Eudora Welty (post 2) in “The Optimist’s Daughter,” cites, gratuitously, the forerunner of all modern literature on “the double” and multiple personality

Laurel’s father dies, and after his funeral, she finds that his personal library includes “Hogg’s Confessions of a Justified Sinner” (1, p. 118). The villain of Welty’s novel is her stepmother, “who always saw herself—in the right. Justified” (1, p. 131).

1. Eudora Welty. The Optimist’s Daughter. New York, Vintage International, 1969/1972/1990.

2. Wikipedia. https://en.wikipedia.org/wiki/The_Optimist's_Daughter

Gratuitous Multiple Personality

The above illustrates what I call “gratuitous multiple personality,” which is when a work of fiction makes reference to multiple personality that has no apparent reason, in either plot or character development, for being included, suggesting that its presence may reflect the author’s psychology.

Hogg’s Confessions of a Justified Sinner

Wikipedia. https://en.wikipedia.org/wiki/The_Private_Memoirs_and_Confessions_of_a_Justified_Sinner