It's a natural reaction for a lot of people, isn't it?
As far as I'm aware, no. I've never heard of someone clapping by reflex. It is a conscious decision.
On that note, clapping after a movie is perfectly fine.
If you clap DURING, leave the theater. You aren't welcome there, and your a disruption. It's the same as yelling "look out he's right behind you!" in a horror movie, and similar ilk.
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