Darwinian medicine (the application of Darwinian ideas to our understanding of disease) is relatively new, to the extent that most doctors are not familiar with the term. In this book two psychiatrists (one a Jungian analyst, the other a former National Health Service psychiatrist and Senior Lecturer in Psychological Medicine) propose a new model of psychiatry based on Darwinism.
Although both authors are medically qualified, they disagree with the prevailing 'medical' model for psychiatric disorders. While not questioning the view that there are genetic and biological influences at work in disorders such as depression and schizophrenia, they think that mental function and dysfunction cannot be adequately understood unless they are seen in an evolutionary context. Many psychological disorders, they argue, are manifestations of ancient adaptive strategies that are no longer always appropriate in modern circumstances.
The authors consider psychological disorders in three broad categories. The first is disorders of attachment and rank. Depression, they suggest, is connected with loss in personal relationships: for example, loss of an attachment figure (such as a maternal figure) or to loss of 'rank' in a social group. Other disorders discussed in this context include personality disorders, obsessions, anxiety and phobias, and eating disorders. Phobias are obviously easy to explain in evolutionary terms, but some of the others are perhaps more difficult to accommodate.
The second category is what the authors call spacing disorders. By this they mean dysfunctional states characterized by difficulty in forming and maintaining personal relationships and in functioning appropriately in a social group. Schizophrenia is the characteristic disorder found in this category but it lies at one end of a spectrum of symptoms. Most psychiatrists today regard schizophrenia as a primarily biological disorder, with a genetic component, but this raises the question: why has selection not eliminated the gene or genes responsible?
The theory advanced by Stevens and Price is that people with a schizoid personality (but not full-blown schizophrenia) are suited to taking on the role of charismatic leader, and such leaders, they argue, would have been advantageous in early hunter-gatherer societies because they would facilitate the splitting up of groups which were tending to grow larger than the optimum size (about 50 individuals). The trouble with this theory, as the authors recognize, is that it depends on the idea of group selection, which is pretty unfashionable today. However, they think that opponents of group selection have seriously underestimated the rate at which human groups split and the frequency with which groups became extinct in earlier times.
The third category to be considered is reproductive disorders, including homosexuality and sadomasochism. Actually, Stevens and Price don't regard homosexuality as a disorder, since they think it is adaptive in certain circumstances and is widespread among many male primates. They manage to construct a reasonably plausible evolutionary explanation for this apparently non-adaptive behaviour, at least so far as male homosexuality is concerned; they have little to say about female homosexuality, probably because relatively little seems to be known about this. Sadomasochism, again, is mainly a male preoccupation and is therefore discussed in that context.
One weakness of the book is that, although it is intended to put forward a scientific theory, it makes few predictions by which that theory could be tested. The nearest it comes to it is in the penultimate chapter, where the therapeutic implications of the evolutionary view of human psychology are considered. However, this discussion is fairly short and serves mainly to indicate possible directions for the future. I was also rather unhappy about one of the supporting arguments, namely Paul Maclean's theory of the 'triune brain'. The idea that the human brain is really composed of three brains (reptilian, old mammalian, new mammalian) which potentially conflict with one another is unsatisfactory on a number of grounds and doesn't accord with recent brain research. But this idea is not essential to the authors' main thesis, which deserves to be taken seriously.
Human beings, after all, are the product of the evolutionary process, and it is surely self-evident that their mental functioning must be conditioned and shaped by evolution. Stevens and Price make a good case for the view that this approach gives a better understanding of psychiatry than does the current medical model.