Dissociative identity disorder (in the past called multiple
personality disorder) is a rare condition in which a person has
two or more separate personalities. The person often does not
know that the other personalities exist. And he or she cannot
recall events that occur when the other personalities are
active. The condition is not due to substance abuse or another
medical condition. Dissociative identity disorder is likely
linked to severe childhood trauma. The person's defense
mechanism separates different aspects of his or her own
personality in order to deal with the physical and emotional
pain. Eventually, new "personalities" emerge and establish their
own lifestyles in the same individual. Professional counseling
is usually the main treatment for this condition. The goal is to
slowly merge the different aspects of the personalities together
(integration).