Nesse ^{Reference Nesse1} argues that psychiatry requires both proximate and evolutionary explanations to become a fully fledged biological science. He thinks that mental disorders such as schizophrenia and depression would benefit from posing the question of whether low mood and variable social ability were adaptive traits in times long gone and are no longer of evolutionary advantage in our current environment.

I think that Nesse's approach is as laudable as it is flawed. Evolutionary psychology proposes that most if not all human psychological traits are complex adaptations which have evolved under selective pressures. Richardson convincingly shows that the claim that all our psychological capacities have been selected for the purpose of accomplishing a particular task is too strong and that the empirical evidence required to support this claim is necessarily historical. ^{Reference Richardson2} The problem is, however, that the required historical evidence is hard or impossible to come by – we simply do not know what psychological traits were prevalent let alone advantageous to survive in a Pleistocene environment about which we also have little information.

For evolutionary psychology to be regarded as a credible theoretical framework it will have to be examined against standards of scientific enquiry used in other evolutionary fields such as evolutionary biology. Archaeopteryx was thought to be able to fly as it possessed feathers and claws to allow it to perch on trees. ^{Reference Dennett3} However, fossil records also showed that archaeopteryx lacked a sternal keel and that its awkward tail would have been likely to impede flying. Further comparative analysis showed that archaeopteryx was neither likely to perch nor be able to fly and refuted the conclusions arrived at by reverse engineering as proposed by Dennett.

Evolutionary psychology relies mainly on reverse engineering as this strategy requires comparatively few historical data but risks arriving at erroneous conclusions. Buller ^{Reference Buller4} shows this to be the case for evolutionary explanations of the existence of cheater detection modules or gender differences in jealousy.

This is not to say that evolutionary psychology cannot offer an exciting and innovative framework for scientific inquiry into common mental disorders such as depression and schizophrenia but that we have to be aware of its current theoretical and methodological shortcomings and the need for further conceptual work. I agree with Geaney ^{Reference Geaney5} that further advances to understanding human behaviour and mental disorder would be best served by interdisciplinary cooperation whether based on evolutionary theory or not.