The “Clever Hans Phenomenon” revisited

In the first decade of the 20th century, a horse named Hans drew worldwide attention in Berlin as the first and most famous “speaking” and thinking animal. Hans solved calculations by tapping numbers or letters with his hoof in order to answer questions. Later on, it turned out that the horse was able to give the correct answer by reading the microscopic signals in the face of the questioning person. This observation caused a revolution and as a consequence, experimenters avoided strictly any face-to-face contact in studies about cognitive abilities of animals—a fundamental lesson that is still not applied rigorously.