The police use lie detectors all the time (and rely on them to a surprising degree).
But they will tell you that it's not the machine that's critical, it's the person charged with the interview. Basically, you can't just give the machine to any chump to ask questions, it needs to be given to a skilled interviewer - someone who can trap you or catch you off guard.
Even so, I'm always surprised how much the cops rely on it. Beat a lie detector, you're pretty much cleared in the eyes of the police. But of course, your very willingness to take the test is telling in itself.
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