How Lie Detectors Work

Instructor: Mandy Gerhard

Mandy has taught a variety of age groups, from early childhood through adult learners. Mandy has a master’s degree in education.

Different ways to detect deception has been used by a variety of cultures throughout human history. This lesson explains how lie detectors work and explores the accuracy of lie detector results.

Lie Detection

Individuals undergoing a lie detector test have been seen in a variety of films and in television shows. You will often see someone who is hooked up to a machine with jagged lines printing on paper. What exactly are these lines and how do they determine if someone is telling the truth? In this lesson, we'll define the term lie detector, explain how lie detectors work, and evaluate the accuracy of their results.

What Is a Lie Detector Test?

A lie detector test is also known as a polygraph test. The purpose of a polygraph test is to investigate deception in an individual. In other words, it's designed to determine whether or not a person is lying. The polygraph is an instrument that records an individual's physiological response to a series of structured questions. Examiners review the changes in a person's physiological response to questions in order to make a determination as to whether or not an individual is telling the truth. Polygraph tests are used for three main reasons: investigations that are event specific (such as in a crime investigation), employee screenings, or pre-employment screenings.

How Does It Work?

The polygraph is an instrument that has sensors that are attached to the individual who is taking the test. These sensors range in number from four to six, depending on the type of machine. The sensors typically record a person's breathing rate, pulse (heart rate), blood pressure, and perspiration. The measurement of these areas is called physiological responses. Breathing rate is typically measured by a pneumograph, a coil wrapped around a person's chest that measures frequency and intensity of a person's respiration. Heart rate and blood pressure is measured by the use of a blood pressure cuff. In order to measure perspiration, or electrodermal response, electrodes are connected to a person's fingers. These electrodes can be disposable or small metal plates.

Originally, the data that was collected during a polygraph test was recorded using analog instruments. The data would print out on a moving sheet of paper with several pens moving to report the data. Due to this process, the record of an individual's responses during the actual polygraph test is called a polygraph chart. Most of the polygraph tests today use computerized recording systems to collect the data.

A polygraph examination has different phases. The first phase is a pretest phase. During this time, paperwork to administer the test is completed. The examiner also explains the polygraph test process and answers any questions the individual taking the test may have. The questions that will be used for the actual exam are also reviewed during this phase to ensure understanding of the actual questions.

The next phase is referred to as the chart collection phase. During the chart collection phase, the examiner administers questions and collects a number of polygraph charts. The number of questions asked and charts collected varies by case.

The final phase is the data analysis phase. During this phase, the information collected through questioning is reviewed and analyzed. The examiner at this point makes a decision as to whether or not deception was present. At this time, the examiner will give the individual being tested an opportunity to explain their responses to some of the questions when needed.


According to the American Psychological Association, the validity of polygraph testing is still debatable. Polygraph testing is considered very controversial. One of the main underlying theoretical issues is that a specific pattern of physical reactions has not been shown to be unique to deception only. However, the American Polygraph Association claims that the polygraph test is highly accurate.

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