Natural history of the present

The potential for evolutionary theory to transform psychiatry

As an evolutionary biologist I get to spend my days thinking, researching and writing about ‘the most important idea anyone ever had’. That’s what the philosopher Daniel Dennett called evolution by natural selection in Darwin’s Dangerous Idea.

That idea should permeate every field that deals with living organisms, from agriculture to zoology, taking in economics, medicine, psychology and sociology along the way. Yet 154 years since Darwin first revealed the mundane but powerful process whereby evolution occurs, that permeation is nowhere near complete.

In recent years I’ve enjoyed taking the time to depart from my primary research field – how sex, reproduction and the conflicts they stoke shape animal life histories and ageing – in order to explore the frontiers between evolution and economics, psychology and anthropology.

Which is why I was excited to discover today a paper by a group of 12 psychiatrists, entitled “The crisis of psychiatry – insights and prospects from evolutionary theory” published in February in the World Psychiatry Journal.

Psychologists have been building a sophisticated evolutionary understanding of so-called “normal” human behaviour for some decades now. But the study of psychiatric conditions has tended to focus more on proximate mechanisms to the exclusion of the evolutionary adaptive origins of those conditions.

The authors “propose that a contemporary and scientifically satisfying understanding of psychiatric conditions requires … taking into consideration the influence of environmental contingencies and natural selection in sculpting not just brain based mechanisms and processes germane to clinical neurosciences, but also diverse characteristics of behaviour.”

The paper is so short (three pages) and readable that I highly recommend you read it to absorb their full argument. The authors illustrate the importance of understanding both psychiatric ‘normalcy’ and ‘disorder’ as parts of continuous distributions of continuously varying traits. This variation may be introduced by environmental circumstances, selection operating on genetic variation and by interactions between genes and environment.

An example comes from a particular “short” allele (i.e. form) of the serotonin transporter coding gene. Carriers, when exposed to adversities in early childhood, experience greater risk of depression in later life. But those carriers with emotionally secure upbringings are significantly less likely to develop depression in adulthood.

This idea, of interactions between gene and environment, features among the most prominent evolutionary explanations for why natural selection doesn’t simply eliminate all but one “most successful” form of each gene, leaving no genetic variation in a population.

We can’t expect to find single genes underlying most complex traits, but the example proves the principle that whether a gene’s psychiatric effects are positive or negative can depend on the environment.

This illustration of how genes and environment interact also, in admirably few words, transcends the pointless old dichotomy between biology and experience.

I had heard the psychiatrist Randolph Nesse, a great champion of Darwinian Medicine, lecturing about the evolutionary basis of anxiety and depression, but I had no idea just how far evolutionary thinking was seeping into the usually mechanism-obsessed field of psychiatry.

I’m going to enjoy keeping an eye on new developments in this field.