The Clever Hans Phenomenon

By Karen Harris

A horse flicks water off its hoof at Windsor Racecourse on July 19, 2021 in Windsor, England. (Alan Crowhurst/Getty Images)

Animals are much smarter than we give them credit for, but just how intelligent and teachable are they? In the late 1800s, a horse named Clever Hans wowed the public by selecting the correct answers to math problems, but was he really a wonder beast, a hoax, or something else altogether?

Wilhelm Von Osten

Born in 1838, Wilhelm von Osten was a math teacher by trade but a lifelong student. He was especially interested in phrenology, the now-debunked field of studying the size and shape of the human skull to determine intelligence and other traits. Osten wanted to prove the principles of phrenology applied to animals as well, and while his experiments with a cat and a bear didn't pan out for what can only be assumed to be a number of variably painful reasons, he finally seemed to succeed with a horse named Hans, who appeared to count by tapping out numbers with one of his front hooves. Soon, he was delivering the solutions to basic math problems.

By 1891, Clever Hans, as Osten had begun calling him, became a German national sensation, and eventually, media as far away as The New York Times ran front-page stories on the supposedly gifted horse. Osten never tried to profit off Hans, staging the shows at no cost to the enormous crowds that gathered to watch. Still, the authorities assumed there had to be something fishy (or rather, horsey) going on, and it wasn't long before Germany's Board of Education started questioning the validity of Hans's skills.

Clever Hans performing in 1904. (Karl Krall/Wikimedia Commons)

The Clever Hans Phenomenon

The Board of Education put together a task force of scientists, trainers, and other professional to investigate Osten's claims and concluded in 1904 that he didn't use hand gestures or other fraudulent means to manipulate the horse, but one psychologist, Oskar Pfungst, suspected that wasn't the end of the story. After he got permission from Osten to test Hans, he went a step further than the Board's commission—literally. He asked Osten to step back while he quizzed Hans, who correctly solved most of the problems but performed worse than usual. The farther away Osten stood, however, the fewer problems Hans could correctly solve. Next, Pfungst asked him to present Hans with questions Osten himself didn't know the answers to, after which the horse's correct response rate plummeted to zero.

It turned out that Osten was indeed not intentionally signaling the answers to Hans, but the horse picked up on subtle changes in his body language to determine the correct answer. For example, Hans could tell when he approached the right answer because the tension in Osten's body suddenly decreased. The effect, which is observed in humans as well, has become known as the Clever Hans Phenomenon. People, especially those involved in behavioral studies or other situations where their responses are important, often respond to unintentional cues from the person questioning them rather than the question itself.

Like it? Share with your friends!

Share On Facebook

Karen Harris


Karen left the world of academic, quitting her job as a college professor to write full-time. She spends her days with her firefighter husband and four daughters on a hobby farm with an assortment of animals, including a goat named Atticus, a turkey named Gravy, and a chicken named Chickaletta.