Clever Hans the wonderhorse

At the turn of the last century, a horse named Hans was thought capable of complex intellectual tasks such as arithmetic, reading, spelling, telling time and even understanding the German language. Hans, der Kluge Hans, or clever Hans as he is more commonly known, was owned by a Mathematics teacher from Berlin, Wilhelm von Osten. During four years of careful and deliberate instruction, von Osten went to great lengths to educate his equine pupil and understand the horse’s expressions: (…) “his chief mode of expression was tapping with his right forefoot. Hans also expressed himself by means of movements of the head; thus “yes” was expressed by a nod, “no” by a deliberate movement from side to side; and “upward,” “upper,” “downward,” “right,” “left,” were indicated by turning the head in each particular direction” (Pfungst, 1911).

However, Hans counted-out most of his answers by tapping his forefoot on the floor appropriate number of times. Numbers were simply tapped-out, so 7+3 would be tapped 10 times. Von Osten had encoded the alphabet as “A”=one tap; “B”=two taps and so forth, assigning numbers to letters. The horse spelled out answers to questions by tapping its hoof the number of times corresponding to each letter, forming words and sentences. For this task the crafty mathematician had (…) “translated a large number of concepts into numbers.” (Stumpf, 1911) The letters of the alphabet in small German script, where arranged on a chart, with numbers below them indicating the row, and the place occupied by the letter in any particular row. Tones of the musical scale were taught using a small child’s organ with the diatonic scale C^1 to C^2. For teaching colour discrimination a number of coloured cloths were used.

Placards with written words were placed in front of Hans, he would step forward pointing with his nose to any of the words that were required for the answer, at times even spelling words successfully.This was done by the aid of a table devised by Mr. von Osten, in which every letter of the alphabet, as well as a number of diphthongs had an appropriate place which the horse could designate by means of a pair of numbers. Thus in the fifth horizontal row “s” had first place; “sch” second, “ss,” third, etc.; so that the horse would indicate the letter “s” by treading first 5, then 1, “sch,” by 5 and 2, “ss” by 5 and 3. Upon being asked “What is this woman holding in her hand?” Hans spelled without hesitation: 3, 2; 4, 6; 3, 7; i. e., “Schirm” (parasol). At another time a picture of a horse standing at a manger was shown him and he was asked, “What does this represent?” He promptly spelled “Pferd” (horse) and then “Krippe” (manger).” (from Pfungst, 1911)

Herr Schilling, an African explorer with high scientific standing, tested the abilities of Hans. Herr Schilling would approach Hans in his stall accompanied by another person: at Schillings request the second person would intently think of a number between 1-20, keeping the same secret. Schillings would then ask Hans to tap the answer while he withdrew from the performance, remaining absolutely passive. More often than not, Hans would tap the right response even if the response was unknown to Schilling.(Sanford, 1914)

Knowledge of the extreme intellect allegedly possessed by this quadruped, caused much mental turmoil, heated debates, newspaper headlines, attracting the masses and the attention of eminent scholars and scientists alike. A number of explanations for the curious abilities of the “wundepferde” or wonder-horse as he was often called, were suggested by all walks of life including von Osten’s own. The math teacher and proud owner of the wundepferde was convinced that Hans was an educated animal not merely a trained one (Sanford, 1914). After four years of laborious and dedicated education, von Osten had no doubt that the abilities of Hans were due to a thorough education by way of the same pedagogical tools used with school children. The completely opposing view was also suggested, and it was that Hans was merely trained, and his so called answers were only feats of associated repetition with certain conditions serving as cues. Another group smelt fraud, suggesting intentional signalling, a trickery of sorts to fool the masses.Last, but not least, there were suggestions that did not limit themselves to the ordinary sort of explanation, preferring an extraordinary one instead: Response to radiant heat emanating from the questioner; electric apparatus on the ground acting on the horse’s hoof, etc…

Von Osten who had never charged a penny for his demonstrations, was dismayed at the accusation of fraud from the press and appealed to the local school board to investigate the veracity of the horse’s ability as well as the viability of his teaching methods.


The German Department of Education urged by von Osten himself, put together a commission, known as the Hans Commission. This group, led by the Philosopher and Psychologist Professor Carl Stumpf, were called together to investigate (…) ” whether or not there is involved in the feats of the horse of Mr. von Osten anything of the nature of tricks, that is, intentional influence or aid, on the part of the questioner.” (Pfungst, 1911) The Hans Commission was comprised of a Circus Manager (Paul Busch), a Count (Otto zu Castell-Rüdenhausen), Teachers (Dr. A. Grabow; Robert Hahn), Zoologists (Dr. Ludwig Heck; Dr. Oscar Heinroth) a Veterinarian (Dr. Miessner), retired Military (Major F. W. von Keller; Major General Th. Köring) and Physiologist (Prof. Nagel), amongst some others.

