Nesse’s research focuses on the evolutionary origins of depression. Why does depression exist at all? If it’s stayed in our gene pool for so long, he argues, there must be some evolutionary benefit. Nesse believes that depression may be an adaptive mechanism meant to prevent us from falling victim to blind optimism—and squandering resources on the wrong goals.11 It’s to our evolutionary advantage not to waste time and energy on goals we can’t realistically achieve. And so when we have no clear way to make productive progress, our neurological systems default to a state of low energy and motivation. During this period of mild depression, Nesse theorizes, we can conserve our resources and search for new, more realistic goals. But if we persist in pursuing unattainable goals? Then, Nesse proposes, the mechanism kicks into overdrive, triggering severe depression.