Clever Hans: The Horse Who Could Count, or Could He?

Clever Hans was a horse in Berlin who seemed to do arithmetic, tell the time and spell words by tapping his hoof. A panel of experts found no trick in 1904. Then in 1907, psychologist Oskar Pfungst discovered the real answer: Hans was reading tiny, unconscious movements in the faces and bodies of the people around him.

The Smartest Horse in Berlin

Wilhelm von Osten spent years teaching Hans as if he were a schoolchild. Ask Hans what five plus three was, and he tapped his hoof eight times. He seemed to answer questions about dates, time and spelling too. Crowds filled the courtyard to watch, and newspapers named him Clever Hans.

The Experts Found No Trick

In 1904, a commission of scientists, a circus trainer and others examined Hans. Von Osten was not secretly signalling him, and Hans answered even when other people asked the questions. The commission found no deliberate trick.

How Was Clever Hans Doing It?

Oskar Pfungst ran careful experiments. When the questioner did not know the answer, Hans failed. When Hans could not see the questioner, he failed. As Hans tapped closer to the right number, people tensed slightly, and when he reached it they relaxed, often with a small lift of the head. Hans stopped tapping at that moment. Nobody was doing it on purpose. Most people did not know they were moving at all.

Why Clever Hans Still Matters

Hans was not a genius at numbers, but he was remarkable at reading people. Scientists now call this the Clever Hans effect: an animal or a person picking up unintended cues from an experimenter. Researchers design studies so that the person testing does not know the answer, and they do it with Hans in mind.

Frequently Asked Questions

Could Clever Hans really do math?

No. He was responding to small, unconscious cues from the people watching him.

Who figured out Clever Hans?

Psychologist Oskar Pfungst, in experiments published in 1907.

What is the Clever Hans effect?

When a subject picks up unintended cues from an experimenter, which is why many scientific tests are run so the tester does not know the answer.

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Sources

• Oskar Pfungst, Clever Hans (English edition): the experiments and findings
• Nature, 1904: contemporary coverage
• Samhita and Gross, peer-reviewed review: the Clever Hans phenomenon today

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Cover photo: Karl Krall via Wikimedia Commons, public domain.