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Clever Hans: cueing and the observer effect

Wilhelm Von Osten, a German high school mathematics instructor, thought that people dramatically underestimated the intelligence and reasoning skills of animals.

A man of science, he set out to test his hypothesis by teaching mathematics to a horse called Hans. Hans quickly showed real aptitude; soon he could read a number written on a board and tap his hoof with the correct number of times.

Von Osten moved from reading and counting to basic arithmetic and again Hans mastered the new tasks, learning how to correctly answer a variety of math problems including basic square roots and fractions. From 1891, Von Osten toured Germany, showing Hans’ skill before large crowds.

Now able to put questions to Hans orally, Van Osten would ask questions such as “If the first day of the month is a Wednesday, what is the date of the following Monday?” Hans would tap his foot six times. “What is the square root of sixteen?” Four taps.

Von Osten also demonstrated that Hans could answer simple questions in German, having learned to tap once for A, twice for B, and so on, and that he could also tap out the time. Though he made occasional mistakes, the horse was right almost 90% of the time: by some accounts, he had the mathematical skills of an average fourteen-year-old.

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When doubts were raised, Von Osten happily agreed to allow Germany’s board of education to conduct an independent investigation. In 1904, after extensive testing, the Hans Commission – which included two zoologists, a psychologist, a horse trainer, several schoolteachers and a circus manager – concluded that no trickery was involved. So far as they could tell, the horse’s talents were genuine.

The Commission then passed the investigation to a young psychologist, Oskar Pfungst. Pfungst designed a careful set of experiments and began testing Hans.

He soon noticed that Hans performed well when questioned under his normal conditions. But the horse’s accuracy dropped when the questioner stood farther away than normal. And if the questioner didn’t know the answer to a question or was concealed from him, Hans’ accuracy plummeted to nearly zero. Pfungst hypothesized that Hans’ cleverness relied on him having a close, unobstructed view of a person who knew the correct answer.

With that in mind, Pfungst began watching the questioners, and he noticed that as Hans tapped his hoof in response to a question, their breathing, posture, and expression showed subtle signs of increasing tension, tension which disappeared when Hans made the correct tap. Innocently and without realizing they were doing so, Pfungst concluded, the questioners were giving Hans a cue when to stop tapping.

Discovery of the unconscious cueing

Pfungst had discovered ‘unconscious cueing’, the influence of researchers’ subtle and unintentional signals on their subjects, and it is now recognized as widespread in research involving human subjects as well as animals.

Unconscious cues introduce a form of bias into experiments, leading subjects to give answers that seem right to the researchers. Blinding and double blinding trials are one response. In a blind drug trial of a new drug, for instance, one group of participants receives the drug while a second group receives a placebo.

Participants do not know which group they are in. Still, the doctor administering the drug may give some subtle, unconscious, cue about which drug the patient is getting: perhaps they check the reactions of participants in the active group just a little more carefully, or are more ready to attribute those of participants in the inactive group to something other than the drug.

Double-blind experiments – in which neither researcher nor participant – knows which group a participant is in - also keep the researcher in the dark, preventing them sending unconscious cues.

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Logical and Critical Thinking

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