Clever Hans Effect

The Clever Hans effect describes how a person, animal or system under study can appear to demonstrate an ability, such as arithmetic or comprehension, that is actually produced by unconsciously picking up on subtle cues given off by an observer who already knows the correct answer. The effect is named for Clever Hans, a horse exhibited in early twentieth century Germany that appeared to perform arithmetic by tapping out answers with a hoof. The psychologist Oskar Pfungst investigated the horse and found that Hans answered correctly around eighty-nine percent of the time when he could see a questioner who knew the answer, but only about six percent of the time when he could not. Pfungst determined that as the horse's taps approached the correct number, the questioner's posture and facial expression tended to change in ways consistent with rising tension, which visibly released once the horse produced the correct final tap, giving Hans an unintentional signal to stop. The term Clever Hans effect is now used to describe how a researcher can unintentionally bias the outcome of an experiment through subtle, involuntary cues given to a human or animal subject, and it underlies the case for blinding procedures and minimizing direct experimenter-subject interaction in rigorous experimental design across psychology and other fields.