Some people may be more vulnerable to the effects of stress because an area of their brain is smaller than average.
Soldiers sometimes develop PTSD
Scientists have found an area of the brain called the anterior cingulate cortex is more likely to be small in people who have developed symptoms of post traumatic stress disorder (PTSD).
The anterior cingulate cortex is known to play a role in regulating emotion.
PTSD is a disturbing psychological condition where people who have lived through a stressful experience relive painful memories against their will.
This can lead to feelings of isolation and a sense of losing control - patients sometimes turn to alcohol or other drugs as they attempt to get rid of the flashbacks.
Previous studies linked PTSD to structural abnormalities in the brain - in particular in the hippocampus, an area involved in long-term memory.
However, these studies yielded inconsistent results, and few areas other than the hippocampus have been thoroughly investigated.
To determine if other brain regions are structurally affected by PTSD, a team led by Dr Hidenori Yamasue at the University of Tokyo used a recently developed technique known as voxel-based morphometry to study the whole brain.
The scientists recruited 25 survivors of the 1995 Tokyo sarin gas attacks, nine of which suffered from PTSD.
After performing brain scans, the researchers found that the left anterior cingulate cortex was significantly smaller in subjects with PTSD compared to those without the disease.
And the smaller it was, the greater the severity of PTSD symptoms.
No other significant differences in grey or white brain matter were found between the two groups
The scientists suggest that abnormalities in the anterior cingulate cortex may interfere with emotional regulation, in some cases increasing the chances of developing PTSD.
Simon Meyerson, a consultant psychologist and expert in PTSD, said the structure of the brain may be one factor in the development of PTSD.
But he said it was a complex condition, probably caused by many factors.
"PTSD is caused by a psychological blast to the mind, and it is the personality, not the physique that begins to crumble," he told BBC News Online.
"The physical nature of the brain may be one ingredient, but it is not likely to be a major factor. The structure of the personality - whether it is solid or fragmented - is likely to be more important, and that is governed by all sorts of things, including learning and social interaction."
The research is published in the journal Proceedings of the National Academy of Sciences.