Evolutionary psychiatry: foundations, progress and challenges
Dr Gurjot Brar and Prof. Henry O'Connell review Randolph Nesse's recent article
Introduction
The upcoming June volume of the World Psychiatry Journal (the psychiatry journal with the largest impact factor) includes a special article dedicated to Evolutionary Psychiatry by one of the field’s most eminent physicians, Randolph M. Nesse. (see article here and twitter thread by Nesse himself here).
This landmark paper is likely to become a ‘go-to’ resource in the field for current and future psychiatrists. In the article, Nesse eloquently traces the ‘foundations, progress, and challenges’ of the discipline, illustrated with various examples. It is likely to be an equally enjoyable and informative read for established evolutionists and newcomers alike. Nesse has already demonstrated in 'Good Reasons for Bad Feelings' (2021) the depth of his knowledge and his ability to describe evolutionary ideas in engaging and accessible ways for readers from all backgrounds.
Here we provide some highlights which intend to pique the interest of readers, of course, encouraging all to review the full paper by Nesse themselves.
Current Frameworks (Nosology)
As referenced by Wakefield in his editorial of the same volume, now appears to a be a timely opportunity to ‘examine the strengths and limitations of evolutionary psychiatry’. Current frameworks that classify mental disorders have been accused of stagnating and failing to deliver on promises made decades ago. Other ‘competitors’ including the Research Domain Criteria (RDoC) and Hierarchcal Taxonomy of Psychopathology (HiTOP) which, whilst noble in their vision, do not deliver on providing a unifying framework that ties together seemingly different fields such as genetics, anthropology, neuroscience, behavioural science, psychology, etc. factors related to the origin, development (ontogeny) and evolution (phylogeny) of humans. Additionally, there are challenges in determining whether mental disorders should be seen as a spectrum or as discrete categories. An evolutionary approach recognises, for example, that selective pressures exerted on genes and alleles over time commonly produce the most robust trait in a given population in a normal distribution. Thus the ‘tail-ends’, can represent traits leaving individuals potentially vulnerable to disorders. Nesse illustrates this ‘sweet spot’ phenomenon with the example of the wingspan of birds travelling across seas. Too large or too small of a wingspan reduces chances of successfully navigating through storms.
Nesse begins his paper by tracing a brief history of evolution and describing what it is (and what it is not) capable of explaining currently. He deftly dispels the group selection theory error often made by evolutionists claiming mental disorders can be explained by benefits to the group.
W.B. Haldane’s quote in reference to whether he would sacrifice his life for his brother “No, not for one brother. But I would for two, or for eight cousins”
Touching on sexual selection and individual differences, Nesse then moves through various evolutionary theories proposing to explain mental disorders such as stochastic (random, unintended) variations, frequency dependent selection and trade-offs. Species-wide vulnerabilities from the stochasticity of evolution include the path of the urethra through the prostate gland and the path of the recurrent laryngeal nerve. The brain with its countless trillion neuronal connections is likely to also be vulnerable in this regard and no exception.
Giving seven evolutionary explanations for disease vulnerability (see Table 1.), Nesse takes the reader through concise and informative details regarding each including species-wide vulnerabilities resulting from genetic drift and path dependence and the ever-popular mismatch theory of bodies versus environment. A developmental history of evolutionary psychiatry then brings us up to speed from Darwin to Dawkins and McGuire and Troisi to Abed and St John Smith’s latest edited book on the subject. Following this, Nesse tackles emotions and disorders, a lifelong interest intersecting with much of his work. In it he describes the possible utility of negative emotions equating them to symptoms such as fever and pain making the persuasive case to map emotions to situations rather than functions.
Nesse then approaches ‘Anxiety and its disorders’ which contain the explanation of the ‘smoke-detector principle' and hypophobia. Reviewing the leading evolutionary explanations for low mood and depression is next on the agenda, concluding on framing depression as extreme versions of overlapping states shaped to cope with different unpropitious situations. Does low mood increase fitness? How is ‘kindling’ relevant to depression?
Moving on to substance use and abuse, novel evolutionary questions are posed such as why do plants make psychotropic drugs? Are human motivations to use substances an epiphenomenon of motivations shaped for other reasons, or are they adaptations shaped because taking drugs increases fitness?
If you encountered a lion at the watering hole two nights in a row, it is best to stay home. If getting water is essential, it will be wise to go with friends, make the trip short, and be on alert and ready to flee at the least hint of danger
Nesse writes “evolutionary psychiatry's ability to uncover functions for seemingly negative emotions is both its greatest strength and weakness.” By acknowledging that these emotions are evolved adaptations, psychiatry can overcome the error of treating all symptoms as if they were disease manifestations. However, it must also be careful not to make the mistake of viewing illnesses like panic disorder and schizophrenia as adaptations, as this can be equally problematic for the field.
As pollution from the soot made tree trunks darker during Victorian times, light-coloured moths were easier prey for birds to spot and eat. As a result, darker moths had more offspring and became more common over generations (Cook et al., 1986). Although this is a well-known example, natural selection generally works to maintain the status quo rather than cause change, opting for the parsimonious route wherever possible.
It's important to understand that evolutionary medicine doesn't necessarily explain diseases themselves, but rather the traits that can make our bodies more susceptible to developing them. Some examples of these traits include the narrow birth canal, the way the windpipe opens into the pharynx, and the tendency for our immune systems to sometimes attack our own tissues.
Sexual selection is displayed in mating behaviour and competition. A large portion of human behaviour is shaped by competing for mates which can lead to potentially dire circumstances. The tail of the peacock and magnificent antlers on deer encumber these animals at the cost of increasing mating frequency and thus gene transmission.
