Oskar Pfungst

Born in 1874, Oskar Pfungst was a German comparative biologist and psychologist. He is most closely associated with the investigation of Clever Hans, the horse that seemed able to solve arithmetic problems and answer questions.

Working in Berlin as a volunteer assistant in the laboratory of philosopher and psychologist Carl Stumpf, Pfungst tested Hans under controlled conditions. He showed that the horse was not actually doing mathematics, but was responding to tiny, unintentional signals from the questioner. That finding became one of the classic lessons in careful experimental design.

Pfungst later wrote about the case in Clever Hans: The Horse of Mr. von Osten, a study that remained influential well beyond animal psychology because it highlighted how expectation and observer effects can shape results. He died in 1933.