he 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