Assigning a “purpose” to any feature of evolution is an invitation to make a fool of one’s self. Evolution is an inherently random process. The desire to use evolution to tell “just so” stories often results in seemingly scientific explanations that are completely bogus. But I’ll bite, and risk looking like an idiot, because this is such an interesting topic.
A better version of this question might be “why are there disease states that are susceptible to the placebo effect?”. Placebo effects have been reported for a wide variety of conditions, even for seemingly mechanical procedures like knee surgery[1] . But those are individual studies, and the variance between studies tends to be large. A meta-analysis (from 2010 and possibly now outdated) could find significant and consistent placebo effects only for pain, nausea, phobia and asthma[2] . I think we can all agree that these are very real conditions, but ones that are influenced by psychological states.
The key to thinking about the placebo phenomenon is to recall that we are intensely social animals. Yes, there are placebo effects in animals[3] , but these reports always involve highly social animals such as dogs. There are, to my knowledge, no reports of placebo effects in snakes or insects or plants.
Having a placebo-susceptible condition allows the sufferer to place a claim on the resources of the group. The same could be said, of course, for conditions that are not susceptible to placebo cures, such as infections or broken bones. The key difference is that the placebo-susceptible maladies are curable. And they are curable by activities that tend to strengthen social bonds within the group: food-sharing, rituals, intimate conversation. Illness and cure create webs of mutual obligation that increase group cohesion and reduce conflict.
The selective advantage of the placebo effect – if there is one – is at the level of the group[4] , not the individual. Like recreational sex, it is a means of creating and strengthening social ties. A prediction of this view is that placebo-susceptible illnesses are interchangeable with other group-strengthening activities. They should be more prevalent in individualistic societies, and less prevalent in more socialized societies.
And indeed, there are reports that Americans are more susceptible to placebo effects than other nationalities and that placebo susceptibility is increasing over time[5] . No doubt many other factors are involved, but this observation is consistent with an inverse relationship between individuality and placebo effects.
It’s possible that placebos are a quirk of neuronal wiring. But a better bet is that they are a consequence of our extreme sociability. Placebo-responsive illnesses weaken individuals but strengthen societies.
Featured on Forbes and Apple News
Footnotes
[1] Arthroscopic Partial Meniscectomy versus Sham Surgery for a Degenerative Meniscal Tear — NEJM
[2] Placebo interventions for all clinical conditions
[3] Placebo effect in canine epilepsy trials.
[4] Individual versus Group in Natural Selection
[5] Increasing placebo responses over time in U.S. clinical trials of neuropathic pain.