“Gratuitous Multiple Personality”: Novel’s unacknowledged signs of multiple personality may not seem intentional, but could they have been the ulterior motive?

Previously, my explanation of gratuitous multiple personality in a novel has been that it inadvertently reflects the author’s own multiple personality.

But what if it was not inadvertent? How do I know it was not the author’s ulterior motive? Perhaps it was inadvertent in some novels, but the ulterior motive of others. Or perhaps it was inadvertent for some of the author’s personalities, but the ulterior motive for others.

All I know is that a good many novels have unacknowledged signs and symptoms of multiple personality.