The polygraph or so-called lie detector measures physiological responses to stress experienced by a subject during the course of an interrogation. The instrument monitors three physiological states: (1) cardio-vascular responses manifested by changes in blood pressure, and pulse rate, (2) galvanic skin resistance which lowers as perspiration increases, and (3) breathing patterns that respond to changes in tension. Changes in any of these patterns can be detected as the subject experiences emotional reactions. The theory behind the polygraph assumes that people encounter measurable physiological changes in the act of deception. The heartbeat increases, blood pressure goes up, breathing rhythms change, and perspiration increases. All of these reactions are recorded on a moving chart for analysis by a trained polygraph technician.
The physiological connection with deception was assumed in the 18th Century. English novelist, Daniel Defoe suggested that "Guilt always carries fear around with it, there is a tremor in the blood of a thief, that, if attended to, would effectually discover him." (Defoe, 1730, quoted in Gale, 1988). In 1915, Harvard Psychologist William Marston devised an instrument to monitor the blood pressure of a subject under interrogation. Medical student John Larson (1921) came up with the first true polygraph, adding a measure of respiration along with blood pressure. In the 1930s, Leonarde Keeler integrated Larson s instrument with measurement of electrical skin conductivity into a single machine (Block, 1977). Keeler's instrument remains in controversial use today in forensic and employment practice.
Supporters of the polygraph claim that it is one of the most accurate means available to determine truth and deception, (American Polygraph Association, 2002). But polygraph credibility has yet to become accepted by the scientific community. A major study by the National Academy of Sciences (NAS) (2002) found that while polygraph data is reliable, it lacks validity. Reliability is a measure of consistency, suggesting that the results are the same across different times, places, subjects, and conditions. Validity is a measure of appropriateness, suggesting that the test actually measures what it purports to measure. The NAS study found that if there were 10 spies among 10,000 government employees, the lie detector would catch eight of them, but 1,598 loyal staff workers would also be falsely accused of deception. If the polygraph tests were adjusted to a much lower sensitivity, only 41 people would be wrongly accused, but eight of the 10 spies would escape detection (Fienberg, et. al., 2002).
Because of such obvious problems, use of the polygraph is practiced only at the fringes of legal and forensic practice, but it is in active use today. The polygraph is utilized more for its utilitarian value to extract information than for its ability to measure truth or lies (Lykken, 1984). Armed with a deceptively "scientific" instrument, an investigator can be perceived with an ability to read the mind of his subject. The ethical use of lie detection has been rationalized for its ability to extract information, even though the instrument cannot accurately discriminate between truth and lies. In this sense, Kant s categorical imperative yields to Mill's utilitarian ethic. The end of truth justifies for the modern detective the means of lying. Technical deception is practiced as a means of extracting reluctant truths.
Title: The Polygraph
Author: Martin Ryder
American Polygraph Association (2002), What is a Polygraph? Online document: http://www.polygraphplace.com/docs/information.shtml#polygraph accessed November 17, 2003.
Block, Eugene G. (1977) Lie detectors: Their history and use. New York: David McKay Company.
Gale, Anthony (1988) The Polygraph Test Lies, Truth and Science; 1988 p. 158.
Fienberg, Stephen E. and others on The Committee to Review the Scientific Evidence of the Polygraph (2002), The Polygraph and Lie Detection. National Academy of Sciences, p. 6. Available online: http://www.nap.edu/catalog/10420.html?onpi_newsdoc10082002 accessed Nov. 17, 2003.
Lykken, David Thoreson (1984) Polygraphic Interrogation in Nature 307: 681-684
Marston, W. (1924). A theory of Emotions and affection based upon systolic blood pressure studies. American Journal of Psychology. Vol.35, pp.469-506.