Clever Hans Case Study

As Denyse O’Leary will likely discuss in her new series “Animal Minds,” a number of research studies claim to show that under some circumstances animals are capable of abstract thought. I believe that those conclusions are wrong, and that animals are not capable of abstraction.

Animals think particularly, not abstractly. That is, animals think about particulars (individual things) and about perceptions connected to the particulars, but they are not capable of abstracting universal concepts from particulars and contemplating the universal concepts in isolation from individual things that evoke them. For example, my dog certainly thinks of my kindness and responds by wagging her tail, but there is no evidence she is capable of contemplating kindness as a concept, abstracted from my particular behavior.

That is not to say that animals can’t be clever. Animals can be very clever — sometimes more clever than humans. Try to build a spider web yourself. But animal cleverness is always tied to particular things and situations and instincts. A spider builds a very effective web to catch his meals. There is no evidence the spider contemplates the beauty of spider webs or that it understands “web” as a metaphor for a complex set of associations.

In animal cognition research, it is of the utmost importance to distinguish clever perceptive thinking from abstract thinking. Animals can be trained to do very clever perceptive things, and some of these clever things seem like abstract thinking. A parrot could be trained to recite Godel’s Theorems, but the parrot knows nothing of mathematical logic. To be trained in perceptual cleverness is not to think abstractly.

The pitfall of mistaking training in perceptual thought for abstract thought is clearly illustrated by a famous example in animal cognitive research — the case of Clever Hans.

Wilhelm Von Osten was a German mathematics teacher and phrenologist at the turn of the 20th century who was convinced that animals were capable of abstract thought. He tutored several animals in the hope of demonstrating the truth of animal reason. The cat and the bear didn’t turn out well (he actually tried), but he claimed to have success with Hans, an Arab stallion. Osten drew mathematical problems on a blackboard, and, after a while, the horse seemed to understand some of what was written, and could even answer mathematical questions (by counting with his hoof). Hans responded to a “6” on the blackboard with six taps of his hoof. He responded to “What is the square root of nine” with three taps. He was able to answer correctly consistently, over a broad range of problems. He was even able to calculate dates — if asked “if Tuesday is the 4th of the month, what date will the following Friday be?” he would tap his hoof seven times. Osten announced that Hans’ mathematical skills were those of the average 14-year-old child.

Osten exhibited Hans across Germany to large crowds (the horse was even featured in the New York Times). The horse’s accuracy was about 90 percent, and the exhibitions attracted much attention from the scientific community.

In Germany, the educational establishment formed a Hans Commission, staffed by scientists who studied Hans’s abilities. They observed the exhibitions carefully and studied Hans extensively. The Hans Commission concluded that Hans’ abilities were genuine, and there was no evidence for deception. The horse was capable of abstract mathematical reasoning.

However, a leading psychologist — Oskar Pfungst — remained skeptical, and he was invited by the Commission and by Hans’ owner to examine Hans in even more detail. This is what Pfungst found:

  1. When his owner asked Hans a question, he did very well.

  2. When someone other than his owner asked Hans a question, he also did very well.

  3. However, when someone who did not know the answer to the question asked Hans a question, Hans almost never got the answer right.

  4. When Hans was asked a question in such a way that he could not see the person asking the question, Hans almost never got the answer right.

The precondition for Hans’ correct answer was that the questioner had to know the answer and Hans had to be able to see him.

Pfungst carefully studied the questioners. He noted that the questioner always gave subtle verbal or physical clues — a tone of voice at the right moment, a grimace, a tensing of muscles — and Hans’ would tap his hoof until he saw the clue to stop. All of this was quite unconscious on the part of the questioners — volunteers did it, as well as Hans’ owner. But Hans was not capable of abstract mathematical reasoning. He was capable of perceiving remarkably subtle particular clues. He was trained to respond to particular clues. This perhaps should come as no surprise in a horse — an animal that was selected, bred and trained to respond to the subtle cues of a human rider.

Hans was exceptionally skilled at perceiving cues from humans. He had no abstract mathematical ability at all.

This sublime perception of unconscious cues is familiar to mentalists who can do amazing feats of perception on audience members who are unaware that they are subtly signaling their thoughts.

The Clever Hans problem is well known to researchers in animal cognition, but it haunts the field. The difficulty is this: animals are capable of prodigious feats of perception — think bloodhounds who follow vanishingly subtle scents for miles — and animals are capable of high levels of training. Subtle perception reinforced with meticulous training can give the appearance of abstract thinking when in fact none is present.

An example of behavior that would suggest abstract thought would be Hans’ apparent ability to calculate dates using the concept of a calendar. If an animal were to respond correctly, given a date and day of the week, to a question about the date of an ensuing day of the week, the inference that the animal was able to abstract the concept of “calendar” from the present and calculate according to a calendar would be reasonable. But in the absence of perceptual clues no animal has ever done it, or has ever done anything like it.

The Clever Hans effect is not merely an effect caused by the ability of the animal to see the trainer — it is an effect that can taint computer and flash-card training, even when the trainer is not visible to the animal. Just as a parrot can be trained to recite the words of a complex abstract theory, an animal can be trained to group objects according to type, color, shape, use, etc. Yet there is no reason to infer that the animal understands the concepts of the categories–the animal is merely responding to perceptual clues. Color, size and shape are cues just like grimaces.

