The observer effect in psychology, also known as the Hawthorne effect, refers to subjects altering their behavior when they are aware that an observer is present. This applies when a psychologist observes his patients or when a person is aware that he is being recorded.
The original study regarding the Hawthorne effect took place in 1950, and hypothesized that awareness of being observed could temporarily improve worker behavior and increase productivity, because people tend to show increased motivation when they are shown special attention. After the study, productivity decreased and returned to normal. The observer effect is evident in television documentaries in which people are aware they are being filmed. It is possible to avoid the observer effect during studies by using unobtrusive methods so that the subject is unaware he is being observed.
The observer effect is one form of reactivity. Reactivity can also refer to subjects altering behavior based on perceived expectations of the observer. For example, the experimenter effect occurs when the observer tells subjects about their expectations, and the subjects act accordingly. The Pygmalion effect refers to students changing their behavior to meet teacher expectations. Both of the latter effects can be caused by pre-existing bias or stereotyping.