Depressive disorder from the evolutionary standpoint
Marcin Piotr Nowak

A depressive episode is a common psychiatric disorder, yet, according to theories of evolution, traits negatively impacting fitness are deleted from population by natural selection, which is not observed in this case. To solve that paradox, evolutionary hypotheses of depression have been proposed, namely:
- immunologic hypothesis (depressive episode as prolonged reaction to infection);
- attachment hypothesis (depressive symptoms resemble the behaviour of a child detached from the mother);
- social rank hypothesis (competition with dominant individuals is not profitable for ones of lower social position);
- social navigation hypothesis (depressive symptoms have social rumination function – a depressive person is actually intensely thinking about solving his/her problems – and social motivation function – bargaining for help);
- incentive hypothesis (major depression is maladaptive, but fear of depression – like physical pain informing about trauma – helps to avoid danger).
Some of the formulations in the listed hypotheses are inconsistent with modern evolutionary synthesis, e.c. describing evolutionarily unstable strategies. This provokes questions: If depressive symptoms are part of human nature and enhance fitness, should they be treated? And which of the hypotheses is correct? Firstly, when some trait is natural and improves fitness, it does not necessarily have to improve the clinical state and well-being of the patient. What is more, the fact that “something is” does not entail that “something has to be.” To answer the second question, the mechanism of preadaptation is proposed, in which some adaptation can change to new functions. Depressive symptoms could appear as a part of response to infection or maternal detachment and, when the social structure of the human group became more sophisticated, adapt to perform new functions.