a rare mental disorder in which two or more independent and distinct personality systems develop in the same individual. Each of these personalities may alternately inhabit the person's conscious awareness to the exclusion of the others. In some cases all of the personalities remain mutually unaware of the others' existence. In a more common form of the disorder, there is one personality that basically dominates the person's conscious awareness. This personality cannot remember what happens during the time a subordinate personality is in control (see amnesia), but a subordinate personality may be aware of the dominant personality's existence and actions and may even comment upon and criticize the dominant personality as if it were another person. Usually the various personalities differ markedly from one another in outlook, temperament, and body language and give themselves different first names. The various personalities may also exhibit different handwriting and electroencephalogram readings and perform differently on projective tests.

This condition is extremely rare, and only a few hundred cases have been reported. Dissociative identity disorder is widely viewed as resulting from dissociative mental processes—i.e., the splitting off from conscious awareness and control of thoughts (thought), feelings (feeling), memories (memory), and other mental components in response to situations that are painful, disturbing, or somehow unacceptable to the person experiencing them. The failure to form a distinct personality can thus be seen as a way of coping with or escaping from inner conflict, which in turn is frequently triggered by some trauma experienced early in life, such as being abused as a child.

Treatment is aimed at integrating the disparate personalities back into a single and unified personality. To do this, the dominant personality must gradually be made aware of the existence of the others, a process that is usually possible only after the trauma that originally caused the dissociation is brought to conscious awareness and thus defused.