Dissociation is about switching in and out of hypnotic or semi-hypnotic states without control.

When a toddler, newborn to four years old, is traumatized so severely, either physically, psychologically, or emotionally, that the toddler goes into shock, that state of shock is exactly a state of hypnosis. Usually the dysfunctional family traumatizes the toddler throughout the first five years of life. So personalities develop within that constant state of shock that are not the same as the original personality or host, if you will.

When the child comes out of these hypnotic or semi-hypnotic type states, automatic amnesia occurs just like it occurs if a mental health professional puts someone under deep hypnosis! I have a book on sleep talking by Dr. Arthur Arkin which has documented this phenomenon.

That is what dissociation is about. How different the personalities are depends on various factors like the severity of abuse, the type of abuse: physical, emotional, or psychological, the personalities of the abusers, the child genetic personality tendencies, etc.

The reason that the mental illness is not diagnosed in childhood is related to the dysfunctional family who usually are totally insensitive to the child's feelings and do not communicate with the child appropriately.