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)