Nesse’s article, “Is Depression an Adaptation?”, suggests that “low mood” and depression may have the adaptive function of decreasing motivation in situations in which the likelihood of a successful outcome is low or when pursuing further action could be potentially dangerous. This hypothesis seems somewhat problematic, especially applied to depression.
According to this article, “low mood” is within the range of normal experience, and it seems likely that it decreases motivation to pursue unreachable goals. But how do we define depression? Sure, the DSM-V has a definition of depression for us, but that definition is based on arbitrary criteria (number/duration/frequency of symptoms) that change with every new edition of the manual. How, then, could someone claim that depression is an adaptation if there isn’t even a clear, objective way to define it? Maybe I’m being overly critical here, but branding depression as an adaption could have serious implications in terms of the way in which people who suffer from clinical depression are treated and/or perceive their condition.
Those who suffer from severe depression don’t only have their mental health to worry about. Depression can also have grave physical health consequences. On top of that, there are thousands of depression-related suicides each year (although the number of suicides is low in comparison to the the total number of people who suffer from depression). However, the adverse health effects of depression and, especially, the incidence of suicide do not make sense from an adaptive perspective, because both of these things could result in a reduction in fitness.
To conclude, low mood is quite possibly an adaptation that allows us to give up on pursuits that could have negative costs to us in the future, but depression, a prolonged and intensified version of that low mood, is potentially maladaptive. We should be careful not to conflate the two.