The theme of two brothers or mysterious doubles as contrasts in temperament or moral nature is literally as old as written literature itself, appearing in the first written story of the hero (c.

2500 B.C. or earlier) about Gilgamesh, king of Uruk, and his wild companion, Enkidu. Pagan mythologies abound with sets of often opposed brothers: Osiris and his murderous brother Set in ancient Egypt, for instance, or Ahura Mazda, ancient Persian god of light, and his evil twin Ahriman, god of darkness, both born of the androgynous Ur-principle, Zurvan (Time). In the Hebrew-Christian tradition, of course, the first two brothers, Cain and Abel, feature in the first murder story, and have many literary descendants, such as Steinbeck's East of Eden (1952).

With the nineteenth century and the popularization of depth psychology, the concept of the unconscious mind contributed a new slant to stories of demonic possession...