The polygraph records physiological arousal (heart rate, blood pressure, respiration and skin conductivity), not deception directly, and its accuracy has never been validated to a standard most courts accept. United States courts have almost uniformly excluded polygraph results at trial, and trained subjects can learn countermeasures that defeat the test, yet the technique remains in operational use for screening and investigative leads.
What would resolve this A physiological correlate of deception, distinct from generalized arousal, validated across diverse populations and shown to resist trained countermeasures in blind, controlled studies.
OpenPsychophysiology and forensic psychologyWikipedia: Polygraph