A natural experiment in psychology is a psychological experiment in which the subject is studied in his natural circumstances, for instance in his place of work on in his home. Natural experiments require minimal intrusion by the researcher into the behavior being observed.
Natural experiments are preferable primarily because the data is considered to be more authentic when the observer intrusion is minimal. The subjects of the experiment are impacted less strongly by the study, reducing risk of harm. Subjects are not typically aware that they are being studied.
A classic example of a natural experiment is the Hodges and Tizard Study, in which the development of children was documented from just after childbirth into early adulthood.