Clever Hans

A particularly fertile ground for scientific self-deception lies in the field of animal-to-man communication. Time and again, the researcher's expectation has been projected onto the animal and reflected back to the researcher without his recognizing the source. The most famous case of this sort is that of Clever Hans, a remarkable horse that could apparently add and substract and even solve problems that were presented to it. He has acquired immortality because his equine spirit returns from time to time to haunt the laboratories of experimental psychologists, announcing its presence with ghostly laughter that its victims are almost always the last to hear. Hans's trainer, a retired German schoolteacher named Wilhelm Von Osten, sincerely believed that he had taught Hans the ability to count. The horse would tap out numbers with his hoof, stopping when he had reached the right answer. He would count not just for his master but for others as well. The phenomenon was investigated by a psychologist, Oskar Pfungst, who discovered that Von Osten and others were unconsciously cuing the equine prodigy. As the horse reached the number of hoof taps corresponding to the correct answer, Von Osten would involuntarily jerk his head. Perceiving this unconscious cue, Hans would stop tapping. Pfungst found that the horse could detect head movements as slight as one-fifth of a millimeter. Pfungst himself played the part of the horse and found that twenty-three out of twenty-five questioners unwittingly cued him when to stop tapping.  Pfungst's celebrated investigation of the Clever Hans phenomenon was published in English in 1911, but his definitive account did not prevent others from falling into the same trap as Von Osten. Man's age-old desire to communicate with other species could not so easily be suppressed.

from the book Betrayers of the Truth Fraud and Deceit in the Halls of Science by William Broad, Nicholas Wade