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Why are people afraid of things that they know are harmless or non-existent? For example: Why do horror films/stories leave people scared after they have finished? Someone may see a harmless spider somewhere that isn't even on their property, and still be scared of it. What is the phenomenon behind this?

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closed as too broad by Why Do You Think That Is True, Community, Krysta, Josh Gitlin Aug 30 '13 at 21:10

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs.If this question can be reworded to fit the rules in the help center, please edit the question.

The question mixes several distinct phenomena together and needs a clearer focus: (1) A movie with potentially real events (psychopathic murderer killing you) will remind you of that real danger. Fear is a natural reaction to witnessing a fearful event. [It has been shown that people today are more afraid of becoming victims of statistically rare crimes, because media represent them as happening constantly everywhere.] (2) Being afraid of a "harmless" spider seems an evolutionary remnant. Many spiders aren't harmless. (3) Being afraid of the dark might be due to a loss of perception. etc. – what Jul 27 '13 at 15:58
I'd like to try and help you clarify this question if you'd like. Please feel free to ping me in chat if you'd like. For now, I've put it on hold. – Josh Gitlin Aug 30 '13 at 21:11