Who showed that Clever Hans responded to subtle human cues rather than doing math?

Oskar Pfungst showed that Clever Hans responded to subtle human cues rather than actually doing mathematics.

Clever Hans was a German horse presented in the early 1900s as able to solve arithmetic problems by tapping his hoof. In 1904, a commission found no obvious trickery, so psychologist Carl Stumpf asked his assistant Oskar Pfungst to investigate the horse more rigorously.

Pfungst varied whether Hans could see the questioner and whether that person knew the answer. Hans performed well when he could see a knowledgeable questioner, but his accuracy collapsed when the questioner was hidden or unaware of the correct answer. Pfungst concluded that Hans watched tiny, involuntary changes in posture and facial expression as his taps approached the expected number.

The episode became the source of the “Clever Hans effect,” a warning that researchers can unintentionally cue animals—or influence experiments—in the direction of an expected result. Hans was not necessarily being deceptive; he was exceptionally good at reading people.