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.

Anxiety disorders make sense only in the evolutionary context of the origins and functions of normal anxiety. Anxiety is an adaptation that adjusts diverse aspects of individuals in ways that increase fitness in dangerous situations. Subtypes were partially differentiated by different dangers. Anxiety is not fully differentiated from other aversive emotions, especially low mood. Anxiety disorders result when regulation systems fail. Explaining them requires considering five possible reasons for vulnerability. However, much harmful anxiety arises from normal mechanisms. These insights are valuable in the clinic, and they suggest new research initiatives.

The 2009 celebrations of Darwin's birth and the publication of The Origin of Species were grand not only because his discoveries changed biology, but also because they are continuing to benefit society. Nowhere is this more evident than in medicine and public health. You might think that Darwin's discoveries would have been fully applied long ago, but a deep fracture in the intellectual landscape has prevented medicine from making full use of evolutionary biology. This is changing fast. Scientists are now recognising that diseases need evolutionary explanations as well as explanations based only on the body's mechanisms.

The field that tries to understand why natural selection has left the body vulnerable to diseases is called Darwinian medicine. Also called evolutionary medicine, it applies every aspect of evolutionary biology to every problem in medicine and public health. It has grown quickly since 1991 (Williams and Nesse, 1991). Major edited volumes have illustrated the opportunity in areas from infectious disease epidemiology to genetics, anatomy and physiology. The most significant ones are already in second editions (Stearns and Koella, 2007; Trevathan et al., 2007). They are being widely read and studied except, it seems, by physicians. Most doctors never take a course in evolutionary biology before medical school, and evolution is not part of the usual medical curriculum.