Dissociative Identity disorder (D.I.D.) is another mane for Multiple Personality Disorder. Many people including professionals believe this disorder is extremely rare. After doing my dissertation and writing a book on the subject, I have discovered that D.I.D. is not so rare. I have personally worked with over 20 clients with this diagnosis.
D.I.D. is caused by extreme, ongoing abuse prior to the age of seven. It is the minds way of coping with extreme psychological or physical trauma. Individuals with D.I.D. are not crazy; they have just been hurt badly. Actually, if given the right treatment, a person can be cured and integrate into one person or personality if they choose.
What happens is the mind is able to dissociate (disconnect) from the present abuse and another part of the mind them can take over, gradually creating alter personalities to cope with the abuse. There can be dozens of alters that the mind creates to help the individual survive. Therapy will them work with each alter to help bring about healing. This can then lead to integrating back as one personality.