With his hands tied behind his back and his feet bound together, Charles Alexander Morgan III splashed into the water of the Olympic-size pool at the Navy Diving and Salvage Training Center in Panama City, Fla. with no equipment—except, that is, a dive mask that hung uselessly by its strap from between his clenched teeth. As the water engulfed his body, air bubbles shot towards the surface and he sank quickly to the pool’s bottom. But he didn’t panic. Having spent much of his childhood in the water, Morgan, who studies the biology of stress and post-traumatic stress disorder (PTSD) at Yale University School of Medicine, was surprisingly calm about the particular predicament in which he currently found himself. And it’s a good thing too, as staying relaxed in this situation is really the only way to keep from blacking out.

Many of the military trainees who complete the same task during their 1-month training course are not so fortunate. Morgan had recently come to the training center to look for physiological correlates of stress, trying to identify those individuals better equipped to deal with stressful situations. Trainees are subjected to several dangerous underwater missions, including the one that Morgan tried out himself, during which many soldiers lose consciousness. Once revived, they are given 60 seconds to recover and jump back in the pool, otherwise they have failed the course and are asked to leave. Fifty percent of participants are cut by the end of their first week.

Morgan measured the levels of the steroid hormone dehydroepiandrosterone (DHEA)—implicated in how organisms cope with stress—in the trainees both before and immediately following their final underwater navigation exam: a nighttime underwater swim from 3 miles offshore in the ocean to a target on the beach, in which they cannot resurface until they reach the shore, where they are judged based on their time and distance from the target.

He found that high levels of both DHEA and DHEAS (the sulfated derivative of DHEA) predicted superior performance. Furthermore, trainees with higher DHEA levels following the test reported fewer symptoms of dissociation—a common response to trauma involving feelings of detachment from oneself and the world—which again translated into better performance in the underwater task ( Biol Psychiatry , epub June 3, 2009). “It’s the first time I’ve seen a really solid behavioral correlation between DHEA in human beings,” says neuroscientist Joe Herbert of the University of Cambridge.

Morgan first noticed DHEA’s role in a military situation in 2000 when he was conducting experiments at the Army’s Fort Bragg survival school training program in North Carolina. There, in a simulated prisoner-of-war camp, soldiers are subjected to coercive interrogations employing sleep and food deprivation, among other techniques. Again, Morgan found that people with higher DHEA fared better ( Arch Gen Psychiatry , 61:819–25, Aug. 2004).

Although the data are merely correlative, it does raise the possibility that DHEA is mitigating the neurotoxic effects of cortisol, the “stress hormone,” says psychiatrist Owen Wolkowitz of University of California, San Francisco. Cortisol levels rise in times of acute stress, causing a boost in energy, memory, immunity, and pain tolerance, all of which help the organism deal with the demands of the stressful situation. On the other hand, prolonged exposure to high levels of cortisol and other glucocorticoids (such as in a combat situation) can kill nerve cells in the brain.

In mice, administration of DHEA attenuated or even blocked the neurotoxic effects of glucocorticoids in the hippocampus. However, “in humans [this mechanism] is much more speculative,” says Wolkowitz. Animals have significantly lower DHEA levels than people, and while some studies report reduced DHEA levels in people suffering from disorders caused by stress, such as anxiety, depression, and PTSD, these studies do not address the neurobiology that might underlie the relationship between DHEA and stress resistance.

Morgan hopes to initiate clinical trials with DHEA—sold as an over-the-counter dietary supplement to diminish the effects of aging—in military personnel. If DHEA does “help buffer against the negative impact of stress,” he says, “[it could be] an easy way to sustain performance in people who are deployed.”

that personality is probaly also based on genetics, as I suspected. That is, there are personality genes in humans and other higher mammals, just as there may be intelligence genes. It is not politically correct, I know, but science has nothing to do with political correctness.