A polygraph, most commonly named to as the lie detector, is a machine used by law enforcement and private investigators to test people's physiological responses to specific questions. The polygraph does not detect lies despite its colloquial name, and most polygraph inspectors say they are testing for false positives, not intentionally testing lies.
Why a Polygraph?
Lie detectors are used according to the theory that most people don't lie or cheat without a little bit of anxiety or nervousness. This comes from the idea that most people feel bad about lying or fear that lying will get them caught or in trouble. It is this fear and guilt that causes anxiety and restlessness. When a person has these feelings, he or she has involuntary physiological changes that are difficult to detect, which can in theory be detected using a polygraph.
The psychological systems that lie detectors focus on are heart rate, blood pressure, respiration rate, and sweat volume. Lying is usually accompanied by an increase in heart rate and blood pressure, an increase in respiratory rate as measured by a pneumatic meter, and an increase in sweating as measured by a change in voltage. Skin resistance due to increased electrolytes in sweat.
Because these physiological signs can accompany other physical conditions, such as illness, alcohol, drug use, or taking certain medications, polygraph tests may not be effective. The Baseline Question is asked during all polygraph tests to eliminate pre-existing elevated physiological signs.
The results of the polygraph test have been found to be fundamentally inconclusive in court because there is a fear that the jury will unquestionably believe all the results of the test. However, the polygraph can be used if both parties agree to its validity.