The Science Behind the Printing Machine

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A polygraph is not a lie detector, but rather a tool that measures a person’s physiological changes in response to questions asked by the examiner. A heightened physiological response to a particular question can help detect deception. The physiological changes shown during a polygraph test are the result of a person’s autonomic nervous system reacting when they know they are lying. This autonomic response is called the fight or flight response.

When faced with a stressful situation, a person’s physiology will automatically change. For example, if you’re driving down a highway a little faster than the speed limit and suddenly 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 may begin to sweat, and your blood pressure and heart rate increase.

You didn’t tell your body to make these changes; it happens automatically. This response is your body preparing you to fight or flee (flight). The polygraph is based on the same concept. The polygraph captures the physiological changes a person experiences when they fear being caught in a lie.

Consequences of lying

For the polygraph to be accurate, there must be a consequence for the person lying. We all tell little white lies in our lives that probably won’t engage our autonomic nervous system. For example, if one’s spouse asks how one likes a meal one has worked hard to prepare, one is likely to say that one likes it, even if one does not believe the dish tastes good. This is to avoid hurting your husband’s feelings and not seem ungrateful.

The fear of getting caught must have significant consequences in order to achieve the most accurate polygraph results. For job applicants (pre-employment screening), there is a fear that they may not get the job if the applicant is caught in a lie. For current employees (personnel security clearance), the fear of losing their security clearance, job, or becoming the target of a criminal investigation is high. For subjects of criminal or counterintelligence and counterterrorism investigations, there is the fear of being accused and convicted of a crime and possible imprisonment by lying. All of these consequences will provide enough concern to open trials.

Answers recorded by polygraph

Polygraph monitors record three responses of the autonomic nervous system. First is the electrodermal or sweat gland activity. Two small plates are attached to the fingers of the subject, measuring the electrodermal activity of the subject.

Second, the polygraph monitors and records the subject’s blood pressure and heart rate. A blood pressure cuff is attached to the patient’s upper arm, forearm, or leg to measure increases or decreases in blood pressure and heart rate.

Third, the polygraph monitors and records the test person’s breathing rate. Two pneumograph tubes are attached to the subject’s chest and waist to monitor his breathing in two separate areas.

When the tests lie, the activity of his sweat glands tends to increase. As a result, the subject’s blood pressure and heart rate will also increase. Counterintuitively, examinees’ breathing rate decreases when the examinee is lying. The breathing rate decreases because the test gets more oxygen in the lungs to fight or flee (flight). However, because the subject is sitting and not participating in flight (running) or fighting, the breathing rate actually slows down because excessive oxygen is not needed.

Accuracy of the polygraph

Various studies have found the accuracy rate of the polygraph to be between 87 and 90 percent. The American Polygraph Association found that when the polygraph is used for an event-specific (single-trial) test, the overall accuracy rate is 89 percent. In repeated polygraph examinations, the accuracy is 85 percent. The combination of all validated polygraph techniques, excluding the final scores, yielded an accuracy of 87 percent. (American Printing Association, 2011)

This blog is co-authored by David Young, who is a retired FBI polygraph examiner.

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