The Clever Hansel Effect

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The Clever Hans Effect is the phenomenon where an experimenter inadvertently signals to an experimental subject, whether human or animal, the correct answer. To control for this effect, researchers "blind" their experimental trials as much as possible. Ideally, they are double-blinded, meaning neither experimenter nor experimental subject knows which arm of the experiment they are in.

The name comes from a horse named Clever Hans, who was famous for being able to do simple math. He indicated the correct answer by stamping his hoof up to the number. Later, it was discovered that the horse had been reading a demeanor change in the handler when it arrived at the right answer, and was not actually adding and subtracting. Hansel, of course, comes from Hansel and Gretel.