Evolutionary advantages of clinical depression

Evolutionary analyses examine the ways in which depression as a response to certain environmental stimuli may act as an adaptive advantage and increase genetic fitness, either of the individual or the society as a whole. See, e.g., Nesse 2006

The psychic pain hypothesis
Psychic pain, such as depression, is analogous to physical pain. The function of physical pain is to inform the organism that it is suffering damage, to motivate it to withdraw from the source of damage, and to learn to avoid such damage-causing circumstances in the future. Analogously, depression informs the sufferer that current circumstances, such as the loss of a friend, are imposing a threat to biological fitness, it motivates the sufferer to cease activities that led to the costly situation, if possible, and it causes him or her to learn to avoid similar circumstances in the future. Proponents of this view tend to focus on low mood, and regard clinical depression as a dysfunctional extreme of low mood. See, e.g., Nesse 2000 and Keller and Nesse 2005; see also Hagen and Barrett n.d..

Rank theory
Rank theory: If an individual is involved in a lengthy fight for dominance in a social group and is clearly losing, depression causes the individual to back down and accept the submissive role. In doing so, the individual is protected from unnecessary harm. In this way, depression helps maintain a social hierarchy. This theory is a special case of a more general theory derived from the psychic pain hypothesis: that the cognitive response that produces modern-day depression evolved as a mechanism that allows people to assess whether they are in pursuit of an unreachable goal, and if they are, to motivate them to desist. See, e.g., Nesse 2000.

Honest signaling theory
When social partners have conflicts of interest, 'cheap' signals of need, such as crying, might not be believed. Biologists and economists have proposed that signals with inherent costs can credibly signal information when there are conflicts of interest. The symptoms of major depression, such as loss of interest in virtually all activities and suicidality, are inherently costly, but, as costly signaling theory requires, the costs differ for individuals in different states. For individuals who are not genuinely in need, the fitness cost of major depression is very high because it threatens the flow of fitness benefits. For individuals who are in genuine need, however, the fitness cost of major depression is low because the individual is not generating many fitness benefits. Thus, only an individual in genuine need can afford to suffer major depression. Major depression therefore serves as an honest, or credible, signal of need. See, e.g., Hagen 2003, Watson and Andrews 2002.

Social navigation or niche change theory
The social navigation, bargaining, or niche change hypothesis suggests that depression, operationally defined as a combination of prolonged anhedonia and psychomotor retardation or agitation, provides a focused sober perspective on socially imposed constraints hindering a person’s pursuit of major fitness enhancing projects. Simultaneously, publicly displayed symptoms, which reduce the depressive's ability to conduct basic life activities, serve as a social signal of need; the signal's costliness for the depressive certifies its honesty. Finally, for social partners who find it uneconomical to respond helpfully to an honest signal of need, the same depressive symptoms also have the potential to extort relevant concessions and compromises. Depression’s extortionary power comes from the fact that it retards the flow of just those goods and services such partners have come to expect from the depressive under status quo socioeconomic arrangements.

Thus depression may be a social adaptation especially useful in motivating a variety of social partners, all at once, to help the depressive initiate major fitness-enhancing changes in their socioeconomic life. There are extraordinarily diverse circumstances under which this may become necessary in human social life, ranging from loss of rank or a key social ally which makes the current social niche uneconomic to having a set of creative new ideas about how to make a livelihood which begs for a new niche. The social navigation hypothesis emphasizes that an individual can become tightly ensnared in an overly restrictive matrix of social exchange contracts, and that this situation sometimes necessitates a radical contractual upheaval that is beyond conventional methods of negotiation. Regarding the treatment of depression, this hypothesis calls into question any assumptions by the clinician that the typical cause of depression is related to maladaptive perverted thinking processes or other purely endogenous sources. The social navigation hypothesis calls instead for a penetrating analysis of the depressive’s talents and dreams, identification of relevant social constraints (especially those with a relatively diffuse non-point source within the social network of the depressive), and practical social problem-solving therapy designed to relax those constraints enough to allow the depressive to move forward with their life under an improved set of social contracts.

Bargaining theory
This theory is similar to the honest signaling, niche change, and social navigation theory. It basically adds one additional element to honest signaling theory. The fitness of social partners is generally correlated. When a wife suffers depression and reduces her investment in offspring, for example, the husband's fitness is also put at risk. Thus, not only do the symptoms of major depression serve as costly and therefore honest signals of need, they also compel social partners to respond to that need in order to prevent their own fitness from being reduced. See, e.g., Hagen 1999, Hagen 2003.