Dr. Roger Pitman of Yale University in New Haven, Conn., and Dr. Kiyoto Kasai of the University of Tokyo in Japan said that although it is tempting to conclude that brain abnormalities linked to post-traumatic stress disorder were caused by the traumatic event, it is also possible that they were pre-existing risk factors that increased the risk of developing PTSD.

The researchers measured the gray matter density of the brains of combat-exposed Vietnam veterans, some with and some without PTSD, and their combat-unexposed identical twins using magnetic resonance imaging.

The study, published in Biological Psychiatry, found that the gray matter density of the pregenual anterior cingulate cortex -- an area of the brain involved in emotional functioning -- was reduced in veterans with PTSD, but not in their twins who didn't experience combat.