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Oskar Pfungst book is a detailed piece of investigative journalism looking into Clever Hans, an Orlov Trotter horse that was claimed to have performed arithmetic and other intellectual tasks. Pfungst details the results of many experiments, by the end of which he is able to explain exactly how Hans did it. This classic example of the observer-expectancy effect is still relevant today, as the "Clever Hans effect" has to be taken into account whenever a study of animal intelligence takes place.