Clever Hans (The Horse of Mr. Von Osten) / A contribution to experimental animal and human psychology

by Oskar Pfungst

Another group of errors was characterized as stubbornness on the part of Hans, such as his persistence in repeating an incorrect response, or his repetition of a former correct answer in response to later questions where it was perfectly senseless. During a demonstration before a large number of persons, I held a slate with the number 13 upon it within the horse's view and also within view of the spectators. I, myself, did not know what number was written on the slate. Having been asked to tap the number, Hans responded by tapping 5. The grand-stand shouted "Wrong!" I asked Hans to try again. Four times in succession he answered 5. At another time Mr. von Osten and I each whispered a number (7 and 1, respectively,) into the horse's ear and asked him to add the two. Three times in succession he tapped 11. After the test had been repeated in accordance with "procedure with knowledge" and a correct response had been received, we tried once more a test of "procedure without knowledge". Again, he responded with an 11. On a third occasion, I asked Hans to tap 5. He responded with a 4 and then, correctly, with a 5. Thereupon, I asked him to tap 6. Again, he responded with a 4. Then I asked him to tap 7. Once more he responded with a 4, and only when I proceeded to count aloud did he tap 7 correctly. I had him repeat the 7 and then went over to 9. Promptly he responded with another 7. In these cases, which by-the-way were not very frequent, we have to do, not with stubbornness on the part of Hans, but with the persistence of that number in the consciousness of the questioner. Modern psychology has recognized this tendency of ideas, which have once been in consciousness, to reappear on other occasions even though they are wholly inappropriate. It has been termed "perseverative tendency." (

Perseverationstendenz).[21]

While the errors thus far discussed appeared sporadically in long series of correct responses, there still might be observed at times a massing of errors, usually at the beginning of a day of experimentation or at the beginning of a new series. We were regularly told that Hans always had to have time to adjust himself to new circumstances. The records often showed comments such as these: "After a number of practice tests the horse appears particularly well disposed", or "Hans, at first inattentive, does not respond. Suddenly he gets the hang of things". Different questioners who worked with the horse required different lengths of time to obtain proper responses. Some needed a quarter of an hour, others scarcely half a minute. I, myself, found that in the degree in which I learned to control my attention, in that degree did this phenomenon tend to disappear, but would reappear the moment I became indisposed. From this we see that, instead of attributing all sorts of mental characteristics, such as stubbornness, etc., to the horse, we should lay them to the account of the questioner. As a matter of fact we find that this "getting into the sweep of things", i. e. the overcoming of psycho-physical inertia, has long been known in the case of man and has been experimentally determined and called "Anregung" (excitation) by the psychiatrist, Kraepelin,[22] and his pupil, Amberg.[23] A massing of errors toward the end of a long series occurred only when the questioner was fatigued. There was nothing which had to be interpreted as fatigue or as indisposition on the part of the horse, (except in the few cases of very large numbers, cf. [page 67]). To be sure, Mr. von Osten always offered these two excuses. That they were without warrant is shown by the fact that Hans, after appearing indisposed or fatigued while working with one questioner, would nevertheless react promptly and correctly a moment later for some other experimenter, and furthermore, when working with me, the number of his correct responses would rise or fall with my own mental disposition.

Finally, I would here note a rather interesting observation for which I am indebted to Mr. Schillings and the Count zu Castell. They had noticed, independently of each other, that the horse would often fail to react when for any length of time he was given problems dealing with abstract numbers, even though they were of the simplest kind; but that he would immediately improve whenever the questions had to do with concrete objects. They believed that Hans found applied mathematics more interesting, and that abstract problems, or those which were altogether too elementary, bored him. The Count zu Castell furthermore noticed that the responses tended to be more correct as soon as he had the horse count objects which he, himself, (Castell) could see during the test. Quite in accord with this is the statement to be found in the report of the September-Commission, in which we find this note in a discussion of the arithmetical problems (not involving visible objects), which the gentlemen already mentioned had given the horse. "The horse responded with less and less attentiveness and appeared to play with the questioner." Here again, that was looked for in the animal which should have been sought in the man. Mr. Schillings was capable of intense, but not continued concentration and it was he who was bored, and not the horse. And it was the Count zu Castell and not the horse that found it necessary to invoke the aid of perceptual objects to bring his attention to the proper height of concentration.

The reader will see that thus far I have supposed the horse to be a never-failing mechanism and that I have placed all errors to the account of the questioner. The horse never failed to note the signal for stopping and therefore never was the immediate cause of an error. It is not to be denied that now and then he would cease tapping spontaneously and in this way would become the cause of an error. We have no data on this point, but undoubtedly the horse's share in the total number of errors was very slight.

(b.) Another source of error was faulty computation on the part of the questioner. The questioner made the signal for stopping when the expected number of taps had been reached. The horse faithfully mirrored the miscalculation of the questioner. I have knowledge of only one such case. The journals report that once Mr. von Osten, when someone called to his attention that Hans had indicated the wrong day of the week, replied: "Yes, you are right, it was not Thursday, but Friday," whereupon Hans being asked again, promptly responded correctly. This appeared to the reporter in question as proof of the subjective influence of Mr. von Osten upon the horse.

