To the evolutionarily oriented clinical psychiatrist, the discipline of behavioural ecology is a fertile basic science. Human psychology discusses variation in terms of means, standard deviations, heritabilities, and so on, but behavioural ecology deals with mutually incompatible alternative behavioural strategies, the heritable variation being maintained by negative frequency-dependent selection. I suggest that behavioural ecology should be included in the interdisciplinary dialogue recommended by Keller & Miller (K&M).

Keller & Miller (K&M) assert that mental disorders could not have evolved as adaptations, but they fail to make their case against the theory of the evolutionary origin of bipolar disorder that I have proposed (Sherman 2001). Such an idea may be unorthodox, but it has considerable explanatory power and heuristic value.

The target article shows that the application of the evolutionary theory to psychopathology should not necessarily consist in finding hidden adaptive benefits for each psychiatric syndrome. However, in rejecting lax adaptationism, Darwinian psychiatrists should not forget that the search for adaptive behavioral polymorphisms can be a powerful antidote against the normative attitude of mainstream psychiatry and its growing tendency to medicalize human diversity.

Suicidal behavior is an interesting blank space in Keller & Miller's (K&M's) population genetical account on explaining the existence and persistence of common, harmful, heritable mental disorders. I argue that suicidal behavior is yet another of these disorders. It may well be consistent with all three evolutionary models considered by K&M.

Three reservations about Keller & Miller's (K&M's) argument are explored: Serious validity problems afflict epidemiological criteria discriminating disorders from non-disorders, so high rates may be misleading. Normal variation need not be mild disorder, contrary to a possible interpretation of K&M's article. And, rather than mutation-selection balance, true disorders may result from unselected combinations of normal variants over many loci.

Keller & Miller's (K&M's) treatment of disorders usefully avoids diagnostic minutiae; but it needs more real-world constraints. Classifying processes by their evolutionary age helps to clarify both evolution and current function. Evolutionarily old, optimised, normative processes deserve special recognition, because they can be studied in animals and computers, and because they provide the machinery through which disorder-related polymorphisms act.

First kudos, followed by some friendly badinage, and then renewed appreciation and a look ahead. This commentary is meant to clarify main arguments, redress incorrect attributions, and strengthen an excellent contribution that draws further attention to the importance of evolutionary epidemiology. Keller & Miller (K&M), despite significant errors, have done well to further systematize the evolutionary epidemiology of psychopathology.

This response (a) integrates non-equilibrium evolutionary genetic models, such as coevolutionary arms-races and recent selective sweeps, into a framework for understanding common, harmful, heritable mental disorders; (b) discusses the forms of ancestral neutrality or balancing selection that may explain some portion of mental disorder risk; and (c) emphasizes that normally functioning psychological adaptations work against a backdrop of mutational and environmental noise.