Social navigation hypothesis of depressive disorder disproven

Evolutionary psychiatry tries to explain paradoxical high psychiatric disorders and symptoms prevalence by means of evolutionary biology, but testing their hypotheses is often problematic. Social Navigation Hypothesis (SNH) belongs to the evolutionary hypotheses of depressive disorder. It assumes two mechanisms of increasing fitness by depressive signs and symptoms: social rumination function (depressed person is thinking intensely, trying to solve his/her social problems; in contrast to research demonstrating that depressed person usually manages with social challenges worse than healthy people) and social motivation function. The latter postulates that depressive behavior, by diminishing the fitness of surrounding people, forces them to help for depressed person to terminate the fitness-reducing episode. Social motivation function is discussed according to John Maynard Smith’s idea of evolutionarily stable strategy. On that base two simple mathematical models are constructed. SNH can theoretically describe an evolutionarily stable strategy (the precondition is the relation between the duration time of the episode and remission given by a certain formula), but the prediction of SNH (episode duration shortened with the patient’s age) is contrary to epidemiological data. Presented models, based on simplistic mathematical assumptions, don’t take into account kin selection and inclusive fitness. SNH cannot explain the high prevalence of depressive symptoms and depressive disorder. Falsification was possible only due to the mathematical formulation of the previously descriptively formulated hypothesis.