Kelleher et al ^{Reference Kelleher, Jenner and Cannon1} argue that evolutionary theories of psychosis provide a valuable theoretical framework for the investigation of non-clinical psychotic phenomena and that the findings of such research would generate new insights into the aetiology, nosology and treatment of psychosis. They rely mostly on Nesse's ideas of cliff-edged fitness ^{Reference Nesse2} and Dodgson & Gordon's work on hypervigilance hallucinations ^{Reference Dodgson and Gordon3} and think that there must be an evolutionary advantage to non-clinical psychotic symptoms – why would they otherwise be so prevalent in the population?

I would like to add two further possible explanations. The first is that non-clinical psychotic symptoms are neither advantageous nor disadvantageous and may have been passed on alongside other fitness enhancing phenotypes. The second is that non-clinical psychotic symptoms are disadvantageous but their negative effects are diminished by being coupled to advantageous phenotypes. Fodor & Piattelli-Palmarini ^{Reference Fodor4} call this free-riding and argue that this is a counterexample to natural selection as proposed by Darwin (although Darwin was very well aware that non-adaptive processes play an important role in evolution).

Evidence to support one or the other evolutionary theory of mental illness will be hard to come by – I have argued elsewhere that, owing to its necessarily historical nature, it will be difficult to arrive at credible causal explanations. ^{Reference Treffurth5}

Even if evidence were available, there remains a considerable explanatory gap – why do some (young) people who experience non-clinical psychotic symptoms develop a full-blown psychotic illness with significant functional impairment? One of the aims of evolutionary psychiatry is to define mental disorder in value-free terms. However, when assessing a patient's ability to function, values do come into play, as has been shown by Fulford. ^{Reference Fulford6} I think that whichever way one might argue the case, arriving at a psychiatric diagnosis is unlikely ever to make do without a notion of dysfunction.

As for the direct clinical utility of an evolutionary theory, I cannot see how knowing that psychotic symptoms might have been advantageous in times long gone is of huge benefit to patients or, for that matter, clinicians, and I think that current and possibly future treatment strategies work just as well without taking recourse to an evolutionary perspective.

I would like to thank Professor Ebmeier for his helpful suggestions and thoughts on evolutionary psychiatry.