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).