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Clever Hanssound icon

In a nutshell: Clever Hans was a horse that some people thought could do math in his head and understand German. One group of people tested him and found these claims were true. Another scientist tested Hans and found the claims weren't true.

Clever Hans (German: Kluger Hans) was a German horse who seemed to be able to do math problems in his head, tell time, name people, and answer questions by tapping his hoof. When asked to add 3 + 2, Hans would tap his hoof five times. If asked my name, Hans would have tapped twice for 'B', paused, tapped 15 times for 'O', paused, and then tapped twice again.

Hans could even answer hard written questions. Was he really able to read?

"If the eighth day of the month comes on a Tuesday, what is the date of the following Friday?” Hans could figure that out. He answered by tapping his hoof eleven times. This was one smart horse!

Hans's owner, Wilhelm von Osten, began showing his clever horse to the public in 1891. He did so for about twenty years. Many people, including some scientists, were sure that Hans was really doing some heavy duty thinking. They were certain that his hoof tapping was not a trick. Clever Hans became famous.

Clever Hans

The German board of education sent a group of people (the Hans Commission) to study Hans. Carl Stumpf, a philosopher and psychologist, led the commission. A few school teachers, a veterinarian, a director of the Berlin zoo, and a few others joined Stumpf.

The Hans Commission listed some possible explanations for what Hans could do. Maybe he could really understand both spoken and written language. Maybe he could do math in his head. Maybe he was acting on cues his owner was giving.

Animal trainers have known for a long time that many animals can be trained to perform tricks on cue. The animals don't really understand human languages, but they can be taught to connect sounds and movements with food. With enough training, the connection can become so strong that the animal will respond to the sound or movement alone, even though no treat follows. We call this operant conditioning, a method that grew out of the work of Russian scientist Ivan Pavlov (1849-1936).

The idea that animals have evolved from common ancestors was new in 1891. (Charles Darwin's theory that animals evolved by a process of natural selection was published in 1859.) Maybe we don't know enough about horses, pigs, dogs, or gorillas to say that they don't understand math or language. After all, they have brains like we do and maybe we don't understand them because they don't speak our language.

The commission made up some tests to rule out one or the other of the ideas. Could the horse read and understand human speech or was he responding to cues? The tests showed that Hans wasn't being cued. Since there was no trickery or signaling going on, the commission said that Hans could do math and he could read and understand German.

a skeptical scientist tests Hans again

Repeating an experiment is one of the key methods in science. If there was no signaling going on, then other scientists should be able to test Hans and get the same results. If many scientists test a claim (called a hypothesis) and get the same results under different testing conditions, science considers the hypothesis confirmed. Since it is always possible that the hypothesis will fail the next test, no single test confirms a hypothesis with absolute certainty. If a hypothesis passes many hard tests under many different conditions, then scientists consider the hypothesis probably true. No number of confirmations, however, proves a scientific hypothesis with absolute certainty.

Oskar Pfungst (1847-1933), a biologist and psychologist, tested Hans again. He found that the Hans Commission had not done a very good job. Pfungst proved that Hans was responding to very slight movements of those watching him. Hans was especially sensitive to small head movements by his owner. Pfungst also found that the movements by those watching Hans were not done on purpose to cue the horse. If asked to add 3 + 2, for example, the horse would start tapping. When he got to five taps, von Osten (or others watching Hans) would lean forward very slightly and that was the cue for the horse to stop tapping.

How did Pfungst figure this out?

He set up a test where the horse could be asked questions but could not see von Osten or others who were watching. He put blinders on Hans. When the horse couldn't see anyone, he didn't respond. In another test, the horse didn't respond when the one asking the question didn't know the answer to it. But the horse did give the right response when the one asking the question did know the answer to it, even if von Osten was not present. That last test ruled out the hypothesis that von Osten knew he was signaling Hans. He wasn't cheating. He really believed his horse was doing math and reading letters. The evidence collected by Pfungst led the scientific community to hold that Hans was responding to slight movements rather than understanding written and spoken words. Von Osten, however, kept on believing that his horse could read, do math, and understand German. He was making a little bit of money showing off Hans and he kept on with his pony show.

one scientific discovery leads to another

One of the beautiful things about science is how one discovery leads to another. One of the things Pfungst found is the ideomotor effect: making slight movements without being aware of making them. (See the entries on dowsing and the Ouija board for other cases of the ideomotor effect.)

While testing Hans, Pfungst discovered that animals respond to movements around them that can barely be seen. He also found that people aren't always aware that they are moving and giving cues to animals (or other persons). We now know that humans also respond to movements or sounds without being aware of it. We call this giving of signals without awareness unconscious signaling. This discovery has had a major effect on how experiments should be done when they involve either people or animals.

We now know that humans are unconsciously aware of many things right before their eyes. A botanist might see a rare flower out of the corner of his eye and not be conscious of having seen it. Later, he finds himself thinking of that rare flower. Then, he sees the flower and says "wow, I was just thinking of that flower." Isn't that amazing? Yes. Scientists call it "sensing without seeing."

We also know that sometimes we don't see what is right before our eyes. If a person in a gorilla suit walked through a scene you were watching on a video, you think you'd notice. Right? Scientists have found that about half the viewers don't see the gorilla if they're asked to watch a video and count how many times players in white shirts pass a basketball to each other. The person in the gorilla suit walks right through the scene, yet many viewers don't see him. Viewers pay so much attention to the people passing the ball that they don't see the gorilla. Scientists call this inattentional blindness.

A trainer might think his drug-sniffing dog is a genius because in tests he always sniffs at the bowl with the drugs in it. But maybe the trainer is pointing the dog to the right bowl without even being aware of it. The trainer might be giving an unconscious signal, however slight, to the animal. The animal may get 100% on the test but not really be able to sniff out drugs!

A trainer might think his cancer-sniffing dog is a genius because in tests he always sniffs at the box with the cancer cells in it. But maybe the trainer is pointing the dog to the right box without even being aware of it. The trainer might be giving an unconscious signal, however slight, to the animal. The animal may get 100% on the test but not really be able to sniff out cancer!

Scientists who test human subjects might also be giving or getting slight cues during a test. They must be careful not to trick themselves because of the Clever Hans effect.

further reading

The Animal Communication Project "The Clever Hans incident chilled enthusiasm for human/nonhuman communication research for decades. No one today assumes horses can understand human language. But many assert great apes can. The 1960s and 1970s saw a resurgence of attempts to communicate with animals—chimpanzees, orangutans and gorillas this time—until the Clever Hans phenomenon struck again. One researcher analyzed videotapes of 'conversations' between trainers and his subject, a chimpanzee named Nim, who had learned some word signs derived from American Sign Language. The frame-by-frame analysis revealed trainers unconsciously prompting and modeling each word, and Nim imitating. Unaware of their own prompting, the trainers had credited Nim with producing sentences. For the second time, enthusiasm for human-language training for animals waned. The few researchers currently pursuing these studies keep Clever Hans in mind, doing their best to design scientifically 'clean' experiments."

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Last updated 10-Jul-2013

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