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.
Mathematical models developed by scientists at the University of Bristol are providing new insights into why the placebo effect exists and when it should occur. Their research is published today in the journal of Evolution and Human Behaviour. 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.
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