The Clever Hans Effect

In Berlin in a courtyard surrounded by high apartment buildings in the summer of 1904, there is a man and his horse.  The man is Herr von Osten who taught mathematics to grade school children.   His horse is Hans and the two are obviously very close and von Osten is very gentle with his horse.  Herr von Osten asks Hans questions, usually mathematical ones, and the horse answers by tapping out his right foot on the ground.  A shake of the head is zero.  When the answer is large, Hans taps quickly, when the answer is small he taps slowly, as though he knew the answer in advance. Von Osten gave Hans vegetables as rewards during the sessions.  Hans, the clever horse, was a popular topic of the day for Berliners and this resulted in a commission being formed to determine whether Hans’ behavior was due to some trickery or real intelligence.  A commission of respected persons was organized to investigate the story including Oskar Heinroth (a respected researcher and teacher of K. Lorenz).  All the members of the September Commission were enormously impressed with Hans’ abilities.  They tested Hans by having someone other than von Osten ask questions.  Hans was able to answer questions given by unfamiliar persons and the Commission concluded that there was no trickery and that no signs or cues were being given.  A student of one of the members of the Commission followed up with a study of Hans’ abilities.  He showed that Hans performed best only with certain persons present. He also concluded that whenever the questioner knew the solution nearly all of the horse’s answers were correct, but when the answer was unknown to the questioner, the horse’s responses were not correct.   He also showed that Hans performed much better when he could see the questioner.  He ultimately concluded that minimal movements of the head on the part of the questioner were providing the horse with cues.  After asking the question, the questioner, without knowing it, and involuntarily, bent his head and trunk slightly.  As soon as the desired number of taps was given, the questioner would make a slight upward jerk of the head. After Hans had ceased tapping, the questioner would raise his head and trunk to their normal position.

The lessons of Clever Hans are important for the study of behavior.  The learning abilities of animals are impressive, although they may not always be learning what you think they are learning!