Research on post-traumatic stress disorder (PTSD), which can haunt former soldiers and survivors of catastrophes, is shedding light on who may be more vulnerable, and how best to treat each case.

"Interestingly, there are some individuals who, when confronted with extreme stress, their hormone profile is rather unique," said Deane Aikins, a psychiatrist at Yale University in Connecticut, at the annual meeting of the American Association for the Advancement of Science (AAAS) which ends Monday.

"It doesn't reach the same peak as the rest of us. So we are ready to scream in our chair, and there are certain individuals who just don't get as stressed. Their stress hormones are actually lower and the peptides that down regulate that stress are quite higher," he explained.

Aikins released the results of his research on groups of US military personnel including special forces, using simulators to expose them to highly stressful situations.

"So certain people are cooler under pressure and they perform very, very well during these periods of time while I may be screaming for help."

He said that the research suggest "we can start predicting who are the individuals that will have this very unique cooler hormonal profile under high stress.

"And we can do this maybe two weeks ... prior to the stressful event, who is going to do very well," he said.

Fighting two wars, the US Defense Department has crashed headlong into the difficult fallout from PTSD and the related high emotional and healthcare costs. It estimates that 20 percent of troops returning from Iraq have PTSD.

According to Karestan Koenen, a public health professor at Harvard University in Massachusetts, although huge numbers of people live through traumas -- such as rape, natural disasters or other violent crime. But not everyone develops PTSD.

"For example for rape, about 50 percent of women developed PTSD and why is that? One issue is the severity of the trauma but we also found that early childhood and genetic factors influence who develop PTSD much later in life," she said.

She said she has worked with "a cohort from New Zealand and we have been looking at these people evolved from birth to their 30s and we have been looking at what predicts who develops PTSD if exposed to trauma later."

She and her colleague found factors such as low IQ early as age 5, difficult temperament at age 3 and then family environment factors such as growing up in poverty, having a depressed mother, moving often or being separated from parents at a young age all could help predispose people to developing the condition.

Also "some people have genetic variants that make them more vulnerable to the effects of trauma," so now "basically we know now that PTSD is not just about the trauma, that early child characteristics, family environment and genetic factors influence how people respond to horrible events during their life," she said.

Another study released here on Vietnam veterans showed that those who suffered injuries in a certain area of the brain did not develop PTSD, said Jordan Grafman, of the National Institutes of Health National Institute of Neurological Disorders and Stroke, in Bethesda, Maryland.