The Persistent Problem of Tricky Test Animals: The Clever Hans Effect

"Clever Hans" was a horse that toured Germany around the turn of the 20th Century allegedly capable of doing arithmetic (by tapping a hoof a number of times corresponding to the correct answer to a math problem). Though the spectacle pleased thousands of German onlookers, it was later proven by psychologist Oskar Pfungst that the horse didn't know arithmetic at all but was instead able to read the cues of the onlookers as their excitement appreciably mounted as the horse neared the correct answer. However, after thoroughly studying Hans well enough to direct performances himself, Pfungst discovered something frightful: he himself couldn't contain his excitement enough to keep from tipping off the horse - for what could be more delightful than a counting horse?!?. Thus, the resulting "Clever Hans Effect" has less to do with clever animals than stupid researchers. The effect would go on to manifest itself with studies of the irresistible charms of Washoe the chimp and Rico the Border Collie.