Why is a polygraph not an exact tool for testing the sincerity and honesty of a subject?
Polygraph is used to determine intentional deception regarding a
very specific issue. It is not used to determine general honesty,
sincerity or integrity. Part of what makes polygraph testing work
is "fear of detection" by a person trying to conceal involvement in
or knowledge of a specific event. This fear of detection triggers a
reaction in the sympathetic nervous system (SNS) which polygraph
examiners are trained to identify. The frequency, location and
intensity of these reactions in a polygraph exam are used to
develop a numerical score, and this score is used to determine the
probability of deception. The more specific the questions, the more
reliable the results will be. You can not ask someone "Are you an
honest person?" because this is subjective to the point of view of
the person taking the test. - Michael Martin (President, Global
Polygraph Network)