In 1904, a retired mathematics teacher named Mr. von Osten had a horse named Clever Hans.
One fine morning, von Osten decided to teach his horse to add. "What is three plus four?" he would call to his pupil, and Clever Hans would paw the ground seven times, no more, no less. Was Hans indeed Clever, or was von Osten some kind of clever con artist?
A Con? Or A Very Smart Horse?
The Case of the Clever Horse was investigated by psychologist Oskar Pfungst. He found that if Hans could see von Osten while doing his sums, the horse always got the right answer, but if von Osten was out of sight, three plus four might equal five, or eight, or twelve.
Von Osten swore, however, that he wasn't playing some kind of trick. What was going on?
As it turned out, von Osten was giving Hans subtle visual cues without even realizing it. As he asked a question, he leaned forward ever so slightly, starting the horse a tapping.
When the answer was reached, von Osten stood somewhat taller, naturally proud of his pupil. Hans relied on these unintentional cues, not his brains, to produce the right answer.
Clever Hans illustrates an important concept in psychology, the concept of "double blind" testing procedures. This means that the person asking questions in a psychological test shouldn't even know what the test is about, or what the "right" answers are.
Examiners in the know can bias a test to give the results they want, not only for clever horses, but for humans as well. An inadvertent smile or frown can easily persuade someone to answer in a certain way.