Except that it doesnt work like that. What if you're nervous not about the questions relating to a crime, but simply the fact that you're jammed in a small, bright room with cops who accusing you of a crime? Or what if you're nervous that they might get a bad reading and think you're lying when you're telling the truth? What if you clench your anus at the wrong moment? I guarantee you an innocent person is going to be plenty nervous and stressed going into a polygraph, and that's exactly what the polygraph is trying to measure.