When the placebo works

News that a study in the New England Journal of Medicine found sham surgery to be just as beneficial as regular knee operations for arthritis seemed to be another blow to modern science.

Many doctors agreed that patients who had arthroscopic surgery had not benefited greatly. However, rather than being an indictment of unnecessary surgery, this research provides further evidence of one of the great mysteries of science – the placebo effect.

The new study, by government researchers at the Houston Veterans Affairs Medical Center and Baylor College of Medicine, showed that patients who received the sham surgery could walk faster than they had been able to before the operation for about two years (until their knee's continuing deterioration caught up with them). In fact, all the participants, whether or not they had the real surgery, demonstrated physical improvements. The issue is therefore not one of whether or not the real surgery was ineffective – it proved reasonably effective at what it claimed to do – but just how the "placebo effect" can produce results that are just as good.

The effect seems to hold true for psychological as well as for physical maladies. In May, researchers announced that in an analysis of 96 studies of the effectiveness of antidepressant medications, more than half showed that people who received sugar pills responded just as well to the "treatment" as those who took the real medicines. Along with the surgery study, this research suggests that the placebo effect is genuine and that people's minds and bodies react as if they have received beneficial treatment when in fact they have received only the appearance of treatment.

The placebo effect is widely used in medical practice as well as controlled trials of active-ingredient medicines. It is part of a long dispute in Western science, largely growing out of the Cartesian philosophical tradition that divided the human experience into mind and body. Once we acknowledge that many illnesses are associated with psychological states such as fear, stress, or depression, the notion that psychological reassurance or belief might do the trick becomes more reasonable. The question of whether the placebo effects are "real," and if so, by what mechanism, has challenged medical science, since the phenomenon hints at the efficacy of spiritual healing and many "alternative medicine" methods.

Unfortunately, not all evidence supports the existence of a placebo effect. The strongest scientific criticism against it was published in 2001, when a Danish study reviewed 114 previous studies that compared the effectiveness of real treatments to both placebos and no treatment at all. It found that the placebo effect was either nonexistent or wildly overblown. A little skepticism was actually long overdue. The original basis for the placebo effect was a 1955 paper by an anesthesiologist that claimed, without much substantiation, that 35 percent of patients got better with a dummy pill. The number entered medical folklore without further scrutiny.

On the other hand, the Danish study was not as powerful as it seemed at first. It was not an independent test of the placebo effect versus no treatment, but a review of other studies. Remember, the absence of evidence does not equal evidence of absence. It may mean there was no effect, or it may signal only that effects weren't detected.

Further, the definition of a placebo used in this analysis was broad. It included a psychiatric study of "nondirectional neutral conversations," (or, in English, vague chats) as opposed to talk therapy. Should informal conversations with a doctor, no matter how reassuring, fit the definition of a placebo trial? Given that placebo advocates note greater effectiveness depending on the type of placebo – those given by injection are more effective than drinking colored liquid, while people given a placebo said to be morphine report less pain than patients told they were receiving generic aspirin – counting nebulous conversations as placebos probably understates the impact.

The two more recent studies provide powerful evidence that the placebo effect is noisily alive. Perhaps the most convincing evidence for its existence was revealed in February, when Science magazine announced that Swedish and Finnish researchers had shown that placebos activate the same brain circuits as painkilling drugs. It may be that the placebo response is actually part of all painkilling treatments. These studies show that we dismiss the power of the placebo at our peril.

• Iain Murray is director of research at the Statistical Assessment Service (STATS), a nonpartisan research organization.