..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 only answered 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 probably depend on the detection of small postural changes, and this may be why Hans so easily picked up on the cues given by von Osten (who seemed to have been entirely unaware that he was providing such cues). However, the capacity to detect such cues is not confined to horses. Pfungst proceeded to test the hypothesis that such cues would be discernible, by carrying out laboratory tests in which he played the part of the horse, and human participants sent him questions to which he gave numerical answers by tapping. He found that 90% of participants gave sufficient cues for him to get a correct answer.Clever Hans - Wikipedia, the free encyclopedia
""CLEVER HANS" AGAIN.; Expert Commission Decides That the Horse Actually Reasons."
. The New York Times. 1904-10-02