In 1907, in Berlin, Oskar Pfungst, an experimental psychologist, carried out an admirably skillful investigation of the seemingly remarkable intellectual feats of a horse dubbed "Clever Hans" by his flabbergasted onlookers.1 In doing so, he made a very important, lasting contribution to the methodology of animal behavior studies. He provided us with an awareness of the effects of unconscious cuing and of experimenter expectancy. Pfungst's work, together with Pavlov's observation that the performance of laboratory animals improves as the result of improvement in the human investigator's skill in conducting the experiments, has been invaluable for those who are engaged in research into human as well as animal behavior. His discoveries also have been very useful to those who are interested in such phenomena as placebo effects, hypnosis, psychic reading, and the "magic tricks" of illusionists.2

Just one year later, in Vienna, Sigmund Freud participated in