How to Read a Polygraph Test

Have you ever watched someone take a polygraph (lie detector) test in a movie or on reality TV and wondered how it works? If you apply for a job with the FBI, you'll undergo a lie detector test yourself. The theory behind polygraph is that certain physiological responses reveal whether someone is answering questions with the truth or lies.

How Polygraph Works

Polygraph is the field of forensic science that is able to determine truthful or untruthful responses to questions by monitoring a subject's cardio, sweat glands and breathing reactions when a trained polygraph examiner asks a subject carefully devised questions. Everybody has an autonomic nervous system that lets their body's organs be separately stimulated or relaxed to cope with life's demands and stresses. The response to fear is a type of autonomic response. In most cases, lying increases heart rate, causes sweating and alters breathing patterns.

Read More: Conditions That Can Cause a False Positive on a Polygraph Test

Taking a Polygraph Test

An accurate polygraph test takes up to two hours and should be carried out by a trained examiner. Several sensors are attached to the person taking the test. The signals from the sensors are recorded by a polygraph machine on a single piece of moving paper. Most polygraphs record the person's breathing rate, pulse, blood pressure and perspiration. Some polygraphs also record arm and leg movement. At the start of the test, the examiner asks the person a few basic questions known to be true and false, to set the norms for her signals. For example, if the answer to the question "Are you an alien?" is "Yes," this provides the reading for how she reacts when lying. The answer "No" to the question "Are you the President of the United States?" provides the reading for how he reacts when telling the truth. These serve as the base readings. The examiner then asks the real questions, which are based on a central issue devised to manipulate a person's autonomic fear response. The process is repeated several times to ensure accuracy, and the results of different categories of questions are combined and compared to reach an overall conclusion of truthfulness or deception.

Reading a Polygraph Test

The process of reading a polygraph chart is an exact science requiring formal training from a polygraph school. However, when you understand the basic principles, it's possible to distinguish truths from lies. In general, a significant change like a faster heart rate, higher blood pressure or increased perspiration indicates that the person is lying. During the test, this is shown as a large line drawn on the paper. A smaller line (or no line at all) signifies a truth. However, stressful memories or fear can also cause large lines, even when the person is telling the truth. A polygraph never gives 100 percent accuracy, but experienced, trained examiners can use their professional judgment as well as the test results to reach a highly reliable conclusion.


  • Formal training from a polygraph school is required to read a polygraph test with the highest possible level of accuracy, but knowing the basics of how the test works lets you recognize truths and lies.

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