How Do Polygraph Tests Work?

The Machine

Whether you view polygraph tests as "fake" science or a legitimate means to discover whether an individual is telling the truth, polygraph machines and polygraph examiners are used every day as a tool in law enforcement. "Lie detector" is a misleading term for a polygraph machine, since the machine only measure changes in respiratory rate, blood pressure and sweat, or a lack thereof, on a person's fingertips. A statement from the American Civil Liberties Union asserted, "The 'lie detector' does not measure truth-telling; it measures changes in blood pressure, breath rate and perspiration rate, but those physiological changes can be triggered by a wide range of emotions."­ Some polygraph machines also monitor a subject's arm and leg movements. Most new polygraph machines are digital and use a computer monitor, rather than the older, analog machines that spewed scrolls of graphed paper, which was observed and marked by the polygraph examiner during the course of the examination.

Hooking Up

To monitor changes in blood pressure, a blood pressure cuff is placed on the subject's arm. Changes in blood pressure are relayed via electronic signals in a digital polygraph machine, and a sound-sensitive bellows in an analog machine. The bellows activate a pen which indicates the changes on a scrolling piece of paper. Air-filled rubber tubes are placed around a subject's upper chest to monitor the changes in respiratory rate, and use the same bellows/pen/scrolling paper principle as the blood pressure cuff. Because humans tend to sweat more when they're under stress, sensors called "galvanometers" are placed on a subject's fingertips to check for changes in the volume of perspiration on the surface of the skin. As a person's skin becomes more moist, the galvanometers register electricity conducted through the skin, hence indicating possible deceptive behavior.

The Examiner

Skilled polygraph test examiners feel that deception or untruths can be determined despite the stress of a person being hooked to the polygraph machine itself. A polygraph test cannot take place without a polygraph examiner, and the test results greatly depend on the examiner's skill. Examiners must pass a rigorous ten-week course that includes classes in psychology, ethics, human physiology, data analysis and more. An internship follows successful completion of the coursework, during which the examiner must conduct a minimum of 25 reviewed polygraph tests before ever being allowed to examine actual cases.

Resources

• The Truth About Lie