrators of violence, because they're just seen as bad people," Coccaro said. "If we don't do something to understand and help the people who are more aggressive than the rest of us, we're still going to have victims."

Current treatments that help people with IED include serotonin modulators, such as selective serotonin re-uptake inhibitors (anti-anxiety drugs), which could raise the emotional threshold for an outburst. Cognitive behavioral therapy might also help people better control their reactions in potentially triggering situations, Coccaro said.

Follow Ashley P. Taylor @crenshawseeds. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.