Split Personality
This is a psychiatric disorder that causes the sufferer's personality to divide into sub personalities, also known as alters. The alters have their own memory, and don't know of each other's existence. Consequently, the sufferer has episodes of amnesia.
The development of MPS is almost always triggered by severe abuse, including sexual abuse. During the traumatic experience the person becomes dissociated from the experience, becomes 'not there', and an alter personality takes over the pain. For every traumatic experience an alter is created.
Those of us who do not suffer from this disorder just "don't get it"!