Researchers in the US looked at 40 pairs of identical twins where one twin had seen combat in Vietnam and the other had stayed at home.

More than 40% of the Vietnam veterans suffered from post-traumatic stress disorder, and a clear link was found between the size of a region of their brains, the hippocampus, and which veterans were affected.

Previously it was thought the hippocampus, which is in a central part of the brain, had been damaged by the disorder. But the US researchers found that the stay-at-home siblings of PTSD-suffering veterans also had smaller than average hippocampi.

In other words, say the scientists in the latest online edition of Nature Neuroscience, some GIs sent into combat were doomed to suffer the harsh consequences of PTSD.

People with the disorder ex perience flashbacks and nightmares.

PTSD sufferers display emotional numbness or outbursts and problems with memory and concentration. Sometimes their symptoms are complicated as they turn to drugs such as alcohol in a form of self-treatment. It affects not only victims of war and natural disasters but those who have suffered violence, sexual abuse, bullying and other situations of extreme stress.

Much work will need to be done before the new findings are widely accepted, but if they are, they may lead to brain screening in recruitment for jobs like soldiering or police work which involve experiencing extreme events.

The PTSD study, led by Mark Gilbertson of the Veterans Administration Medical Centre in New Hampshire, tried to account for factors like the tendency towards alcoholism among sufferers.