Narrator: This is Science Today. While it's clear that stress can make depression worse, it's never been clear why this is. Owen Wolkowitz, a professor of psychiatry at the University of California, San Francisco says it's been thought that in people who are genetically predisposed to depression, stress can - by some mysterious way - cause alterations in neurotransmitters in the brain, such as seritonin and dopamine.

Wolkowitz: What our research is looking at is, is there an understandable biological connection between stress and subsequent disregulation of neurotransmitters that then leads to depression?

Narrator: Wolkowitz found intriguing data backing the theory that stress may trigger depression in a study of patients with Cushing's Disease. In this syndrome, there's a spontaneous increase in levels of a stress hormone called cortisol.

Wolkowitz: So it's not in response to stress, it's just a primary event. About ninety percent of those patients develop symptoms reminiscent of major depression, so it stands to reason that cortisol or some steroid that's being affected there can actually cause depression.

Narrator: For Science Today, I'm Larissa Branin.