Observer Effect (Psychology)

In psychology and research methodology, the observer effect, more formally called reactivity, is the general phenomenon in which people alter their ordinary performance or behavior simply because they are aware that they are being observed or studied, a change that can run in either a positive or a negative direction and that poses a threat to a study's external validity, since the behavior recorded may no longer represent how the same people would act when unobserved. This sense of the term is distinct from the unrelated observer effect described in physics, which concerns how the act of measurement can disturb a physical system, though both share the common idea that observation is not always a neutral, non-interfering act. Reactivity in psychology encompasses several more specific, individually named effects that describe particular ways this general phenomenon plays out: the Hawthorne effect, in which participants change their performance simply because they know a study is paying attention to them; the John Henry effect, in which control-group participants specifically alter their behavior out of rivalry with an experimental group; the experimenter effect, in which a researcher's own expectations are subtly and often unconsciously communicated to participants, causing those participants to behave in ways that fulfill those expectations; and the Pygmalion effect, in which students shift their own performance to match what a teacher expects of them. Because reactivity can compromise the validity of directly observed self-report and behavioral measures alike, researchers often turn to unobtrusive research methods, which gather information without the participant's awareness of being measured, as a way to reduce or bypass the distorting influence of being watched.