Spirit possession may be broadly defined as any altered or unusual state of consciousness and allied behavior that is indigenously understood in terms of the influence of an alien spirit, demon, or deity. The possessed act as though another personality—a spirit or soul—has entered their body and taken control. Dramatic changes in their physiognomy, voice, and manner usually occur. Their behavior often is grotesque and blasphemous. Justinus Kerner, a nineteenth-century German physician and disciple of the philosopher Friedrich Schelling, describes a demonically possessed woman in his native Swabia:

In this state the eyes were tightly shut, the face grimacing, often excessively and horribly changed, the voice repugnant, full of shrill cries, deep groans, coarse words; the speech expressing the joy of inflicting hurt or cursing God and the universe, addressing terrible threats now to the doctor, now to the patient herself.… The most dreadful...