A promising new drug for depression failed to clear efficacy tests this year, illuminating a decades-old problem in psychopharmacology that researchers say deserves more study: the placebo effect, in which patients receiving a dummy pill do almost as well as those on the drug being tested, thereby wiping out the rationale for the new drug. Drug developers regard it as an occupational hazard that masks the effects of potentially useful compounds. But some psychiatrists and clinical psychologists are fascinated by the power of the placebo effect, viewing it not as a problem but as a source of insight into mental health. And a few go further, arguing that many compounds may be little more than sophisticated placebos themselves.