Post-traumatic stress disorder and traumatic brain injury can increase a person’s anger and hostility and diminish his or her self-control. But the link between those disorders and outright violent behavior is weak and hard to pin down with certainty.

That’s what the research suggests about the effects of post-traumatic stress disorder (PTSD) and traumatic brain injury (TBI), two medical conditions suffered by tens of thousands of U.S. soldiers who fought in the Iraq and Afghanistan wars. That research — which is both voluminous and inadequate — could play a role in the defense of Army Staff Sgt. Robert Bales, who has been charged with 17 counts of murder after a rampage in Kandahar province in southern Afghanistan.

People with PTSD avoid certain activities and environments, are hypervigilant, have intrusive memories and are often depressed. Anger, hostility and aggressiveness are less common symptoms. Headaches, troubled sleep, poor attention and muddled thinking are the hallmarks of mild traumatic brain injury. Impulsive behavior is sometimes seen, too.

The prevalence of PTSD in combat veterans of the Iraq and Afghanistan wars varies from 9 percent to 31 percent, depending on whether severity of impairment is included in the measurement.

Numerous studies have shown that repeated deployment is a “risk factor” for the disorder. A study published this month examined the experience of 66,000 Marines who served in Iraq. Those with two deployments had almost twice the rate of PTSD as those deployed once. People with longer time at home between deployments had half the risk of developing the disorder as those with rapid turnaround times. There’s also considerable evidence that untreated PTSD tends to get worse, not better, over time.

Bales served three tours in Iraq before being deployed to Afghanistan. During one of those tours, the 38-year-old soldier suffered a traumatic brain injury. Bales’s attorney said last week that Bales described experiencing flashbacks, nightmares and persistent headaches. It is unclear when those symptoms began, or whether he has been diagnosed with PTSD.

Veterans with PTSD are two to three times as likely to be physically abusive of their wives and girlfriends as those without the diagnosis. They’re three times as likely to get into fistfights when they go to college. One study showed they are especially prone to “impulsive aggression,” but that “premeditated aggression” — the kind of act Bales is accused of — was far more common in veterans without PTSD than in those with it.

Anecdotal evidence

Hostility and aggression are measured by questionnaires asking soldiers both about their feelings and whether they’ve done such things as hit an “intimate partner,” threatened someone with or without a weapon, been in a fight, or destroyed property. There’s no data about more specific violent acts.

“The closer we get to trying to understand how PTSD relates to extreme violence, the more we get anecdotal,” said Paula Schnurr, deputy director of the Department of Veterans Affairs’ National Center for PTSD, in Vermont.

The effects of traumatic brain injury on future behavior is even more complicated and — on some questions — contradictory.

Head injuries can diminish “executive function” — self-control — especially when the part of the brain just above the eyes, called the orbitofrontal cortex, takes the hit. Injury there “increases the chance of violent behavior by about 10 percent,” said David Cifu, director of the VA’s physical medicine and rehabilitation program, citing an analysis of more than 50 studies.

TBI can worsen the symptoms of people who already have PTSD or who go on to develop it because of, say, a near-fatal bomb explosion. Curiously, however, PTSD appears to be more severe in soldiers who suffer mild head injuries rather than severe ones. The reason may be that prolonged unconsciousness can blot out the memory of the traumatic event.

“We’ve asked [experts], ‘How do we predict violence in a soldier?’ and they haven’t been able to provide us with a good screen,” said Col. Rebecca I. Porter, a psychologist who heads the behavioral health division of the Army surgeon general’s office. “The best predictor of future behavior is past behavior.”

‘Distortion’ in the media

In any case, mental disorders arising from trauma are unlikely to be a big contributor to a person’s violent tendencies, many experts believe. “To pick PTSD and highlight it in the way it’s been played out in the media is a gross distortion and contrary to what we know,” said Matthew Friedman, director of the VA’s national center.

Nevertheless, finding the angriest combat veterans is an urgent priority. A group of researchers at the University of California at Irvine reported in January that a seven-item questionnaire called the Dimensions of Anger Reactions scale given to 3,500 combat veterans seeking treatment was able to identify those in whom anger and thoughts of violence (as opposed to anxiety or depression) predominated.

Three months before soldiers return home from the war theater, commanders fill out a “downrange assessment tool” telling their counterparts in the United States about unusual stresses a soldier may be under — combat experience, domestic problems, disciplinary issues. Upon arrival, everyone is screened for PTSD (among other things), and again 90 to 180 days later.

Soldiers are also screened for PTSD and TBI problems before they are sent overseas. Porter, the Army psychologist, said that although neither diagnosis precludes deployment, soldiers with severe symptoms or under active treatment for the disorders wouldn’t be sent.

“There’s a misperception that PTSD is debilitating for anyone who has the diagnosis. There are actually varying degrees of how it impacts one’s life,” she said.

Neither the Army surgeon general’s office nor Central Command could provide data on what fraction of soldiers scheduled to be sent to the war theaters in the past 10 years were judged undeployable because of PTSD or TBI, or whether the percentage has gone up over time.

However, Porter said the percentage of combat soldiers evacuated for behavioral health reasons has remained relatively stable over time. That suggests “the soldiers we are sending into theater are able to complete their mission and are at least as resilient now as they were at the beginning of the wars.”