What is Polygraph?

Legal Definition
A polygraph, popularly referred to as a lie detector, measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions. The belief underpinning the use of the polygraph is that deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers.

The polygraph was invented in 1921 by John Augustus Larson, a medical student at the University of California, Berkeley and a police officer of the Berkeley Police Department in Berkeley, California. The polygraph was on the Encyclopædia Britannica 2003 list of greatest inventions, described as inventions that "have had profound effects on human life for better or worse."

The efficacy of polygraphs is debated in the scientific community. In 1991, two thirds of the scientific community who have the requisite background to evaluate polygraph procedures considered polygraphy to be pseudoscience. In 2002, a review by the National Research Council found that, in populations "untrained in countermeasures, specific-incident polygraph tests can discriminate lying from truth telling at rates well above chance, though well below perfection". However, the review also notes that generalization from these findings to uses of polygraphs for screening is not justified: "polygraph accuracy for screening purposes is almost certainly lower than what can be achieved by specific-incident polygraph tests in the field." Effectiveness may also be worsened by countermeasures.

In some countries, polygraphs are used as an interrogation tool with criminal suspects or candidates for sensitive public or private sector employment. US law enforcement and federal government agencies such as the FBI and the CIA and many police departments such as the LAPD use polygraph examinations to interrogate suspects and screen new employees. Within the US federal government, a polygraph examination is also referred to as a psychophysiological detection of deception (PDD) examination.

Polygraph testing is designed to analyze the physiological reactions of subjects. However, research has indicated that there is no specific physiological reaction associated with lying and that the brain activity and mechanisms associated with lying are unknown, making it difficult to identify factors that separate liars from truth tellers. Polygraph examiners also prefer to use their own individual scoring method, as opposed to computerized techniques, as they may more easily defend their own evaluations.

The validity of polygraph testing is again called in to question with the relevant-irrelevant testing technique, designed to gauge reactions of subjects against crime questions and other non-crime related questions. Studies have indicated that this questioning technique is not ideal, as many innocent subjects exert a heightened physiological reaction to the crime relevant questions.

The control question test, also known as the probable lie test, was developed to combat the issues with the relevant-irrelevant testing method. Although the relevant questions in the probable lie test are used to obtain a reaction from liars, the physiological reactions that "distinguish" liars may also occur in innocent individuals who fear a false detection or feel passionately that they did not commit the crime. Therefore, although a physiological reaction may be occurring, the reasoning behind the response may be different. Further examination of the probable lie test has indicated that it is biased against innocent subjects. Those who are unable to think of a lie related to the relevant question will automatically fail the test.

Polygraph examiners, or polygraphers, are licensed or regulated in some jurisdictions. The American Polygraph Association sets standards for courses of training of polygraph operators, though it does not certify individual examiners.
-- Wikipedia
Legal Definition
1. A device to detect lies. 2. A machine that copies documents.