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Has everyone heard of "Clever Hans," the mathematical horse owned by an apparently ingenuous German schoolmaster in the early 1900's? (For those who like to know such things there was "Rosa," the mare of Berlin, the dog of Utrecht, and the reading pig of Virginia, but these were cleverly cued vaudevillians.) Anyway, this horse could add, subtract, multiply, divide, read, spell, and solve problems of musical harmony, communicating his answers by tapping his foot. How? The schoolmaster was not a showman, and he made no profit from his marvelous horse. He permitted scores of investigators to look into the talents of his animal.
None could explain the phenomenon until O. Pfungst observed that others had been misled by "looking for in the horse, what should have been sought in the man." He found in the man, by some careful experiments, that he unconsciously and very subtly, cued the horse. The
Titchener JL. Experimenter Effects in Behavioral Research.. Arch Intern Med. 1967;120(6):753-755. doi:10.1001/archinte.1967.00300050109027