Possession, in religious and folk traditions, condition characterized by unusual behaviour and a personality change that is interpreted as evidence that the person is under the direct control of an external supernatural power. Symptoms of spirit possession include violent unusual movements, shrieking, groaning, and uttering disconnected or strange speech. Occasionally a normally pious member of a religious body becomes incapable of prayer, utters blasphemies, or exhibits terror or hatred of sacred persons or objects. Christianity and some other religions allow for the possibility that some of these states have an evil transcendental cause (see exorcism). Most scientific studies treat them as psychophysical manifestations to be dealt with medically or in terms of social psychology. Some conditions historically termed demonic possession have come to be treated as epilepsy, hysteria, somnambulism, schizophrenia, or other organic or psychological forms of illness.
In some traditions, the “possessed” individual becomes ill and is regarded by his community as having committed some spiritual transgression; recovery is held to require expiation of his sin, often by a sacrifice. In other traditions, the “possessed” person is conceived as a medium for the controlling spirit and functions as an intermediary between spirits and men. His major role is usually to diagnose and heal other spirit-afflicted individuals. In this tradition the trance behaviour of the medium is often self-induced (autohypnotic); it may be stimulated by drugs, drumming, or collective hysteria. In his trance the medium appears genuinely insensible to ordinary stimuli.