Placebo

The placebo effect is the observation in scientific clinical studies that in any given trial a proportion of subjects will show health improvements when they are given a dummy drug – such as a sugar pill.

Does evolution explain the placebo effect?

Up until now, the placebo effect has seemed to make no sense…if people are able to heal themselves unaided, why does the body wait for a sugar pill to prompt recovery? Now, mathematical modelling carried out by researchers from a British university suggests that the placebo effect evolved because the immune system holds back in conditions of stress in order to save energy, but then kicks into higher gear when cues from the exterior environment indicate that it is safe and nourishing.

Since the immune system is energy-expensive, a full, sustained response could dangerously drain an animal’s energy reserves, especially in conditions where the external environment is harsh. As long as an infection is not life-threatening, it is prudent to wait for a sign that fighting it will not endanger the animal by draining its energy reserves.

The model revealed that, in challenging environments, animals lived longer and sired more offspring if they endured infections without mounting an immune response. In more favourable environments, it was best for animals to mount an immune response and return to health as quickly as possible. The results show a clear evolutionary benefit to switching the immune system on and off depending on environmental conditions.

This model may explain why therapeutic interactions that include warm interpersonal relationships are capable of generating a healing response, even in the absence of an active therapeutic agent, since they signal a beneficial environment.

(Understanding the placebo effect from an evolutionary perspective. Evol Hum Behav. 2012 Aug 28. [Epub ahead of print]).