Also known as a lie detector test. Despite the common name, what test actually does test for is deceptive reactions to carefully phrased questions. Based on the theory that most people do not lie or deceive without some sort of physiological reaction, anxiety or nervousness. Heart rate and blood pressure are measured by a cardiograph. Perspiration is measured by the change in the electrical resistance of the skin due to the increase of electrolytes that are found in sweat, and breathing rate is measured by a pneumograph.
Because these physiological signs can accompany other physical states such as illness, alcohol, drug use, or the ingestion of certain medications, polygraph exams can be inconclusive. Baseline questions are asked during all polygraph exams in order to eliminate any existing elevated physiological signs. Polygraph examination results are not court-admissible because they are considered fundamentally unreliable by the court. Polygraphs can, however, be submitted in court if both parties agree to its validity.