"Fear, jealousy, anger. Name an emotion, and you can find an evolutionary reason why it exists. But why depression? What role does it play? Is it an adaptation that helped our survival, or is it a disorder? For insight, we welcome: Randolph Nesse, emeritus professor of psychiatry at the University of Michigan, and author of "Good Reasons for Bad Feelings: Insights from the Frontier of Evolutionary Psychiatry; Maryanne Fisher, professor of psychology at Saint Mary's University in Halifax; and Paul Andrews, professor of evolutionary psychology at McMaster University."