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Compendium January 2012 (Vol 34, No 1)

Guest Editorial: The “Clever Hans Effect”

by James R. Coffman, DVM, DACVIM

    Most people do not realize how acutely perceptive horses can be. For veterinarians, understanding equine perception can facilitate optimal application of equine medicine. The following story of a horse named Clever Hans can help make all of us who work with horses aware of the nonverbal messages we send.

    At the turn of the 20th century, a German mathematics teacher named Wilhelm von Osten owned a horse named Hans. As a teacher, phrenologist, and closet mystic, Von Osten believed that animals could think. Whether animals could think was a major issue of the day. Darwin’s revelations were still reverberating in public discourse. The central question was whether animals, having reputedly given rise to humans through evolution, were conscious and could think in the abstract like their human derivatives. Enter Hans.

    Von Osten began teaching Hans mathematics, color and shape identification, and various subjects in general education. Von Osten’s methodology was described, along with a comprehensive account of Hans’ story, in a book by Oskar Pfungst,a who solved the mystery surrounding Hans’ amazing abilities and characterized the “Clever Hans effect,” which becomes the point of the story.

    In short, Von Osten taught Hans to tap his hoof to indicate the correct answer to a question (e.g., “What is the square root of 16?”) and to use his muzzle to indicate which kerchief was red, green, or yellow. Correct answers were recognized with a food reward. After 4 years of education, Hans was performing at a phenomenal level, so Von Osten began to exhibit him widely. Hans’ fame spread on both sides of the Atlantic. Von Osten clearly believed that Hans’ abilities and the demonstrations of them were genuine. This was supported by the fact that Von Osten never charged a fee for demonstrations.

    Thousands of people observed Von Osten and Hans. While skepticism was widespread and people speculated at length that Von Osten must be deluded or running a scam, no one was able to find the slightest evidence of such. In September 1904, the German Board of Education appointed a commission to investigate the Hans phenomenon. After a year and a half of scrutiny, this investigative body announced that it was unable to find any evidence that Hans’ apparent abilities were not genuine.

    Enter Professor Oskar Pfungst, a comparative biologist and psychologist, and his colleagues, who performed well-planned experiments on Hans. Their findings still reverberate today. Pfungst and colleagues tried changing questioners, but Hans could still deliver the right answers. Then they tried asking Hans questions to which the interrogator did not know the answers, and Hans’ proficiency plummeted. When questioners were positioned behind a screen, Hans failed again. Pfungst determined that Hans could answer almost all questions correctly regardless of who the questioner was as long as the questioner knew the answer and was in Hans’ line of sight.

    At this juncture, some wondered whether Hans was psychic like several supposedly psychic horses of the period, including Lady Wonder and Beautiful Jim Key, which seemed to read the minds of questioners. However, these horses could not “read minds” unless the questioner was visible to them.

    These findings led Pfungst to begin observing the questioners as closely as he did Hans. Pfungst soon realized that, in every instance, the questioner had one or more extremely slight changes in posture or facial expression when Hans approached the right answer. These signs of expectation were replaced with traces of relaxation when the correct answer arrived. These nuances of body language were so slight they had been missed by a very large number of observers, including the German Board of Education and other scientists. Von Osten had conveyed the same kind of signals, accompanied by a food reward, all through Hans’ education. Von Osten had never been aware of doing this, and the thousands who observed him demonstrate Hans’ abilities had never seen the signals.

    Pfungst also became proficient at obtaining correct answers from Hans. Despite realizing that he was conveying barely detectable facial or postural signals in association with correct answers, Pfungst was surprised to find that he was powerless to suppress the signals he was sending.

    Although Hans was not capable of learning mathematics and he, Lady Wonder, and Beautiful Jim Key were not psychic, these horses were extraordinarily astute at reading and responding to the subtlest changes in human expression, gesture, and posture.

    Pfungst’s observations of Hans exemplified observer expectancy and ideomotor movement. In observer expectancy, the observer conveys detectable signs of expectancy of a specific result through nuanced body language, and the subject of observation responds by modifying its performance. For example, by tilting his head or arching an eyebrow, Von Osten conveyed expectance that Hans would tap his foot until Von Osten changed his irrepressible facial or postural signal. Ideomotor movement is without volition, and the individual performing it is unaware of it (e.g., the irrepressible signals that Von Osten conveyed). The combination of these phenomena is now known as the Clever Hans effect and must be accounted for in the design of experiments on humans and animals in which perception and performance play roles. Although Hans did not learn mathematics or color identification, his level of awareness, or consciousness, was far beyond what his observers realized.

    In his book Blink: The Power of Thinking Without Thinking,b Malcolm Gladwell discusses unconscious utilization of thin slices of information and rapid decision making (i.e., within a few seconds). One example is “reading” people’s minds through their facial expressions, which is exactly what Clever Hans, Lady Wonder, and Beautiful Jim Key did.

    The Clever Hans effect—which affects all equine practitioners every day regardless of whether they realize it—and Gladwell’s book are two “dots” that should be connected by everyone interested in increasing his or her awareness of the subtlety of the human-horse interface. Both sides of this communicative interface, in its most nuanced form, are subconscious.

    aPfungst O. The Horse of Mr. von Osten. New York, NY: Holt, Rinehart and Winston; 1911, 1965.
    bGladwell M. Blink: The Power of Thinking Without Thinking. New York, NY: Little, Brown and Company; 2005.

    NEXT: Interpreting Lactate Measurement in Critically Ill Horses: Diagnosis, Treatment, and Prognosis

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