The other thing is, what if something is something you were teased about a lot, so hearing the question triggers a flood of emotions, but it isn't actually true? So that if you tell the truth, it might still say you're lying, because on that one question they asked something sensitive and you responded emotionally in a way you otherwise wouldn't.
Like, for instance, if you are going out with someone in a monogamous relationship. And there's someone else you're attracted to, but have never actually done anything with. But other people have figured out you're attracted to them and love to bug you about it as if you have been going out with them. If you were then asked if you'd cheated on your partner with this other person, I imagine that even if you said "No," you'd still be blushing or whatever, which has to be enough physiological change to register majorly on the test.
I'm thinking of that because I saw someone talking about a show once where they ask people if they've ever cheated on their girlfriend, with a lie detector test, and I was thinking, in that scenario, a person could fail the test if they'd been thinking about someone else but trying really hard to cover it up in an effort to remain faithful, and thus it was a sensitive question, and set off the lie detector even when they were telling the truth.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams