Dissociative Identity Disorder  (Multiple Personality Disorder ) DID is one of the most misunderstood mental illnesses, and also one of the most fascinating.  Unlike the other illnesses I've discussed here, DID is not caused by a chemical imbalance in the brain.  It is, instead, a learned behavior.  It is a highly developed defense mechanism that develops in early childhood to deal with abuse or unpleasantness.  To this day I don't know if I had multiple personalities, or if the people in my head were caused by the schizo-affective disorder.  I may never know for sure.  The fact sheet from NAMI (National Alliance for the Mentally Ill) is below to give you more information. Dissociative Identity Disorder (DID), previously referred to as multiple personality disorder (MPD), is a dissociative disorder involving a disturbance of identity in which two or more separate and distinct personality states (or identities) control the individual's behavior at different times. When under the control of one identity, the person is usually unable to remember some of the events that occurred while other personalities were in control. The different identities, referred to as alters, may exhibit differences in speech, mannerisms, attitudes, thoughts, and gender orientation. The alters may even differ in "physical" properties such as allergies, right-or-left handedness, or the need for eyeglass prescriptions. These differences between alters are often quite striking. The person with DID may have as few as two alters, or as many as 100. The average number is about 10. Often alters are stable over time, continuing to play specific roles in the person's life for years. Some alters may harbor aggressive tendencies, directed toward individuals in the person's environment, or toward other alters within the person. At the time that a person with DID first seeks professional help, he or she is usually not aware of the condition. A very common complaint in people with DID is episodes of amnesia, or time loss. These individuals may be unable to remember events in all or part of a proceeding time period. They may repeatedly encounter unfamiliar people who claim to know them, find themselves somewhere without knowing how they got there, or find items that they don't remember purchasing among their possessions. Often people with DID are depressed or even suicidal, and self-mutilation is common in this group. Approximately one-third of patients complain of auditory or visual hallucinations. It is common for these patients to complain that they hear voices within their head. Treatment for DID consists primarily of psychotherapy with hypnosis. The therapist seeks to make contact with as many alters as possible and to understand their roles and functions in the patient's life. In particular, the therapist seeks to form an effective relationship with any personalities that are responsible for violent or self-destructive behavior, and to curb this behavior. The therapist seeks to establish communication among the personality states and to find ones that have memories of traumatic events in the patient's past. The goal of the therapist is to enable the patient to achieve breakdown of the patient's separate identities and their unification into a single identity. Retrieving and dealing with memories of trauma is important for the person with DID, because this disorder is believed to be caused by physical or sexual abuse in childhood. Young children have a pronounced ability to dissociate, and it is believed that those who are abused may learn to use dissociation as a defense. In effect, the child slips into a state of mind in which it seems that the abuse is not really occurring to him or her, but to somebody else. In time, such a child may begin to split off alter identities. Research has shown that the average age for the initial development of alters is 5.9 years. Children with DID have a great variety of symptoms, including depressive tendencies, anxiety, conduct problems, episodes of amnesia, difficulty paying attention in school, and hallucinations. Often these children are misdiagnosed as having schizophrenia. By the time the child reaches adolescence, it is less difficult for a mental health professional to recognize the symptoms and make a diagnosis of DID.