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 them (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 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."

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 while 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 or 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."