Abstract thinking is not mere memory of similarities between particulars reinforced by training. Worms can be trained to avoid or tolerate encumbrance of a particular kind — that does not mean that the concept “encumbrance” means anything to a worm.

Abstract thinking is the ability to think conceptually without particulars. There is no convincing evidence for abstract thinking in animals. Research that is claimed to show animal abstract reasoning invariably shows sublimely trained perception, not genuine abstract thought.

Animals rely on particulars to think, and their perceptual skills can be prodigious. They do not think abstractly.

Image: Clever Hans, by Karl Krall (Karl Krall, Denkende Tiere, Leipzig 1912, Tafel 2) [Public domain], via Wikimedia Commons.

For Grimm's tale, see Clever Hans (fairy tale).

Clever Hans (in German: der Kluge Hans) was an Orlov Trotterhorse that was claimed to have been able to perform arithmetic and other intellectual tasks.

After an investigation in 1907, psychologistOskar Pfungst claimed to have demonstrated that the horse was not performing these mental tasks, but was watching the reactions of his trainer. He said that he had discovered this artifact in the research methodology, wherein the horse was purported to have been responding directly to involuntary cues in the body language of the human trainer, who had the faculties to solve each problem. The trainer was entirely unaware that he was providing such supposed cues.[1] In reference to Pfungst's study, like anomalous artifacts have otherwise been called the Clever Hans effect and have been considered important knowledge in the observer-expectancy effect and later studies in animal cognition. Hans was studied by the famous German philosopher and psychologist Carl Stumpf in the early 20th century. Stumpf was observing the sensational phenomena of the horse, which also added to his impact on phenomenology.


During the early twentieth century, the public was especially interested in animal intelligence owing in a large part to Charles Darwin's recent publications. The case of Clever Hans was taken to show an advanced level of number sense in an animal.

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.[1] 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. Von Osten would ask Hans, "If the eighth day of the month comes on a Tuesday, what is the date of the following Friday?" Hans would answer by tapping his hoof. Questions could be asked both orally, and in written form. Von Osten exhibited Hans throughout Germany, and never charged admission. Hans's abilities were reported in The New York Times in 1904.[2] After von Osten died in 1909, Hans was acquired by several owners. After 1916, there is no record of him and his fate remains unknown.


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. Philosopher and psychologistCarl Stumpf formed a panel of 13 people, known as the Hans Commission. This commission consisted of 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.[2]

The commission passed off the evaluation to Pfungst, who tested the basis for these claimed abilities by:

  1. Isolating horse and questioner from spectators, so no cues could come from them
  2. Using questioners other than the horse's master
  3. By means of blinders, varying whether the horse could see the questioner
  4. Varying whether the questioner knew the answer to the question in advance.

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. He observed 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 answered only six percent of the questions correctly.

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.[3]

Even after this official debunking, von Osten, who was never persuaded by Pfungst's findings, continued to show Hans around Germany, attracting large and enthusiastic crowds.[3]

The Clever Hans effect[edit]

After Pfungst had become adept at giving Hans performances himself, and was fully aware of the subtle cues which made them possible, he discovered that he would produce these cues involuntarily regardless of whether he wished to exhibit or suppress them. Recognition of this phenomenon has had a large effect on experimental design and methodology for all experiments whatsoever involving sentient subjects, including humans.

The risk of Clever Hans effects is one reason why comparative psychologists normally test animals in isolated apparatus, without interaction with them. However this creates problems of its own, because many of the most interesting phenomena in animal cognition are only likely to be demonstrated in a social context, and in order to train and demonstrate them, it is necessary to build up a social relationship between trainer and animal. This point of view has been strongly argued by Irene Pepperberg in relation to her studies of parrots (Alex), and by Allen and Beatrix Gardner in their study of the chimpanzeeWashoe. If the results of such studies are to gain universal acceptance, it is necessary to find some way of testing the animals' achievements which eliminates the risk of Clever Hans effects. However, simply removing the trainer from the scene may not be an appropriate strategy, because where the social relationship between trainer and subject is strong, the removal of the trainer may produce emotional responses preventing the subject from performing. It is therefore necessary to devise procedures where none of those present knows what the animal's likely response may be.

The Clever Hans Effect has also been observed in drug-sniffing dogs. A study at University of California Davis revealed that cues can be telegraphed by the handler to the dogs, resulting in false positives.[4] A 2004 study of Rico, a border collie reported by his owners as having a vocabulary of over 200 words, avoided the Clever Hans effect by having the owner ask the dog to fetch items from an adjacent room, so that the owner could not provide real time feedback while the dog was selecting an object.

As Pfungst's final experiment makes clear, Clever Hans effects are quite as likely to occur in experiments with humans as in experiments with animals. For this reason, care is often taken in fields such as perception, cognitive psychology, and social psychology to make experiments double-blind, meaning that neither the experimenter nor the subject knows what condition the subject is in, and thus what his or her responses are predicted to be. Another way in which Clever Hans effects are avoided is by replacing the experimenter with a computer, which can deliver standardized instructions and record responses without giving clues.

See also[edit]



External links[edit]

Clever Hans performing in 1904
Wilhelm von Osten and Clever Hans
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