Regardless of the questions the multi-disciplinary commission presented Clever Hans, the horse was able to correctly answer. The commission had no choice but to inform the masses and the media, that no trickery was involved in the feats of the ever cleverest Hans. However the actual origin of the abilities of Hans were not tackled, and they suggested the likeliness that other factors were involved, which ought to be carefully investigated. It was at this point that Stumpf assigned the task of further investigation to his student Oskar Pfungst.

Following the September, 1904 commissions findings The New York Times ran an article on Clever Hans – Berlin’s Wonderful Horse: He Can Do Almost Everything but Talk – How was he taught? The popularity of Clever Hans was now an international sensation attracting even more interest than before.


Dr. E. von Hornbostel, Mr. O. Pfungst and Professor Stumpf investigated the likely origins of the “clever” accomplishments of Hans, through an experimental method. Hans was questioned in a variety of ways and the experimental set-up was modified several times. Hans failed in his responses whenever the solution of the problem that was given him was unknown to any of those present. When objects to be counted or numbers written on a paper where placed before Hans, he failed in response if either the examiner or anyone else had not seen the objects or the numbers scribbled on the paper.

It was thus clear that Hans needed visual aid to solve the problems. If Hans was not in sight of person and especially the questioner, to whom the solutions were known, he gave an incorrect reply. These visual aids which guided Hans, need not be given intentionally. To explain the outcome of the investigation, it was suggested by Stumpf (1911) that the horse had learned to closely attend to the slight unconscious changes in bodily posture of Von Osten, whilst tapping with the forefoot.
“The motive for this direction and straining of attention was the regular reward in the form of carrots bread, which attended it. This unexpected kind of independent activity and the certainty and precision of the perception of minimal movements thus attained, are astounding in the highest degree.” (Stumpf, 1911)

“The movements which call forth the horse’s reaction, are so extremely slight in the case of Mr. von Osten, that it is easily comprehensible how it was possible that they should escape the notice even of practised observers. Mr. Pfungst, however, whose previous laboratory experience had made him keen in the perception of visual stimuli of slightest duration and extent, succeeded in recognizing in Mr. von Osten the different kinds of movements which were the basis of the various accomplishments of the horse. Furthermore, he succeeded in controlling his own movements, (of which he had hitherto been unconscious), in the presence of the horse, and finally became so proficient that he could replace these unintentional
movements by intentional ones. He can now call forth at will all the various reactions of the horse by making the proper kind of voluntary movements, without asking the relevant question or giving any sort of command. But Mr. Pfungst meets with the same success when he does not attend to the movements to be made, but rather focuses, as intently as possible, upon the number desired, since in that case the necessary movement occurs whether he wills it or not. In the near future he will give a special detailed report of his observations, which gives promise of becoming a valuable contribution to the study of involuntary movements. Also he will give an account of our tests and of the mechanism of the various accomplishments of the horse. We must also defer, till then, the disproof of certain seemingly relevant arguments in favor of the horse’s power of independent thought.” (Pfungst, 1911)


In the end an explanation was found to the origin of the remarkable abilities of the Clever Hans. Hans had no need for telepathy or even critical thinking: he was using his uncanny perception as a horse to detect even the involuntary physical responses of von Osten as well as that of the questioners. Hans had learned to identify subtle tensing and relaxing of muscles that occur in someone who is anticipating the correct answer. Thus, Hans would tap his hoof until he saw the subconscious twitch in observers who knew he had arrived at the right spot in the alphabet, and there Hans would stop, oblivious to the semantic content of his actions.

Wilhelm Von Osten never really accepted the Clever Hans explanation, so he and his horse continued to put on their math-and-body-language show throughout Germany for some time, eventualluy ceasing their public performances. Wilhelm von Osten passed away in 1909, and Hans was tranferred to a farm and later sold to jewelry tycoon Karl Krall of Elberfeld. Hans was again in the spotlight in Eberfeld and performed with two other horses; Muhamed and Zarif.

Today, the “Clever Hans Effect” is used to describe the influence of a questioner’s subtle and unintentional cues upon their subjects, in both humans and in other animals, to prevent prejudices and foreknowledge from contaminating experimental results.

Thank you Hans, you clever boy!

Our horses are, as a rule, sentenced to an especially dull mode of life. Chained in stalls (and usually dark stalls at that) during three-fourths of their lives, and more than any other domestic animal, enslaved for thousands of years by reins and whip, they have become estranged from their natural impulses, and owing to continued confinement they may perhaps have suffered even in their sensory life. A gregarious animal, yet kept constantly in isolation, intended by nature to range over vast areas, yet confined to his narrow courtyard, and deprived of opportunity for sexual activity,—he has been forced by a process of education to develop along lines quite opposite to his native characteristics. Nevertheless, I believe that it is very doubtful if it would have been possible by other methods, even, to call forth in the horse the ability to think. Presumably, however, it might be possible, under conditions and with methods of instruction more in accord with the life-needs of the horse, to awaken in a fuller measure those mental activities which would be called into play to meet those needs.
(Pfungst, 1911)