In most systems of medicine, the healer artfully evokes the patient's powers of self-suggestion, which are responsible for whatever healing may occur. This mysterious gift of self-healing is cloaked with an anodyne(止痛的) label, the 'placebo effect', and recognized only as a nuisance likely to confound clinical trials. But the placebo ( Latin for 'I will please' ) and its shadowy twin the nocebo ( 'I will harm' ) are much more than methodological problems: they lie at the heart of every interaction between doctor and patient. How they work no one knows. But the brain rules the body in many subconscious ways, including its control of the body's major hormones and its subtle influence over the immune system. So it's possible that, in ways yet unknown, expectations about health or disease are sometimes translated in to a bodily reaction that fulfils them. The power of these effects is hard to overstate. A rule of thumb is that 30 percent of patients in the placebo half of a drug trial ( i. e. those who unknowingly receive a dummy pill instead of the real thing) will experience all improvement in symptoms. But the proportion may be much higher. Just like real drugs, placebo pills can produce stronger effects in larger doses. Patients will report greater relief when given a larger pill, or two dummy capsules instead of one. Doctors' expectations also contribute to the awesome power of the placebo effect. In a study of tooth extraction, patients were given either a painkiller or sham drugs. Some dentists were assigned to give either drug, without knowing which, but other dentists knew they would be giving only sham drugs. The patients whose dentists thought they had at least a 50-50 chance of giving a painkiller suffered significantly less pain. Presumably, doctors transmit their expectations to the patient through subtle cues, often without knowing they are doing so. Placebo and noeebo ______.