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?