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 inury (TBI), two medical conditions suffered by tens of thousands of U.S. soldiers who fought in the Iraq and Afhganistan wars. That research, which is both voluminous and inadequate, could play a role in the defense of Army Staff Sgt.Roger Bales, who has been charged with 17 counts of murder after a rampage in Kandahar providence in south 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 thinknig 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 th 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 lawyer 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 more likely to be physically abusive of their wives and girlfriends than those without the diagnosis. They're three times more 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.

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's National Center for PTSD, in Vermont.

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