Postscript: Testing a hypothesis In 1891 a man named William Von Osten began showing his horse in public exhibitions. Clever Hans could apparently perform mathematical calculations and would give the answers by tapping his hoof. For example, if he was asked to give the sum of 3 plus 2 he would tap his hoof ve times. More than a dozen scientists observed Hans and were convinced there was no signaling or trickery. They were impressed that Hans performed almost as well without Von Osten as with him. Admirers of Clever Hans' explained his behavior saying that Hans was capable of understanding human speech and performing mathematical calculations. Countless experiments supported this conclusion! However, in 1904 by Oskar Pfungst proposed a different explanation. He suggested that the horse was responding to subtle physical cues in the posture of the questioner. Oskar Pfungst hypothesized that Hans could perceive very subtle postural cues from his listeners that would tell him when to start and stop tapping. He tested his hypothesis in the following experiment. He asked Hans a question to which nobody in the audience knew the answer. If Oskar's hypothesis was correct, then Hans should not be able to see the cues and consequently would not respond correctly. This prediction was supported. Pfungst also tested what happened when the horse couldn't see the person who did know the answer. In this case also, the horse didn't respond correctly. Clever Hans was indeed clever not because he could count and understand English but because he was extraordinarily sensitive to the expectations of people around him. Von Osten and Pfungst had different explanations of Hans' behavior. What did they do differently to test their ideas of how Hans could count