Why do so many people suffer from depression? Research in the U.S. and other countries estimates that between 30 to 50 percent of people have met current psychiatric diagnostic criteria for major depressive disorder sometime in their lives. This staggeringly high prevalence—compared with other mental disorders that affect only around 1 to 2 percent of the population, such as schizophrenia and obsessive-compulsive disorder—seems to pose an evolutionary paradox. The brain plays crucial roles in promoting survival and reproduction, so the pressures of evolution should have left our brains resistant to such high rates of malfunction. Mental disorders are generally rare—why isn’t depression?
This paradox could be resolved if depression were a problem of growing old or a result of our modern lifestyles. Aging cannot explain depression, however, because people are most likely to experience their first bout in adolescence and young adulthood. So perhaps depression is like obesity—a problem that arises because modern conditions are so different from those in which our ancestors lived. But this explanation is not satisfactory, either. The symptoms of depression have been found in every culture that has been carefully examined, including societies such as the Ache of Paraguay and the !Kung of southern Africa—societies in which people are thought to live in environments similar to those that prevailed in our evolutionary past.