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AN OLD medical adage admonishes healers to treat the patient, not the disease. But how a patient treats the disease can sometimes be as important as anything his healer might do. For example, when a patient believes he is receiving treatment, his symptoms often fade miraculously, even if he is only swallowing sugar pills. How such “placebo” effects work has mystified doctors for centuries. At the AAAS meeting this week, some light was shed on the subject.

According to Fabrizio Benedetti, a neuroscientist at the University of Turin, researchers now believe that the placebo effect results from the release in the brain of natural painkillers called endogenous opiates, which are produced when the brain anticipates relief. When people are dosed with a substance that counteracts the effects of these opiates, their response to placebos evaporates. Moreover, the brain also contains a substance called cholecystokinin, which opposes the action of opiates. When patients are given a drug that degrades cholecystokinin, their opiate levels stay higher than normal and their responses to placebos become stronger.

The placebo effect also holds true in reverse. When patients do not know they are receiving treatment, they do not respond to it. Dr Benedetti showed the audience a remarkable video of a patient with Parkinson's disease, an ailment that causes muscle tics and trembling. These symptoms can be alleviated by electrical stimulation of the brain. The video showed that when the patient was unaware that stimulation was being applied, his twitching continued unabated. But as soon as he knew the electrodes had been switched on, his symptoms were reduced.

Even more remarkable is the effect that beliefs can exert on intellectual performance. Claude Steele, a psychologist from Stanford University, has studied differences between the sexes in maths exams. Popular prejudice suggests that women do not perform as well as men in such exams. Dr Steele found that this difference in performance has more to do with differences in confidence than in competence.

He discovered this by administering a maths exam to a set of equally qualified men and women, all of whom were studying the subject at Stanford. As the test was being distributed, one group was informed that it had proved to be sex-neutral: men and women performed equally well in it. To the other group, which acted as a control, no such comment was made.

As popular prejudice predicted men outperformed women in the control group. In the experimental group, however, not only did the women do better than their sisters in the control group, but the men did worse than their brothers there. The result was an equal performance by both sexes. In education as in medicine, it seems, mind acts over matter in yet mysterious ways.