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:
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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.