A polygraph, most commonly referred to as a lie detector, is a machine that is used by law enforcement to test the physiological responses of individuals to certain questions. Despite its colloquial name, the polygraph does not detect lies and most polygraph examiners will say that they do not test specifically for lies, but for deceptive reactions.
Polygraphs are used under the theory that most people do not lie or deceive without some feelings of anxiety or nervousness. This stems from the idea that most people either feel bad that they are lying or are afraid that they will get caught or will be in trouble if they lie. It is this fear and guilt that produces the anxiety and nervousness. When a person feels this way they exhibit difficult to detect involuntary physiological changes that can, theoretically, be detected with a polygraph.
The physiological systems that a polygraph focuses on are heart rate, blood pressure, respiratory rate, and how much a person sweats. Lying is usually accompanied by an increase in heart rate and blood pressure, which is measured by a cardiograph, an increase in breathing rate, which is measured by a pneumograph, and an increase in perspiration, which is measured by the change in the electrical resistance of the skin due to the increase of electrolytes that are found in sweat.
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 and there is a fear that jurors would, without question, believe all results of a polygraph. Polygraphs can, however, be submitted in court if both parties agree to its validity.
Many things can go wrong in investigations and with the gathering of evidence. Read a case where a bad forensic investigation led to a wrongful hanging