The Science Behind the Polygraph Machine

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The polygraph is not a lie detector but rather an instrument that measures a person’s physiological changes to questions posed by the examiner. An elevated physiological response to a particular question can help detect deception. The physiological changes exhibited during a polygraph examination result from a person’s autonomic nervous system response when they know they are lying. This autonomic response is referred to as the fight-or-flight response.

When faced with a stressful situation, a person’s physiology will change automatically. For example, if you drive down a highway a little faster than the speed limit, and suddenly you see a police car behind you with its lights on, your autonomic nervous system will most likely activate. As a result, your breathing changes, you might start sweating, and your blood pressure and pulse rate increase.

You did not tell your body to make these changes; it automatically happens. This response is your body preparing you to fight or run away (flight). The polygraph is based on the same concept. The polygraph captures the physiological changes a person experiences when they are afraid of getting caught in a lie.

Consequences to Lying

For the polygraph to be accurate, there must be a consequence for the person telling a lie. We all tell little white lies in our life that probably would not engage our autonomic nervous system. For example, if someone’s spouse asked how they liked a meal they worked hard in preparing, one would likely say they liked it, even if they did not believe the meal tasted good. This is to avoid hurting the spouse’s feelings and seeming ungrateful.

The fear of getting caught must have significant consequences to achieve the most accurate polygraph results. For job applicants (pre-employment screening), there is the fear that not getting a job is possible if the applicant gets caught lying. For current employees (personnel security screening), the fear of losing their security clearance, employment, or becoming a target of a criminal investigation looms large. For subjects of criminal or counterintelligence and counterterrorism investigations, there is the fear of being charged and convicted of a crime and possibly imprisonment by lying. All of these consequences will provide ample detection apprehension for the examinee.

Responses Recorded by Polygraph

The polygraph monitors record three autonomic nervous system responses. First is the electrodermal or sweat gland activity. Two small plates are attached to the examinee’s fingers, measuring the examinee’s electrodermal activity.

Second, the polygraph monitors and records the examinee’s blood pressure and pulse rate. A blood pressure cuff is attached to the examinee’s upper arm, forearm, or leg to measure the increase or decrease in blood pressure and pulse rate.

Third, the polygraph monitors and records the examinee’s respiration rate. Two pneumograph tubes are attached to the chest and waist of the examinee to monitor their respiration in two separate areas.

When an examinee lies, their sweat gland activity tends to increase. As a result, the examinee’s blood pressure and pulse rate will also increase. Counterintuitively, an examinee’s respiration rate decreases when an examinee lies. The respiration rate decreases because the examinee gets more oxygen to the lungs to fight or run away (flight). However, since the examinee is sitting and not engaging in flight (running away) or fighting, the respiration rate actually slows down because excessive oxygen is not needed.

Accuracy of Polygraph

Various studies have determined that the accuracy rate of the polygraph is between 87 and 90 percent. The American Polygraph Association determined that when the polygraph is used for an event-specific (single issue) test, the aggregated accuracy rate is 89 percent. In multiple-issue polygraph examinations, the accuracy is 85 percent. The combination of all validated polygraph techniques, excluding outlier results, produced an accuracy of 87 percent. (American Polygraph Association, 2011)

This blog was coauthored with David Young who is a retired FBI polygraph examiner.