In possession, the person enters an altered state of conscious and feels taken over by a spirit, power, deity, or other person who assumes control over his or her mind and body. Generally the person has no recall of these experiences in the waking state. Such experiences have a long human history and many religions offer rituals and healings to protect participants from unwanted possession. One of the signs of Christ's divinity was his ability to cast out demons from people who were possessed.
However, the deliberate induction of possession states is part of valued religious rituals in many cultures, and probably the most popular form of union with the divine throughout human history. Possession-oriented rituals have been documented in ancient Egypt, and in the earliest forms of Kabbalistic practice. Possession was a recognized phenomenon in ancient Greece with the Delphic oracle. Possession is a central feature of Haitian voodoo ceremonies where specific deities are invited to 'ride' the bodies of the worshipers during ceremonies, as well as In Balinese ritual drama where the dancers become the entity they are portraying.
Possession also appears in early Christianity in a positive light, particularly in the form of "speaking in tongues." Many contemporary forms of evangelical Christianity consider it desirable to be possessed by the Holy Spirit, with physical manifestations that include shaking and speaking in tongues. St. Paul was worried by the phenomenon, and found it necessary to lecture the Corinthian Christians on the need to carefully manage speaking in tongues:
If therefore, the whole church assembles, and all speak in tongues, and outsiders or unbelievers enter, will they not say that you are mad?. . .do not forbid speaking in tongues, but all things should be done decently and in order (I Corinthians, 14).
POSSESSION AND PSYCHOPATHOLOGY
While possession is a common experience in many cultures, in Western industrialized cultures, such experiences are not normative and may lead to inappropriate diagnoses of dissociative or psychotic disorders. Anthropologist Ruth Inge-Heinz, PhD  who has studied possession experiences in many cultures has commented on the deleterious effects of mislabeling an individual in a state of dissociation as having a mental disorder:
The concept of what constitutes a 'healthy mind' differs considerably from one culture to another...How devastating it can be to affix the label of 'mental illness' to any extraordinary state of consciousness! A dissociative state of mind does not necessarily qualify an individual for being put into a straight jacket. Many dissociative states occur in Southeast Asia, for example, in a culturally conditioned and controlled setting (pp. 28-29).
The DSM-IV lists Dissociative Trance Disorder as a diagnosis requiring further study. The definition includes,
Possession trance, a single or episodic alteration in the state of consciousness characterized by the replacement of customary sense of personal identity by a new identity. This is attributed to the influence of a spirit, power, deity, or other person (p. 729).
The DSM-IV Casebook includes a case example of this in which a woman reports,
"Sometimes God enters my body, which gets hot when I have visions" (p. 420).
In this state, she is presumed by herself and others to be possessed by dead ancestors, and to be able to foresee the future.
The Casebook notes that,
This woman has symptoms that would be considered psychotic if they were experienced by someone from a society that did not share the beliefs of her [Guinean] culture. She believes she has special powers and she has. . .hallucinations. In her local society, however, these phenomena are quite common. Her culture ascribes to her the role of healer and accepts her unusual experiences as normal for someone in that role. Indeed she is a successful healer. . .Local culture would assign her the role as a healer, an her behavior would not be seen as something to be treated (p. 421).
Despite this acknowledgment of the nonpathological nature of her experience in its cultural context, the Casebook, authored by many of the same people who developed the DSM-IV, assigns a diagnosis of a mental disorder to this case: Dissociative Disorder Not otherwise Specified!
Yet possession is also known to be associated with dissociative disorders that are not socially sanctioned and occur outside of the normal part of a collective cultural or religious practice. There is clearly a spectrum of dissociative experiences from nonpathological to pathological. (See Disintegrated experience: the dissociative disorders revisited)
Possessions are dysfunctional when there is impairment in social or occupational functioning or marked distress. The criteria described in Lesson 3 can be helpful in making a differential diagnosis.
ASSOCIATED CLINICAL PROBLEMS
Patients often feel their behavior is beyond their control. Bizarre behavior such as choking, projectile vomiting, frantic motor behavior, wild spasms, and contortions along with grotesque vocalizations can lead to a frightening experience both for the person possessed and for others witnessing it.
A key issue as with most spiritual emergencies is determining whether the person is in the midst of an episode of mental disorder or having a spiritual problem:
Demon possession and mental illness, then, are not simply alternative diagnoses. . .Furthermore, demon possession is essentially a spiritual problem, but mental illness is a multifactorial affair, in which spiritual, social, psychological and physical factors may all play an aetioIogical role. The relationship between these concepts is therefore complex. Differential diagnostic skills may have a part to play in offering help to those whose problems could be of demonic or medical/psychiatric origin. However, spiritual discernment is of at least equal, if not greater, importance in such matters (Professor Chris Cook Demon Possession and Mental Illness: Should we be making a differential diagnosis?).
The differential diagnostic criteria described in Lesson 3 should be used with special consideration for the patient's religious community and its practices. Support for the patient must include social integration of the experience within his/her community. The treatment guidelines in Lesson 5, especially those involving grounding, are especially important in coping with the physical aspects of possession. If the individual is connected with a group whose practices include possession, then collaboration with leaders of that religious community should be part of the treatment plan.