How a 'psychic' horse named Clever Hans changed the way we do science

In the early 20th century, the idea of psychic abilities, such as telepathy, became a subject of fascination for many intellectuals. J.J. Thomson, a renowned physicist, conducted experiments to test mediums, while philosopher C.D. Broad explored the concept of precognition. However, some researchers began to investigate psychic abilities in animals, particularly horses.

Friedrich Canning Scott Schiller, a philosopher at the University of Oxford and a member of the Society for Psychical Research, even wrote about "equine mathematicians" - horses that supposedly demonstrated paranormal mathematical abilities by tapping out solutions to equations. One such horse, Clever Hans, became famous for his mathematical prowess, attracting crowds eager to witness his abilities.

Psychical researcher Carl Stumpf, who was also a friend of Hans' owner, attempted to investigate the horse's abilities but could not find any evidence of supernatural powers. Instead, Stumpf's assistant, Oskar Pfungst, discovered the so-called Clever Hans effect - a phenomenon where the horse's success was due to the experimenter unintentionally providing cues to Hans, rather than the horse possessing any actual mathematical abilities.

This discovery highlighted the importance of experimental design and the need to eliminate potential sources of bias in research. Despite the lack of evidence for these horses' supernatural abilities, the studies conducted by psychical researchers contributed to the development of new experimental techniques, such as randomization, which are now standard in good experimental design.