Clever Hans: the horse that could 'do math' by reading body language

In early-1900s Germany, a horse named Hans appeared to solve arithmetic by tapping out answers with his hoof. Psychologist Oskar Pfungst discovered in 1907 that Hans had no number sense at all: he was reading tiny, involuntary tensing in his questioner's posture that signaled exactly when to stop tapping. The case gave psychology the observer-expectancy effect, the reason well-designed studies now use blind protocols so researchers can't unconsciously cue the results they expect.

— Oskar Pfungst, Clever Hans (The Horse of Mr. Von Osten): A Contribution to Experimental Animal and Human Psychology — Monograph, 1907; English translation 1911
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