For those who refuse a Szaszian perspective and need to define, for therapeutic purposes, what a mental illness is, it is necessary to know what the normal functioning of a human brain is. There is no bypassing this question, on pains, for any psychiatric theory, of losing its relevance, and its object. As for every functional matter, one needs the support of Evolutionary Theory, if one wants to discover natural functions in the brain, not only norms dictated by social concerns or professional agendas. For, if the physician can easily discover the usual effect of such and such an organ on his patient's well-being, only by evolution can he understand why, and how, this organ was actually selected by natural selection. An evolutionary view on mental and (other) physical diseases, is a surprising source of insight on their nature, and sometimes on their hidden adaptive virtues.

Darwinian Medicine is famous for having demonstrated, in a number of cases such as fever, that symptoms often taken to be pathological in nature were in fact tremendously adaptive, or had been at some point in human evolution - though modern medicine frequently smothered their adaptiveness. Darwinian Psychiatry makes the same point concerning mental symptoms : only a few troubles can be reduced to a malfunctioning module (as is the case for some autisms, which are brought by a Theory of Mind deficit). More often psychiatric 'diseases' are but adaptive reactions to some deeper underlying trouble (such an analysis has been brought to explain schizophreniacs' delusions of control). While genuinely biological, darwinian psychiatry also has a humanistic flavor, as it objects the systematic medicalisation and neglect of the patient's adaptive strategies, which plagues the biologically-oriented psychiatry of today.

However adaptive, these mental reactions can misfire, especially when their environment is so strikingly different from the one in which they earned their adaptive value, as is the case of our modern world. Incredible though it seems, McDonald's were quite difficult to find in the Pleistocene, and this is one of the reasons why an innate craving for sugar and calories was very adaptive in those times; such is not the case today. Similarly, the current epidemics of depression has been linked to a serotoninergic mechanism of 'low-profile', adaptive in small primate societies, misfiring, due to the huge transformations of modern life (much more people, much more information, much more chances to realize you are not precisely ruling the rest of the world and being the focus of universal attention!).

Not every maladaptive behavior has to be cured (e.g. homosexuality, or having only one child while you could have nine); conversely, some dangerous traits such as psychopathy (meaning the lack of moral emotions) can be adaptive.

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