The polygraph is not a lie detector, but a device that measures physiological changes in a person in response to the examiner’s questions. A heightened physiological response to a given question can help detect deception. The physiological changes shown in 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 freeway a little faster than the speed limit and suddenly see a police car behind you with its lights on, your autonomic nervous system is likely to kick into gear. 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 getting ready to fight or flee. The polygraph is based on the same concept. The polygraph captures the physiological changes a person undergoes when they are afraid of being caught in a lie.
consequences of lying
For the polygraph to be true, it must have consequences for the person who lies.
We all tell little white lies in our lives that probably wouldn’t impair our autonomic nervous system. For example, if someone’s spouse asks you if you liked a meal that you worked very hard to prepare, you are likely to say that you did, even if you didn’t think the food tasted good. This is to avoid hurting your partner’s feelings and appearing ungrateful.
The fear of being caught must have significant consequences to achieve the most accurate polygraph results. Job seekers fear that they will not get a job if they are caught lying. For current employees (Personnel Security Control), the fear of losing security clearance, employment, or becoming the target of a criminal investigation is very important. For individuals in criminal or counter-intelligence and counter-terrorism investigations, there is the fear of being charged and convicted of a crime and possibly imprisoned for lying. All of these implications will provide the examiner with sufficient discovery.
Answers recorded in polygraph
Polygraphs record three responses from the autonomic nervous system. First is electrodermal or sweat gland activity. Two small plates are placed on the probe’s finger to measure its electrode activity.
Second, the polygraph monitors and records the examiner’s blood pressure and pulse. A blood pressure cuff is placed on the examiner’s upper arm, forearm or leg to measure the increase or decrease in blood pressure and pulse.
Third, the polygraph monitors and records the subject’s breathing rate. Two lung tubes are attached to the examiner’s chest and waist to monitor his breathing in two separate areas.
When a probationer lies, the activity of their sweat glands tends to increase.
As a result, the blood pressure and heart rate of the subject increases. On the other hand, the examinee’s breathing rate decreases when he is lying down. Breathing rate decreases because the subject takes more oxygen into the lungs for fight or flight. However, since the subject is sitting up and not fleeing (getting away) or fighting, the breathing rate actually slows down because the excess oxygen is not needed.
Various studies have found that the accuracy of the polygraph is between 87 and 93 percent. The American Polygraph Association determined that when the polygraph is used for a case-specific (single problem) test, the overall accuracy rate is 89 percent. In multiple polygraph tests, the accuracy is 85 percent. Combining all accepted polygraph methods, excluding results, achieved 87 percent accuracy. (American Polygraph Association, 2011)
This article was written in collaboration with retired FBI polygraph examiner David Young.