What's the link between PTSD, TBI and violence?

Staff Sgt. Robert Bales has been identified as the soldier accused of killing 16 civilians in Afghanistan.

STORY HIGHLIGHTS

Editor's note: Dr. Charles Raison, CNNhealth's mental health expert, is an associate professor of psychiatry at the University of Arizona in Tucson. He has not personally examined the suspect in the Afghanistan mass shootings, Robert Bales, but has used news accounts as the basis for his views.

(CNN) -- Q: Sgt. Robert Bales has been accused of killing 16 Afghan civilians. He served three tours in Iraq before this and his lawyer says he may have been suffering from post-traumatic stress disorder or a traumatic brain injury. What's the link between violence and those disorders?

A: Psychiatrists understand some types of aberrant behavior pretty well and can do things to help resolve it. But, unfortunately, in other instances -- and often the most interesting ones -- we can only mumble generalities that require no special expertise and that offer no hope for a diagnosis or treatment.

Take the case of U.S. Army Sgt. Robert Bales, accused of massacring 16 Afghan men, women and children while they slept unprotected in their village.

The first thing a psychiatrist would want to know is whether the person who committed such a heinous act was psychotic at that time, meaning out of touch with agreed-upon human reality. Did he perform the killings as a result of deeply held false beliefs or in response to hearing voices commanding him to act? If yes, then although the tragedy remains, the psychiatric mystery is solved.

But at this point, although Bales has reportedly told his lawyer he remembers nothing about the night of the massacre, there is no evidence he was psychotic immediately before the killing spree. Nor do we have any evidence at this point that the killings were motivated by some larger political purpose, which might also explain, but certainly not justify, them.

So why would someone who had appeared normal to everyone around him suddenly commit such a hideous act?

Army reviewing PTSD evaluation program

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What's the link between PTSD, TBI and violence?