Clever Hans (The Horse of Mr. Von Osten) A contribution to experimental animal and human psychology

Oskar Pfungst was a German comparative biologist and psychologist. While working as a volunteer assistant in the laboratory of Carl Stumpf in Berlin, Pfungst was asked to investigate the horse known as Clever Hans, who could apparently solve a wide array of arithmetic problems set to it by its owner. After formal investigation in 1907, Pfungst demonstrated that the horse was not actually performing intellectual tasks, but was watching the reaction of his human observers. Pfungst discovered this artifact in the research methodology, wherein the horse was responding directly to involuntary clues in the body language of the human trainer, who had the faculties to solve each problem. The trainer was entirely unaware that he was providing such clues.