Multiple Personality or Dual Personality is a type of Dissociative Identity Disorder
This is a term applied to personalities which are fundamentally immature, and which show marked defects of judgment and an incapacity to learn from experience. Possessors of such personalities are prone to impulsive gratification of their desires without any consideration of other people or the consequences of their acts: they are emotionally unstable and although intelligence may be normal or even superior, quite often it is on the borderline of mental defect. Psychopaths frequently show asocial or criminal traits: they may be amoral, sexually perverse, aggressive swindlers, or drug addicts. Treatment is difficult and usually ineffective, though lesser degrees of psychopathy may be relieved by psychotherapy.
This is an hysterical phenomenon and is allied to the fugue or wandering attack in which an hysteric loses his memory from a certain point and disappears from his usual surroundings. Multiple personality is a marked form of mental dissociation.
Actually, it would be more correct to regard it as the dissociation of large fragments of the one personality. It represents psychologically, the play-acting of some unconscious fantasy and the dissociation is precipitated by some particularly difficult emotional predicament. It is a mechanism for the temporary evasion of reality in a person whose mental synthesis is poor.
By hypnotism, it is possible to explore and uncover the main body of the dissociated personality. Dual or multiple personality is to be distinguished from split personality which is a disintegration of the mind occurring in the disease called schizophrenia.