Why Zebras Don't Get Ulcers

As Sapolsky explains, most of us do not lie awake at night worrying about whether we have leprosy or malaria. Instead, the diseases we fear-and the ones that plague us now-are illnesses brought on by the slow accumulation of damage, such as heart disease and cancer. When we worry or experience stress, our body turns on the same physiological responses that an animal's does, but we do not resolve conflict in the same way-through fighting or fleeing. Over time, this activation of a stress response makes us literally sick.

Robert M. Sapolsky is a professor of biology and neurology at Stanford University and a research associate with the Institute of Primate Research, National Museum of Kenya. He is the author of A Primate's Memoir and The Trouble with Testosterone, which was a Los Angeles Times Book Award finalist. A regular contributor to Discover and The Sciences, and a recipient of a MacArthur Foundation "genius" grant, he lives in San Francisco.