The Amazing Counting Horse

Starting in 1891, William von Osten gave public demonstrations of his amazing horse.  This horse, Clever Hans, could answer mathematical questions correctly every time.  If you asked it the square root of 16, it would stomp its foot 4 times.  It could spell out the answers to questions by stomping the proper number for each letter it needed to spell (1 = A, 2 = B, etc.).  This continued for many years, stupefying audiences all over Germany.

A panel decided to test the horse, and they had other questioners ask the horse mathematical problems, and once more, even without the owner or any audience in the room, Clever Hans got the right answers.

But eventually one skeptic discovered something interesting.  If the questioner stood far away from the horse, the percentage of correct answers fell.  And then he discovered that if the questioner did not know the correct answer, the horse was never able to answer correctly either.

The solution to the puzzle was that Clever Hans was picking up subtle visual clues from the questioner.  So for the square root of 16, the horse would begin counting, and when it reached 4, the questioner would give some indication - maybe a tiny tilt of the head - that this was the right answer.  But it did not matter whether or not the questioner INTENDED to tip off the horse!  Each human questioner had a tell that gave Clever Hans all he needed to know.

It makes you wonder what else animals are able to pick up from our body language..