Ferracuti, Stefano and Sacco, Roberto.  "Dissociative Trance Disorder: Clinical and Rorscharch Findings."  Journal of Personality Assessment.  66.3 (1996).  10 May 2008. .

Ferracuti and Sacco, two psychiatrists, conducted a study on non-psychiatric individuals who believed they were possessed by the devil. From a biomedical perspective, the purpose of their research was to better understand and potentially classify their unique behaviors. After receiving permission from the official exorcist of the Rome diocese, subjects were recruited from weekly exorcisms. Participants, who all strongly followed the Roman Catholic faith, were administered the Dissociative Disorders Diagnostic Schedule, Roscharch Test, and clinical interviews.

Findings suggest that DTD is a distinct clinical manifestation on a dissociative continuum. It shares many personality features with Dissociative Identity Disorder (DID, formerly known as Multiple Personality Disorder), such as altered states of consciousness with a foreign identity, problems in control, psychological complexity, and feelings of guilt. However, those specifically with DTD use extreme dissociation for regenerative purposes, only performing the ‘possessed’ behaviors in a socially accepted, safe, and controlled environment. Otherwise known as an exorcism in the Catholic Church, this setting allows people to reorganize their inner conscious state around an image of “evilness”, thus allowing the expression of inappropriate and unacceptable behaviors. The belief in a state of possession functions as an external control for the low capacity for ego integration and reality distortion manifested by these individuals. In other words, the Church provides coping devices that work as effective therapeutic mechanisms, in which possession serves to fulfill various needs by giving people a chance to let out repressed feelings and develop a more organized ego framework. The exorcist works as a guide in this endeavor, eventually helping people control their socially denied impulses while simultaneously replenishing their faith.

Had the authors not known about the individuals’ religious beliefs, the diagnosis would have been high-functioning neurotic with DID, instead of DTD. They consider the major differences between DTD and DID attributable to traditional cultural attitudes, which influence the belief in possession. Claims of ‘possession’ signify an effort towards ego integration, giving people a sense of security and thus revealing the importance of exorcism as a valuable religious practice.

Their study directly relates to an issue in The Exorcist: if possession is a ‘real’ phenomenon, what does it look like? Also, where does one draw the line between mental illness, where science is most useful, and possession, in which religion offers the best treatment? Ferracuti and Sacco emphasize how DTD can be understood as a psychiatric condition with problems in ego dissociation, potentially treatable through psychotherapy and other biomedical means. However, they also acknowledge the importance of cultural beliefs in shaping its outcome. The possession state exists to those who believe in it, and consequently, many fans were curious enough to reconsider their religious commitment, as well as their views on the causes and remedies of mental psychopathology. This once taboo issue quickly became the center of attention for some time.