According to some estimates, more than half of us will experience a traumatic event in our lifetime: A serious car accident, a physical assault, combat duty during wartime. Yet relatively few will develop the nightmares and panic attacks of post-traumatic stress disorder, or PTSD. Why some and not others? "We don't walk into trauma equally, so we don't all come out of it equally," explained Rachel Yehuda.

A neuroscientist at the Mount Sinai School of Medicine, Yehuda noted there are a number of risk factors for PSTD, including previous trauma, childhood abuse, and a family history of pathologies such as alcoholism and depression. Her own research on cortisol, a hormone secreted by the adrenal gland in times of stress, has raised another possibility that a biological vulnerability to PTSD might actually be inherited. Although PSTD patients have suffered severe stress, they have lower-than-average levels of cortisol in their blood. Yehuda said she thinks she may have the solution to this puzzle: Low cortisol levels may themselves be a risk factor for PTSD.

She cited studies that show that people who had low cortisol levels immediately after being in a car accident were more likely to develop PTSD down the road, and that adult children of PSTD-stricken Holocaust survivors have low cortisol levels. These sons and daugthers may be at high risk of developing the disorder, says Yehuda—but should disaster strike, a blood test may one day single them out for special treatment.