Clever Hans the math horse...

Clever Hans, or Der Kluge Hans, was one bright German horse. Or was he? He could add, subtract, multiply, divide, work with fractions, tell time, keep track of the calendar, differentiate musical tones, and read, spell, and understand German. His owner, Wilhelm Von Osten, would ask Hans, "If the eighth day of the month comes on a Tuesday, what is the date of the following Friday?” Hans would answer by tapping his foot. Questions could be asked both orally, and in written form.

It was later discovered that Hans was picking up subtle cues from his questioners. After formal investigation in 1907, psychologist Oskar Pfungst demonstrated that the horse was not actually performing these mental tasks, but was watching the reaction of his human observers. Pfungst discovered the horse was responding directly to involuntary cues in the body language of the human trainer, who had the faculties to solve each problem. The trainer was entirely unaware that he was providing such cues.

In honor of Pfungst's study, the discovery been referred to as the Clever Hans effect and has continued to be important knowledge in animal cognition.