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A polygraph or lie detector is a device which measures and records several physiological variables such as blood pressure, heart rate, respiration and skin conductivity while a series of questions is being asked, in an attempt to detect lies. A polygraph test is also known as a psychophysiological detection of deception (PDD) examination.

A typical polygraph procedure starts with a pre-test interview designed to establish a connection between the tester and the testee and to gain some preliminary information which will later be used for control questions (see below). Then the tester will explain the polygraph, emphasizing that it can detect lies and that it is important to answer truthfully. Then a "stim test" is often conducted: the testee is asked to deliberately lie and then the tester reports that he was able to detect this lie. Then the actual test starts. Some of the questions asked are irrelevant ("Are you 35 years old?"), others are "probable-lie" control questions that most people will lie about ("Have you ever stolen money?") and the remainder are the relevant questions the polygrapher is really interested in. The different types of questions alternate. The test is passed if the physiological responses during the probable-lie control questions are larger than those during the relevant questions. If this is not the case, the tester attempts to elicit admissions during a post-test interview ("Your situation will only get worse if we don't clear this up"). These admissions are the main goal of the test.

The accuracy of polygraph tests is a matter of considerable controversy. While some claim the test to be accurate in 70% - 90% of the cases, critics charge that rather than a "test", the method amounts to an inherently unstandardizable interrogation technique whose accuracy cannot be established. Polygraph tests have also been criticized for failing to catch actual spies such as Aldrich Ames, who passed two polygraph tests while spying for the Russians.

Several countermeasures designed to pass polygraph tests have been described; the most important of which is never to make any damaging admissions. Additionally, several techniques can be used to increase the physiological response during control questions.

Polygraph tests were invented in the 1930s in the United States and are most commonly applied there. They were often used by employers in an attempt to screen out dishonest job applicants, but this practice was outlawed for most private employers in 1988. Many government agencies still apply routine lie detector tests to screen all employees. While lie detector tests are commonly used in police investigations, no defendant or witness can be forced to undergo the test. A US Supreme Court decision of 1998 left it up to the individual jurisdictions whether polygraph results could be admitted as evidence in court cases.

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