They also reflect whether or not the subject of the test believes the test works. In practice polygraphs are used as a form of psychological torture, used to frighten people into giving confessions.

Of course whether a polygraph can frighten an innocent person enough to falsely confess is another matter, but if the subject is guilty and understands how polygraphs (don't) work, they can deceive the person administering the test very easily. There is no shortage of spies that have passed polygraphs regularly and reliably with flying colors.