The SOCIAL model
The SOCIAL acronym (Social resources, including friends, groups and social status; Occupation and other valued social roles; Children, family and relatives; Income, savings and material resources; Abilities, appearance, health, skills and other personal resources; and Love and sex), developed by Nesse can be used to guide a broader definition of health and wellbeing, allowing psychiatry to pivot away from measuring outcomes as mere changes on questionnaires or rating scales.
Recent research progress has led to a consensus that emotions are adaptive states shaped by natural selection
In his review of the model, Nesse touches on ‘social traps’ and ‘depressogenic’ environments if individuals are trapped pursuing unreachable goals. The standard Porsolt test of how long a drug influences the duration a rat swims, is then argued to be an inadequate proxy for determining depression. Furthermore, Nesse describes an evolutionary informed approach which considers disorders of other emotions, not just mood and anxiety.
Nesse continues with a brief summary of evolutionary perspectives on eating disorders including Abed’s work on the intrasexual competition amongst females and describing the ‘eating control system’ which ensures protection against starvation due to it being the more potent selection force.
Finally, the persistence of deleterious genetic variation is described as a paradox is evolutionary genetics. Here Nesse expounds on the recent work in the field of genetics and outlines that the majority of the risk for major mental disorders such as schizophrenia and bipolar disorder stem from thousands of non-coding alleles with marginal effects. Mutation-selection balance appears to be responsible for the persistence of most disease-causing alleles. In other words, evolution and reproduction will produce mutations which will be selected out at a rate proportional to the reduction in fitness, only to be replaced to maintain the mutation-selection balance. There is now some evidence emerging that the incidence of schizophrenia is declining (McGrath et al., 2008) and that older alleles were more likely to increase the risk compared to newer alleles.
TJ Crow wrote extensively about the possibility that psychosis could be the price we pay for the capacity of language
Instead of arising from common variants with large effects that code for proteins, the majority of the risk instead arises from thousands of non-coding alleles with tiny effects.
The Smoke Detector Principle
Nesse is also well known in the field for the ‘smoke detector principle’ which implies the occasional ‘false alarm’ is acceptable to ensure protection ‘from a real fire’. The cost of a false alarm is low compared to the cost of no or little anxiety (hypophobia) which may be lethal. Just as nausea, vomiting, pain, cough, and fever etc are usually non-life-threatening symptoms, they often point to an underling cause that needs to be addressed. Anxiety in this regard can also be seen as a protective mechanism in certain situations. However, when anxiety becomes excessive and interferes with daily life, it can potentially progress to disorder.
The Cliff-Edge Phenomenon
The ‘cliff-edge phenomenon’ described by Nesse illustrates the precarious nature of some traits which lie in a ‘sweet spot’. Relatively minor variations in allele distribution can leave individuals more susceptible to disorder. When considering fitness functions with a cliff edge, natural selection will push the trait towards the point ‘close to the peak’ that maximizes gene transmission over multiple generations. This happens despite the low fitness experienced by the few individuals with values over the cliff.
This phenomenon is intriguing because it reveals how natural selection can, in some cases, prioritize the long-term success of a species over the short-term well-being of individual members. By optimizing the trait to the point just before the cliff edge, the species as a whole can maximize gene transmission and increase their chances of survival over multiple generations.
Conclusion
Psychiatry describes pathology with little reference to normal functions
Fundamentally, psychiatry seeks to distinguish normal variation from mental disorder. Comprehensive assessment would logically require and benefit from a systems and context based approach with due deference given to phylogenetic mechanisms. Genetic variation driven by natural selection brings about brain variation which in turn leads to behavioural variation and the manifestation of symptoms/syndromes. To truly understand psychiatric disorders, an evolutionary approach becomes essential. Nesse concludes that evolutionary biology as a basic science can make significant contributions to our understanding of mental disorders, providing a missing essential basic framework for integrating findings from disparate fields. It is complementary and not an alternative; it is not a new method of treatment but encompasses all treatments. It is relevant for clinicians, researchers and educators. Finally, Nesse considers the popular question of whether an evolutionary approach in psychiatry represents a new paradigm. In it he describes various reasons for and against, appropriately concluding that it is too early to tell and further work is necessary in furthering the practical applications to understanding and treating mental disorders.
Key Principles
1. Evolutionary psychiatry does not seek to explain diseases/disorders; it seeks to explain traits that leave us vulnerable to disease/disorders
2. Evolution seeks to maximise gene transmission (over e.g. ‘mental stability’ and happiness)
3. Inclusive Fitness refers to direct benefits to the individual and indirect benefits to kin (kin selection)
4. Group selection is unlikely to hold any explanatory power (traits that harm the individual but somehow give benefits to the group)
5. Diseases are unlikely to be useful (and thus not selected for themselves)
6. It is now widely accepted that emotions are adaptive states shaped by natural selection
7. One emotion may have many functions (e.g. anger) and one function may be advanced by many functions
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visiting the webpage of the Evolution and Psychiatry Special Interest Group within the College of Psychiatrists of Ireland
visiting the webpage of the Evolutionary Psychiatry Special Interest Group within the Royal College of Psychiatrists
exploring a Youtube playlist on curated presentations by the Evolution and Psychiatry Special Interest Group within the College of Psychiatrists of Ireland
exploring the Youtube page of the Evolutionary Psychiatry Special Interest Group within the Royal College of Psychiatrists
exploring the Evolving Psychiatry podcast