Former psychologist Frank Tallis on “monosymptomatic delusion” — in which an otherwise sane person latches onto one big outlandish idea. It is possibly responsible for both demonic possession and falling in love. Via New Scientist:
Once a patient came in and said: “I am possessed by a demon.” This guy wasn’t insane, he wasn’t schizophrenic – he just had this particular belief. In my day we called it “monosymptomatic delusion”, but now it would be called something like “delusional disorder”. That’s when you’re completely sound and reasonable in every respect except you have one belief that is absolutely bonkers.
You have to have an openness to it. Lots of people are open to all kinds of spiritual and magical beliefs. An individual could have a perfectly harmless interest in the supernatural but then something happens that triggers this delusion and they get stuck with it, reinforcing it by piling up one misinterpretation after another.
Could anyone end up with delusions like these? Theoretically, yes, in the right circumstances. Maybe we all get such episodes in our lives. It’s not that unusual for people to think they are seriously ill without much evidence. The big one is people suspecting that their spouse is cheating on them. Morbid obsessions about infidelity are relatively common and produce spectacular behaviours, often in individuals who otherwise are OK. In a way, falling in love is kind of monosymptomatic delusion. Even though you’re a rational person, you can engage in all kinds of irrational behaviour because you are fixated on a particular individual.