(c.) When errors in calculation and failures in proper concentration combine, i. e. when the questioner makes a mistake in calculation because he is excited or inattentive and for the same reason does not make the movement, which is the signal for stopping, in accordance with the number which he deems to be the correct answer, then the result is usually wrong, but it may be correct in the few cases in which the two errors exactly compensate each other. Nothing has been so effective in establishing Hans's reputation, nothing has brought him so many followers, as these cases in which he, rather than his mentor, has been in the right. Compared with the mass of cases in which Hans was wrong these latter cases are diminishingly few in number, yet these few made such an impression upon the observers that their number tended to be overestimated. As a matter of fact, I have been able to discover records of only seven such cases. Two of these were reported by the Count zu Castell. On the 8th of September, he entered the horse's stall, alone, and believing it to be the seventh day of the month, he asked Hans the date. The horse responded correctly with 8 taps. At another time he held up before Hans a slate on which were written the numbers 5, 8 and 3 and asked the horse to indicate their sum which in the momentary excitement, vaguely appeared to Castell to be 10. To his chagrin he noticed that Hans continued to tap. Thereupon he intentionally remained motionless until the horse had stopped tapping spontaneously—as he thought—at 16. (The newspapers reported that the numbers to be added had been 5, 3, and 2; that the questioner had expected the answer 11, but that Hans had in three tests always ceased tapping at 10.) In both cases the questioner regarded the answers of the horse as wrong and recognized his mistake when his attention was called to it. I, myself, had the same experience. One time I received in response to the question, "What day of the week is Monday?", the answer 2, although I had expected the answer 1; at another time I asked, "How much is 16 less 9?", and the horse responded with 7 taps, although I had erroneously expected 5. I noticed my mistake only when my attention was called to it by one of those present. Another example is related by Mr. Schillings. A row of colored cloths lay before Hans. Beside them stood an army officer. Pointing to the latter's red coat Mr. Schillings asked the horse to indicate, by means of tapping, the place in the row where a piece of the same color lay. Hans tapped eight times, but Mr. Schillings reprimanded him because the red piece was, as a matter of fact, second in the row. Upon a repetition of the test, Hans again tapped 8. (By some, the facts are recounted as having been the other way round; viz.: Hans tapped 2 instead of 8. This of course would call for a different explanation.) It was noticed that at the place which would be indicated by eight taps there was not a red piece but a carmine colored piece of cloth. A newspaper reports, somewhat vaguely, a sixth case as follows: Hans was asked to spell the name "Dönhoff" and began correctly: "Dö". Mr. von Osten, who somehow began to think of another name, "Dohna", interrupted him and wished to correct him by suggesting o instead of ö (i. e., 2 taps instead of 3). Hans, however,

continued to spell the entire word with the greatest equanimity. He had not erred. A similar experience is reported by Mr. H. von Tepper-Laski, the well known hippologist. Although the details have slipped from his memory, he reports that in the case in question the correct answer was thrice refused by the questioner who thought that the horse's answer was incorrect. Hans, upon being severely reprimanded in a loud and harsh tone of voice, turned about as if disgusted with the injustice of the man and made straight for his stall.—It is clear that in the cases described we are not dealing with accidentally correct responses, for in nearly every case the test was repeated a number of times and the same responses were received each time. As a matter of fact, my own introspection convinced me that the third and fourth cases were surely, and the first and sixth were very probably, due to insufficient concentration on the part of the questioner. Accordingly there is everywhere in these cases a difference of +1 or +2 between the number thought of and the number tapped (see [page 92 f.]). The data in the second and fifth and still more in the seventh case were too meager to warrant an attempt at explanation, for it is not even known whether Hans responded with more or fewer taps than was expected by the questioner. It is unfortunate that a more complete record was not made.

The frequent and intentional attempts of Mr. von Osten to induce the horse to give an incorrect response,—which, by-the-way, were regularly unsuccessful—belong only apparently to this group. Thus he asked, e. g., "2 times 2 is 5, is it not?" "3 times 3 is 8?", etc., but Hans refused to be misled, and responded correctly. This was from the very beginning one of the main arguments for independent thinking on the part of the horse. The actual procedure was as follows, even though the questioner had said "2 times 2 is 5", there still was present in his consciousness the number 4. I, myself, would think either of the first member of the equation, i. e., 2 times 2, in which case Hans would respond with 4 taps or I would have in mind the second member, i. e., 5, in which case he would respond with 5 taps. Never did I succeed in thinking of both at the same time. The association between the thought "2 times 2" and the concept "4" is so close and supported by so many other associations that the attempt to form a new one, that is at complete variance with all these, is futile. One may say "2 times 2 equals 5" but it is impossible to conceive it.