Why hasn’t evolution wiped out our susceptibility to depression? Depression is a highly pervasive condition, with some researchers estimating that 30-50% of people have experienced a major depressive episode at some point throughout their lives.

Evolutionary psychologists seem to believe that not only is our susceptibility to depression not an evolutionary glitch, it could even serve a positive purpose. Researchers have really gotten down into the nitty gritty of depression in hopes of explaining its evolutionary purpose, to the point of looking at the evolutionary significance of depression in pathogen host defense (for those of you who don’t speak science, the research basically suggests that depression actually plays a role in protecting people from dying of infections). On a more tangible level, though, depression can also serve as a function to force us to analyze complex issues, according to psychologists. Evolutionary psychologist Paul Andrews “sees [depression’s function] in the condition’s bouquet of symptoms, which include ‘anhedonia,’ or an inability to feel much pleasure; people who are depressed ruminate frequently, often in spirals; and they get more REM sleep, a phase associated with memory consolidation,” writes New York Magazine’s Drake Braer. Psychologist Laura King expands on Andrews’ postulation by discovering (through decades of research) that “the meaning people derive from difficult experiences depends not on the amount that they’ve suffered, but the extent of reflection — or meaning-making.”

Are you depressed? For the uninitiated, the diagnostic criteria for a major depressive episode include symptoms such as a significant change in weight or appetite, a change in sleeping patterns (manifested in either insomnia or hypersomnia), feelings of guilt and worthlessness, and having suicidal thoughts. So why hasn’t evolution worked its magic to make us immune to depression?