Clever Hans was an arab stallion from Russia. In 1900 he was purchased by Wilhelm von Osten, a retired schoolmaster. Von Osten was convinced that animals possessed an intelligence equal to that of man and spent much of his time trying to prove his point. He had started by trying to teach a cat, a horse and a rather troublesome bear how to do simple arithmetic. Neither the cat nor the bear showed the slightest inclination to fill their heads with math, but the horse, Hans, was of a different calibre altogether.
Von Osten taught the horse to recognize the numbers one to nine, first with the help of skittles. Hans would tap out the correct answer with his hoof. Von Osten then replaced the skittles with numbers on a chalkboard. A sum was drawn up on the chalkboard and Clever Hans tapped out the answer. In a short time Clever Hans could work out reasonably complex calculations including some square root. Scientific observers were astounded and they could see no signs of trickery. After some tests by eminent mathematicians, the horse was considered to have the math ability of a 14 year old schoolboy.
Clever Hans and his owner enjoyed world wide acclaim, but the scientific community remained sceptical and a group of leading professors was asked to subject the horse to five weeks of intense scrutiny, on every test Hans got his sum right. Then one of the scientists, a psychologist from Berlin named Pfungst, had an inspiration. Could Clever Hans get the right answer if the horse alone could see the numbers?
While one of the team wrote down the numbers and left the room, everyone else including the psychologist moved behind the blackboard. Of all of those left in the room, only the horse knew what was inscribed on the blackboard. Clever Hans failed every subsequent test. What the horse had been doing, proposed Pfungst, was to take questions either from his trainer or from other people in the room. Hans was able to spot very subtle movements such as a minute nod of the head and to detect the tension experienced by everyone in the room as he approached the correct answer.
Scientists since have discovered that horses (and dogs) can detect the heartbeat of someone standing nearby. As Clever Hans drew close to correct answer an increase in everyone’s heartbeat indicated the place for him to stop tapping his hoof.