placebo effect

n. The change in functioning or behavior brought about by the administration of an inert substance or nonspecifc treatment which seems to be caused by a belief that a subject has received a treatment. Thus a person who is given a sugar pill and is told it is a powerful sleeping pill may experience sleepiness after taking the pill, while a person who is not told it is a sleeping pill is unlikely to feel sleepy after taking the same kind of pill. Placebo effects account for a significant portion of the effectiveness of many medications and treatments.