A placebo – such as a sugar pill – is a treatment which is not effective through its direct action on the body but works because of its effect on the patient’s beliefs.  But if individuals are capable of recovering without external aid, why do they rely on an external cue?  In other words, why have individuals not evolved the ability to get better immediately on their own?

Members of the Modelling Animal Decisions group in the University of Bristol's School of Biological Sciences built mathematical models of the placebo effect which examine the trade-off between the costs and benefits of an immune response when faced with a health problem.

The work is based on an idea proposed by the theoretical psychologist Professor Nicholas Humphrey.  He proposed that, as it can be beneficial to hold the immune system back from full operation due to uncertainties about the state of the world (such as the possibility of starvation), cues which indicate a change can therefore lead to an altered level of immune response.

The models take this argument even further and demonstrate that the placebo effect is modulated by the patient’s expectations.  Previous studies measuring brain activity using functional magnetic resonance imaging (fMRI) provide experimental evidence which support the models, by showing correlations between the placebo effect and regions of the brain associated with expectation.

The models show why changes to the perceived cost of getting well, the value of being well or external environmental factors can induce the placebo effect.

Dr Pete Trimmer, lead author of the work, said: “The placebo effect comes down to expectations about when to take action.  Waiting for a useless pill before taking action is not optimal.  But the general responsiveness to cues is adaptive, so it is logical for evolved organisms to display the placebo effect.”

The models indicate that under stress it can be better for the immune system to work less effectively.  However, the most important finding of the research is that the particular type of belief in the treatment can lead to positive or negative effects.  The belief that a treatment will cure, without any need for the immune system to do anything, could have deleterious effects on the patient’s health.

Now that a theoretical approach has laid the foundations of understanding the placebo effect, future empirical work may provide insights as to how the placebo effect can be invoked and controlled in a clinical environment.  The Bristol study clearly shows that the focus of future placebo studies should be shifted to the type of belief patients have about their treatment rather than just whether a treatment is helpful or harmful.  A better understanding of the placebo effect may change the code of practice for health practitioners and save human lives.

Paper

'Understanding the placebo effect from an evolutionary perspective' by Trimmer, PC, Marshall, JAR, Fromhage, L, McNamara, JM, Houston in Evolution and Human Behaviour.