Why Do Mental Disorders Persist? Evolutionary Foundations for Psychiatry

Abstract

Discovering why natural selection has left humans vulnerable to mental disorders will make psychiatry more sensible and effective, but defining the appropriate objects and kinds of explanation remains challenging. Asking how a disorder increases fitness is a mistake; disorders are not adaptations and they do not have evolutionary explanations. The correct objects of explanation are the traits that make all members of a species vulnerable to a disorder. Task 1 is to describe the evolutionary origins and functions of the traits involved. Task 2 is to describe the proximate processes that result in the disorder. Task 3 is to discover why natural selection left the traits vulnerable to malfunction. Five main kinds of explanation need to be considered: stochasticity, path dependence, mismatch, trade-offs that benefit the individual and traits that benefit gene transmission at a cost to the individual. Depression, addiction, eating disorders, autism and schizophrenia are used to illustrate the opportunities and challenges of framing and testing hypotheses about vulnerability. Multiple explanations are often needed for a single disorder, frustrating the wish for simplicity. However, recognising the fundamental differences between organic and designed systems offers opportunities for resolving – or at least understanding – some enduring controversies in psychiatry.