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From the Clever Hans wiki article:

Clever Hans (in German, der Kluge Hans) was a horse that was claimed to have been able to perform arithmetic and other intellectual tasks.

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 this artifact in the research methodology, wherein 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.^[1]

In honour of Pfungst’s study, the anomalous artifact has since been referred to as the Clever Hans effect and has continued to be important knowledge in the observer-expectancy effect and later studies in animal cognition.

Hans was a horse owned by a Herr Wilhelm von Osten, who was a gymnasium math teacher, an amateur horse trainer and phrenologist, and something of a mystic.^[1] Hans was taught to add, subtract, multiply, divide, work with fractions, tell time, keep track of the calendar, differentiate musical tones, and read, spell, and understand German. 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. Von Osten exhibited Hans throughout Germany, and never charged admission. Hans’s abilities appeared on page six of the New York Times.^[2]

Both von Osten and Hans were notoriously bad tempered and prone to rage when the horse did not perform well. Pfungst suffered more than one horse bite during his investigation.^[2]

Investigation

Due to the large amount of public interest, the German board of education appointed a commission to investigate von Osten’s scientific claims. Philosopher and psychologist Carl Stumpf formed a panel of 13 people, known as the Hans Commission. This commission consisted of a veterinarian, a circus manager, a Cavalry officer, a number of school teachers, and the director of the Berlin zoological gardens. This commission concluded in September 1904 that no tricks were involved in Hans’ performance.^[3]

The commission passed off the evaluation to Pfungst, who tested the basis for these claimed abilities by:

1. Isolating horse and questioner from spectators, so no cues could come from them
2. Using questioners other than the horse’s master
3. By means of blinders, varying whether the horse could see the questioner
4. Varying whether the questioner knew the answer to the question in advance.

Using a substantial number of trials, Pfungst found that the horse could get the correct answer even if von Osten himself did not ask the questions, ruling out the possibility of fraud. However, the horse got the right answer only when the questioner knew what the answer was, and the horse could see the questioner.

He observed that when von Osten knew the answers to the questions, Hans got 89 percent of the answers correct, but when von Osten did not know the answers to the questions, Hans only answered six percent of the questions correctly.

Pfungst then proceeded to examine the behaviour of the questioner in detail, and showed that as the horse’s taps approached the right answer, the questioner’s posture and facial expression changed in ways that were consistent with an increase in tension, which was released when the horse made the final, correct tap. This provided a cue that the horse could use to tell it to stop tapping.

Even after this official debunking, von Osten, who was never persuaded by Pfungst’s findings, continued to show Hans around Germany, attracting large and enthusiastic crowds.^[4]

The Clever Hans effect

Pfungst made an extremely significant observation. After he had become adept at giving Hans performances himself, and fully aware of the subtle cues which made them possible, he discovered that he would produce these cues involuntarily regardless of whether he wished to exhibit or suppress them. This gives the phenomenon an importance which could hardly be exaggerated. Its recognition has had a large effect on experimental design and methodology for all experiments whatsoever involving sentient subjects (including humans).

Popular culture

Military officer Günther von Kluge was nicknamed Der Kluge Hans (Clever Hans) in admiration of his brilliance. Von Kluge went on to become one of the most able Field-Marshals of the Third Reich.