Clever Hans was an Orlov Trotter horse that was claimed to have been able to perform arithmetic and other intellectual tasks… Hans was a horse owned by Wilhelm von Osten, who was a gymnasium mathematics teacher, an amateur horse trainer, phrenologist, and something of a mystic. Hans was said to have been taught to add, subtract, multiply, divide, work with fractions, tell time, keep track of the calendar, differentiate musical tones, and read, spell, and understand German.
After a formal investigation in 1907, psychologist Oskar Pfungst demonstrated that the horse was not actually performing these mental tasks, but was watching the reaction of his human observers… Using a substantial number of trials, Pfungst found that the horse could get the correct answer even if von Osten himself did not ask the questions, ruling out the possibility of fraud. However, the horse got the right answer only when the questioner knew what the answer was, and the horse could see the questioner.
Pfungst then proceeded to examine the behaviour of the questioner in detail, and showed that as the horse’s taps approached the right answer, the questioner’s posture and facial expression changed in ways that were consistent with an increase in tension, which was released when the horse made the final, correct tap. This provided a cue that the horse could use to tell it to stop tapping. The social communication systems of horses may depend on the detection of small postural changes, and this would explain why Hans so easily picked up on the cues given by von Osten, even if these cues were unconscious.
Pfungst carried out laboratory tests with human subjects, in which he played the part of the horse. Pfungst asked subjects to stand on his right and think “with a high degree of concentration” about a particular number, or a simple mathematical problem. Pfungst would then tap out the answer with his right hand. He frequently observed “a sudden slight upward jerk of the head” when reaching the final tap, and noted that this corresponded to the subject resuming the position they had adopted before thinking of the question.
Both von Osten and Pfungst were notoriously bad-tempered and prone to rage when the horse did not perform well. Pfungst suffered more than one horse bite during his investigation. (wiki)
