Darwinian medicine is a new approach to understanding diseases, disorders, syndromes and, more generally, conditions that cause pain and discomfort to those that have them. Introduced by George Williams, Paul Ewald, David Haig (evolutionary biology) and Randy Nesse (psychiatry), Darwinian medicine is based on the recognition that the evolved architecture of the human body consists of a collection of adaptations: mechanisms that reliably develop in a species-wide fashion, which were designed by natural selection to perform certain adaptive functions. (The heart, liver, and immune system are uncontroversial examples.) Some symptoms or medical conditions represent the proper functioning of these adaptive mechanisms; others are caused by impairments to them.
This perspective applies to any adaptive system, whether in the brain or the rest of the body. The brain is an evolved organ, no less than the heart, and the function of its neural architecture is to make behavior adaptively contingent on information from the environment. As such, it should be comprised of adaptations that process information. Some psychiatric conditions may be caused by impairments to these adaptive systems, whereas others – however painful – may represent their proper functioning.
Darwinian medicine is not just changing how conditions are conceptualized, investigated, and classified; it is also changing how they are treated. Fever, for example, is not a dysfunction in an adaptation: it is one of the body’s defenses against infection. Knowing this, I can intelligently decide whether to treat my fever (and thereby prolong the infection) or endure it but be well earlier. The iron-withholding that characterizes certain forms of anemia is also a defense against infection, in this case by pathogens that require iron to replicate. Before this was understood, iron supplements were given to anyone with anemia; now it is recognized that this worsens the health of many anemic individuals.
In other words, understanding whether a given symptom or syndrome is caused by a functional adaptation, or is a disorder of an adaptation, is not just an academic exercise: It has important implications for treatment. Of course, the fact that a symptom represents the proper functioning of an adaptation doesn’t mean it should not be treated. Because natural selection organized each mechanism to solve a distinct adaptive problem under ancestral conditions, the criteria for whether a mechanism is dysfunctional are supplied by whether the mechanism has become impaired in performing its ancestral function. Because evolutionary function and dysfunction diverge markedly from the ordinary human standards of value that prevail in a given place and time, various mechanisms that are performing their evolved function may cause disturbing outcomes (equally, there can be dysfunctions that are valued by the people who have them). For this reason, many conditions involving the proper functioning of an adaptation may require treatment (if the person with the condition so wishes), and it would be improper to “treat” a dysfunction when its outcomes are valued by the person who has it. For discussion, see
Cosmides, L. & Tooby, J. (1999). Towards an evoluationary taxonomy of treatable conditions. Journal of Abnormal Psychology, 108, 453-464. PDF
How, then, should various psychiatric disorders be seen? Is depression (for example) always caused by an impairment of an adaptive system? Or are there forms of depression that reflect the proper functioning of an adaptation, where the symptoms that cause the depressed person such acute suffering are (like fever) there to solve an adaptive problem of some kind? Ed Hagen’s most recent work addresses these questions.
For Ed Hagen’s website and PDFs, click here Hagen.
Postpartum depression. Ed Hagen, who graduated from the CEP biosocial anthropology program in 1999, presents various research that militates against the hypothesis that postpartum depression is a byproduct of pregnancy hormones (e.g., fathers get it too!); that it results from exhaustion; and so forth. More importantly, he has developed an interesting theory postulating that depression in general – and postpartum depression in particular – results from an evolved adaptation designed for renegotiating cooperative social relationships. On his website you will find papers on the theory, and on his emprical tests of it.
In a nutshell, Hagen’s theory is as follows. Our ancestors (like modern humans) were highly cooperative individuals, who often forged deep cooperative relationships with one another for achieving a common goal. Marriage and childcare are a prime example of this: Cross-culturally, marriage is a publicly recognized agreement between two individuals – a contract, if you will – to have children only with one another and to cooperate in raising them. Sometimes, however, things don’t turn out at all as one hoped: a cooperative arrangement that promised to be beneficial isn’t for some reason – e.g., your spouse is not giving you the help and support that you need. Under these circumstances, you would need to renegotiate the terms of the relationship. One way to renegotiate an unprofitable contract is to “go on strike”: to cease to provide benefits to the other person until that person starts to provide the help and support that you need.
Hagen has proposed that postpartum depression is the expression of an adaptation that causes one to go on strike when a cooperative contract is imposing excessive costs and needs to be renegotiated. It causes a shutdown of the systems that normally cause you to provide benefits to others, and does so in a way that makes the people closest to you solicitous of your needs rather than angry at you for not carrying your weight. While many other theories of depression involve the notion that sad affect is an adaptation designed to inform other decision-making mechanisms in the mind that one’s current environment or relationships are evolutionarily unprofitable, very few theories can explain the most perplexing feature of depression: the inability to act. Yet the inability to engage in purposeful, productive work is exactly what one would expect of a system designed to produce a “labor strike”.
Some people may find this theory shocking – after all, isn’t it awful to propose that a sad, depressed person is callously renegotiating a contract? But someone taking that view is misunderstanding both evolutionary psychiatry and Hagen’s theory. The claim is not that the depressed person has consciously made cold calculations and decided a strike is in order: that’s just not how natural selection designs adaptations. Adaptations should work without one being consciously aware of their function. For example, your heart pumps harder when you run, and you don’t need to know why this happens for it to happen. Indeed, it would be a bad design if pumping harder while running happened only in people who understood that blood brings oxygen to tissues and that tissues need more oxygen when one is running. Similarly, the adaptations that cause depression (if they are adaptations, as Hagen has proposed) can have been designed over hundreds of thousands of years by selection pressures that reflect the functional logic of renegotiating contracts through labor strikes, without the depressed individual understanding this functional logic. All that is necessary is that a depression set in when some part of the mind feels that a cooperative contract is imposing costs, and that the depression lift when the cooperative situation changes for the better (or else when the relationship is abandoned).
For more on Darwinian Medicine, see the following links:
George Williams (who started it all)
George Williams, review in Science
Paul Ewald a pioneer of evolutionary medicine; see, e.g., his book, The evolution of infectious disease.
David Haig: Genomic imprinting and kinship (How conflict within the genome can explain disorders, such as gestational diabetes and pre-eclampsia)
Why we get sick by Randolph Nesse and George Williams (an easy introduction to Darwinian medicine)
Randolph Nesse (Darwinian medicine, Darwinian psychiatry)
Carl T. Bergstrom (Ecology and evolution of infections diseases)
