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Do placebos provide a mental cue to kickstart the immune system?

British scientists have uncovered some interesting insights into the body's healing process that could help explain the placebo effect.

In a new study (subscription required), researchers developed a computer model to test a decade's old theory (.pdf) originally put forth by former London School of Economics psychologist Nicholas Humphrey, PhD. Humphrey proposed that patients subconsciously respond to treatment, even if it's a sugar pill, because of their strong belief in the medicine's ability to fight the infection and aid in recovery without further depleting the body's resources.

A New Scientist story published today describes the latest findings and how the new evidence supports Humphrey's earlier paper:

It all starts with the observation that something similar to the placebo effect occurs in many animals, says Peter Trimmer, a biologist at the University of Bristol, UK. For instance, Siberian hamsters do little to fight an infection if the lights above their lab cage mimic the short days and long nights of winter. But changing the lighting pattern to give the impression of summer causes them to mount a full immune response.

Likewise, those people who think they are taking a drug but are really receiving a placebo can have a response which is twice that of those who receive no pills (Annals of Family Medicine, In Siberian hamsters and people, intervention creates a mental cue that kick-starts the immune response.


Trimmer's simulation is built on this assumption - that animals need to spend vital resources on fighting low-level infections. 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 (Evolution and Human Behavior, The results show a clear evolutionary benefit to switching the immune system on and off depending on environmental conditions.

Previously: Debating the placebo as treatmentThe puzzling powers of the palacebo effect and The increasing power of the placebo
Photo by Lucy Reynell

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