riments showed that Hans responded with a
nod of the head whenever the questioner, while bending forward, chanced
to stand in front of, or to the side of the horse's head, but that he
would begin to tap in response to the same signal, as soon as the
experimenter stood farther back. The difference in the two signals,
therefore, was very slight, and I repeatedly noted that instead of
tapping, as he had been requested, Hans would respond to the Count zu
Castell's and Mr. Schillings' questions by a nod of the head.

If, while standing in the customary position to the right of and facing
the horse, the questioner would turn his head a little to the right--a
movement which, when seen from the horse's position, would appear to be
to the left,--Hans would turn his head to his left. But if on the other
hand the questioner would turn slightly to the left,--i. e. seen from
the horse's position, to the right,--then Hans would turn his head to
his right. And finally, whenever the questioner turned his head first to
the right, then to the left, Hans would respond by turning first to his
left, then to his right. This, according to Mr. von Osten, signified
"zero" or "no". Since this movement could not be executed by the
experimenter while in a stooping position, it can now readily be seen
why it was that Hans, instead of shaking his head, always began to tap
whenever a placard with "O" upon it, was shown to him in the course of
the experiments in which the method was procedure without knowledge on
the part of the questioner. The latter expected the horse to tap, and
therefore bent forward. Like all of the horse's other forms of response,
this, too, was always unsuccessful whenever the questioner stepped
behind the animal. Although Hans had always responded to Mr. von Osten
and Mr. Schillings, and at first also to me, by means of the stereotyped
movement of the head to the right and then to the left to signify "zero"
or "no", I later succeeded in controlling my signals so as to get the
inverted order in the horse's response. In the case of Mr. Schillings
and of Mr. von Osten all of the movements just described were very
minute, and long after the movements, which were effective stimuli for
releasing the process of tapping, were recognized, it was still
exceedingly difficult to discover them in these two gentlemen. The
signal for "zero" and "no" was relatively the most pronounced of the
group in the case of Mr. von Osten, while with Mr. S