This answer originally appeared on Quora on March 21, 2020.

This is an excellent question. Mental illness comes up a lot in the horror genre (including my own works), and there are a few reasons for it. My top five are below, coming from the view of a horror author.

• Mind and reality bending.
• Creating an unreliable narrator.
• Plot twists.
• Making the reader or viewer unsure.
• The sense of losing control over one’s mind can be a terrifying concept.

Mainly, people tend to fear being unable to distinguish between reality and fantasy. You can create an excellent unreliable narrator, and disturb your audience better with someone who may or may not be telling you the truth.