Definition of Multiple Personality (aka Dissociative Identity or Multiple Identity) for Nonpsychiatric Purposes

1. A person has more than one sense of identity or personhood; that is, they have more than one “I”: Ia and Ib (the alternate personality). What gives Ia and Ib their separate senses of identity?

2. Ia and Ib think and act independently and differently. Sometimes Ia is in control. Sometime Ib is in control. For example, they can have a conversation. Sometimes Ia speaks and Ib listens. Sometimes the reverse. They may have different opinions, attitudes, moods, and self-images (as to age, height, sex, etc.).

3. They have separate memory banks. At least one of them will have known and remembered things that the other one hadn't. For example, Ib may tell Ia a story of events that Ia hadn’t known about or remembered. That is, Ia had had amnesia, a memory gap, for the events (real or imagined) that Ib had remembered or imagined.

All the above may be fulfilled when a novelist has a character or another narrator with a mind of its own.

Does Ib, the alternate personality, have to come out and interact with other people? No. It doesn’t have to. Or it may do so rarely. And even if it does so frequently, other people usually don’t realize it, because alternate personalities generally prefer to remain incognito as far as outsiders (people other than Ia) are concerned.

The essential difference between normal multiple personality (which many novelists have) and multiple personality disorder (the mental illness) is that the latter causes the person a significant amount of distress and/or dysfunction. In contrast, some people with normal multiple personality can be very high-functioning, even winning the Nobel Prize.