Which historical example was one of the first demonstrations of the experimenter expectancy effect?

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The horse, Clever Hans, and his mathematical abilities is one of the earliest and most well-known examples of the experimenter expectancy effect. Clever Hans appeared to be able to solve mathematical problems by tapping his hoof, but it was later discovered that he was simply responding to unconscious cues from his trainer. This demonstration showed how experimenter expectations can influence research outcomes.

Which Historical Example Was One of the First Demonstrations of the Experimenter