The Clever Hans Phenomenon


[Updated February 5, 2019]

Clever Hans was a German horse in the early 1900s who was supposedly able to solve math problems and perform other amazing tasks. His owner, math teacher, amateur horse trainer, and mystic Wilhelm von Osten, said Hans could add, subtract, multiply, divide, work with fractions, tell time, keep track of the calendar, differentiate musical tones, and read, spell, and understand German. When given a math problem either orally or in writing, Hans would answer by tapping his hoof.

As a result of the large amount of public interest in Clever Hans, the German board of education appointed a commission to investigate von Osten’s scientific claims. The panel, known as the Hans Commission, consisted of 13 people, including a veterinarian, a circus manager, a Cavalry officer, a number of school teachers, and the director of the Berlin zoological gardens. This commission concluded in September 1904 that no tricks were involved in Hans’s performance.

The commission’s findings were handed off to Oskar Pfungst, a German comparative biologist and psychologist. Using multiple trials, Pfungst found that Hans 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 determined that when von Osten knew the answers to the questions, Hans got 89 percent of the answers correct, but when von Osten did not know the answers to the questions, Hans only answered six percent of the questions correctly.

Pfungst then examined the behavior of the questioner. His examination determined that as Hans’s taps approached the right answer, the questioner’s posture and facial expression showed an increase in tension, then relaxed when the horse made the final, correct tap. This body language provided a cue that Hans used to know when to stop tapping. Pfungst believed that van Osten really thought Hans was answering the questions, and was not deliberately perpetrating a fraud.

Thanks to Clever Hans, today when an animal touted to be brilliant is suspected of responding to the handler’s unintentional cues, it’s referred to as the Clever Hans phenomenon. Also thanks to Clever Hans, researchers created processes such as “double-blind study” for preventing non-human and human animals from responding from unintended cues given by the researchers to their subjects.

Clever Hans may not really have been able to do math, but it was certainly very clever of him to figure out how to read human body language well enough to answer the questions correctly and, no doubt, be reinforced for it.