Here is a horse who lives with his friends in a neighborhood. I would call him a clever horse Hans. Why?
He cannot do arithmetic or talk in a human voice, but his clever gaze or his manner to come to greet me makes his actions in a way very humane. At these moments I talk to him and a strange feeling comes that he can understand me! So let be his name Hans!
In a history of science, a horse named Hans is known for a remarkable capabilities including arithmetic or language skills. Well, the story begins in 1904, in Germany. A school teacher Wilhelm von Osten had discovered that his horse owned is capable to do arithmetic. The clever Hans could subtract, divide and count. Moreover, it means, that Hans should understand German language to fulfill these requests. If you would ask him a question, he would answer in tapping with his leg a code. Consequently, there were a crowd of people who payed money to see this horse, and of course, there were some skeptics who couldn’t believe what they see. Skeptics who observed these performances, noticed that the owner was not giving any clues for Hans. Indeed, there was need for the owner to be near the horse, because he perfectly responded to commands from different people.
But one day the rigorous scientist Oskar Pfungst came to the scene. He took the horse to his lab so the experiments could be carried without any distraction. He tried everything from different questions to different questioners. The most important control, however, was this – none of observers knew the correct answer. How it was done? One of observers would whisper to the Hans a number, for example, „three“ in a way that the other couldn't hear it. Another participant of experiment would whisper „plus two“. That is none of them knew the correct answer. Unfortunately, Hans didn't know the answer either. Asked a question, Hans would go tapping and tapping, indefinitely.
So what is the moral? According to Pfungst, what had been happening was a case of unconscious cuing. As long as the observers knew the correct answer, they would wait until Hans had tapped it, and then lean forward, stop moving their head, or make other un-reflexive movements which would be a sign to Hans that its time to stop. These finding Pfungst confirmed by observing more cues of people who were observing the show of the clever horse, and the results were that over 90 percent of participants provided cues for Hans to stop tapping. Since these findings the term "Clever Hans effect" has been a standard phrase to remind researchers to be more accurate with their study subjects. On the other hand, there is no doubt that the horse Hans who lived in Germany was a very clever animal and a profound observer! That’s what my neighbors eyes reminds me when he with his friends comes to stare at me passing by.
For further readings, the clever Hans effect was discussed widely in a various scientific literature. O. Pfungst has discussed the case of Hans here:
Pfungst , O. (1911) Clever Hans. A Contribution to experimental animal and human psychology. New York: Henry Holt.