The Clever Hans effect was identified in 1907 by psychologist Oskar Pfungst. He demonstrated that Clever Hans, a horse that was claimed to be able to perform arithmetic and other intellectual tasks, was actually observing the reaction of his human observers.
He discovered the effect during research into the validity of the phenomenon, and found that the horse was responding directly to involuntary cues in the body language of the human trainer, who knew the answer to each problem. However, the trainer was entirely unaware that he was providing such cues.
During the research, Pfungst discovered that he would produce the cues involuntarily, regardless of whether he wished to exhibit or suppress them. Recognition of this phenomenon has had a large effect on all experiments involving sentient subjects.
Clever Hans effects are quite as likely to occur in experiments with humans as with animals. For this reason, care is often taken to make experiments double-blind, meaning that neither the experimenter nor the subject knows what condition the subject is in, and thus what his or her responses are predicted to be.