- "Participant-observer effect" redirects here.
The observer-expectancy effect (also called the experimenter-expectancy effect, expectancy bias, observer effect, or experimenter effect) is a form of reactivity in which a researcher's cognitive bias causes them to unconsciously influence the participants of an experiment. It is a significant threat to a study's internal validity, and is therefore typically controlled using a double-blind experimental design.
An example of the observer-expectancy effect is demonstrated in music backmasking, in which hidden verbal messages are said to be audible when a recording is played backwards. Some people expect to hear hidden messages when reversing songs, and therefore hear the messages, but to others it sounds like nothing more than random sounds. Often when a song is played backwards, a listener will fail to notice the "hidden" lyrics until they are explicitly pointed out, after which they are obvious. Other prominent examples include facilitated communication and dowsing.