Experimenter Expectancy Effect

The experimenter expectancy effect is one of the sources of artifact or error in scientific inquiry (see INVESTIGATOR EFFECT). Specifically, it refers to the unintended effect of experimenters’ hypotheses or expectations on the results of their research.

Some expectation of how the research will turn out is virtually a constant in science. Social scientists, like other scientists generally, conduct research specifically to examine hypotheses or expectations about the nature of things. In the social and behavioral sciences, the hypothesis held by the investigator can lead him or her unintentionally to alter behavior toward the research participants in such a way as to increase the likelihood that participants will respond so as to confirm the investigator’s hypothesis or expectations. We are speaking, then, of the investigator’s hypothesis as a self-fulfilling prophecy: One prophesies an event, and the expectation of the event then changes the behavior of the prophet in such a way as to make the prophesied event more likely. The history of science documents the occurrence of this phenomenon with the case of the horse Clever Hans as a prime example (Pfungst, 1911/1965).

The first experiments designed specifically to investigate the effects of experimenters’ expectations on the results of their research employed human research participants. Graduate students and advanced undergraduates in the field of psychology were employed to collect data from introductory psychology students. The experimenters showed a series of photographs of faces to research participants and asked participants to rate the degree of success or failure reflected in the photographs. Half the experimenters, chosen at random, were led to expect that their research participants would rate the photos as being of more successful people. The remaining half of the experimenters were given the opposite expectation—that their research participants would rate the photos as being of less successful people. Despite the fact that all experimenters were instructed to conduct a perfectly standard [Page 357]experiment, reading only the same printed instructions to all their participants, those experimenters who had been led to expect ratings of faces as being of more successful people obtained such ratings from their randomly assigned participants. Those experimenters who had been led to expect results in the opposite direction tended to obtain results in the opposite direction.

These results were replicated dozens of times employing other human research participants. They also were replicated employing animal research subjects. In the first of these experiments, experimenters were employed who were told that their laboratory was collaborating with another laboratory that had been developing genetic strains of maze-bright and maze-dull rats. The task was explained as simply observing and recording the maze-learning performance of the maze-bright and maze-dull rats. Half the experimenters were told that they had been assigned rats that were maze-bright while the remaining experimenters were told that they had been assigned rats that were maze-dull. None of the rats had really been bred for maze-brightness or maze-dullness, and experimenters were told purely at random what type of rats they had been assigned. Despite the fact that the only differences between the allegedly bright and dull rats were in the minds of the experimenters, those who believed their rats were brighter obtained brighter performance from their rats than did the experimenters who believed their rats were duller. Essentially the same results were obtained in a replication of this experiment employing Skinner boxes instead of mazes.

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