A Clever Horse
Popular media are replete with stories of intelligent horses, real and fictional. Flicka, Mr. Ed, Silver, Trigger, Secretariat, and most recently War Horse all stimulate the imagination. Less well known was a horse who lived in Germany in the early1900s who was alleged to be able to do mathematics and who taught us something about scientific inference. The horse was known as "Clever Hans" and earned a place in the history of scientific research.
In 1900, Wilhelm von Osten, a retired German schoolmaster purchased an arab stallion from Russia. Von Osten was convinced that animals had intelligence equal to that of man. He devoted much of his time trying to prove this point with a variety of animals but only a horse fulfilled his expectations. He claimed that Clever Hans was able to answer questions, tell time, solve math problems, and give the names of people he knew by tapping his hoof. The number of taps was used to express numerical answers or to spell words, with one tap signifying "A," two taps "B," and so on. Clever Hans was observed to work out complex calculations, even square roots. A number of scientific investiigations of the horse's prowess revealed no signs of trickery. Some people believed the horse was psychic.
The hoirse achieved worldwide acclaim, yet some scientists remained skeptical. A psychologist from Berlin, named Pfungst, had an inspiration. He and his colleagues decided to test Clever Hans when no one who knew the answer to the problem was present. After a psychologist wrote the problem on the blackboard, everyone left the room. Only the horse could see what was written. Clever Hans turned out to be not so clever. He failed every problem presented to him in that manner. Pfungst concluded that the horse was detecting very subtle movements of the observers, such as a minute nod of the head, when the correct number of hoof taps had been reached. It had since been learned that some horses are even able to detect changes in heartbeat or tension in humans. That animals can accurately respond to non-verbal cues is, itself, amazing.
A great logician, William of Ockham (1285-1349) framed what is now known as Occam's (or Ockham's) law or the law of parsimony. This is a principle (not really a scientific law) urging one to select among competing hypotheses the choice that makes the fewest assumptions and therefore offers the simplest explanation of an effect. To assume that a horse is able to solve mathematical problems or is psychic is far from the simplest explanation. It is far more reasonable to assume that horses as well as humans may be sensitive to non-verbal cues.