A few years ago, scores of middle-aged men took part in a clinical trial of a new drug, intended to ease the symptoms of their enlarged prostates. But this wasn’t just a study of a drug’s power to help. It was also a study of the mind’s power to hurt.

Before getting their prescriptions, half of the men were told about a small risk of erectile dysfunction and other sexual side effects; none of the men had a history of such troubles. Of the men who got the warning, about 44 percent developed these symptoms. But among those kept in the dark, the rate was far lower: only about 15 percent.