Polygraphs have been around for slightly over a century, with the United States federal government officially incorporating lie detector tests into various administrative procedures in 1951.
Despite the raging debate on their legal admissibility, polygraph exams have since become commonplace in many federal and non-federal agencies. A classic use case is the Federal Bureau of Investigation (FBI), which recently announced that it’s now deploying lie detector tests to hunt internal leaks with a view to ramping up its counter-surveillance efforts.
However, for aspiring polygraphers, understanding how lie detector machines work is critical to conducting reliable tests.
Here’s a beginner’s guide to the science behind polygraphs.
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Components of the Polygraph Machine
To comprehend the science behind polygraphs, it’s imperative to familiarize yourself with the distinct components of a lie detector machine.
Lie detectors principally comprise the following hardware:
1. Sensors
There are three major types of polygraph sensors, namely GSR, Motion, and Vasomotor Sensors.
GSR (short for galvanic skin response) sensors measure fluctuations in the skin’s electrical conductivity. They’re also called electrodermal activity (EDA) sensors.
Motion sensors track variances in physical movements associated with lying, while vasomotor sensors focus on changes in the vasomotor center (VMC). VMC is a region of the medulla oblongata that works synergistically with the respiratory center and cardiovascular center to regulate blood pressure and vascular tone.
2. Respiratory Trackers
Alongside sensors, polygraph machines are equipped with devices that track fluctuations in respiratory patterns. Noteworthy ones include respiratory belts and pneumographs.
Both gadgets are worn around the rib cage, helping to detect breathing patterns synonymous with deception. The difference is that pneumographs specifically measure abnormal fluctuations in thoracic electrical bioimpedance.
3. Blood Pressure Cuffs
Blood pressure cuffs track variations in blood pressure.
They’re also known as sphygmomanometers, and wrap snugly around the upper arm.
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What Do Polygraphs Test?
Polygraph machines are designed to detect various physiological responses associated with lying, including heart rate, breathing pattern, and skin conductance. Most of these parameters are controlled by the autonomic nervous system (ANS), a branch of the central nervous system (CNS) that regulates certain involuntary bodily functions.
The ANS falls into two branches, namely the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS).
Studies have shown that the SNS is primarily activated during stressful situations. It’s the ANS sub-branch primarily responsible for regulating the ‘fight-or-flight’ response. In contrast, the PNS promotes mental relaxation and recovery.
Polygraphs target both sympathetic and parasympathetic divisions of the autonomic nervous system. However, the tests specifically track physiological changes synonymous with deception.
Research suggests that activating the ANS can trigger significant variances in skin conductance response (SCR). Anomalous patterns are flagged by the different polygraph sensors, enabling examiners to infer whether the subject is being truthful or deceptive.
Additional findings indicate that ANS arousal may increase cardiovascular activity, substantially increasing heart rate.
Fluctuations in breathing rate may also provide vital clues into an examinee’s credibility. Using pneumographs, examiners can monitor subtle changes in the thoracic girth with each response, indicating the levels of truthfulness.
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Phases of a Polygraph Test
1. Pre-Test
Before conducting the lie detector test, a polygraph examiner must make initial contact with the would-be examinee. The objective is to notify the examinee of the upcoming tests and obtain informed consent.
Pre-test screening also provides an opportunity for polygraphers to establish the examinee’s physiological baselines, which are critical in compiling accurate reports.
2. In-Test
The in-test phase constitutes the actual polygraph test.
Typically, a polygrapher will usher the examinee into a room and go over a few preliminaries to remind the individual about the implications of the process they’ve submitted to. The examinee may be required to reaffirm their consent to and readiness for the test.
Next, the examiner will strap various sensors to relevant parts of the examinee’s body before asking a series of questions. Polygraph questions fall into three primary categories:
- Relevant Questions – Directly address the topic under investigation.
- Irrelevant Questions – Are unrelated to the topic, and mainly help to reestablish the examinee’s baselines.
- Control Questions – Reference similar but less serious past misconducts.
3. Post-Test
During the post-test phase, polygraphers compile a comprehensive report based on the examinee’s responses to the questions. Findings are quantified in percentages, denoting the likelihood of truthfulness.
Before the compilation of a full report, the examinee may be asked to explain some of their responses.
Interpreting Polygraph Results
Polygraphs borrow from the theory that most examinees will lie about crime-related questions than other sets of queries. As such, an inference of deception is reached when there’s a significant pattern in ANS arousal for relevant questions.
A greater pattern of response to control than relevant questions implies a higher likelihood of truthfulness.
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The Bottom Line
Polygraph evaluations are not a foolproof way of establishing guilt or innocence. Instead, results from these tests should strictly be used to impeach or corroborate other pieces of evidence.
It’s also prudent to underscore that polygraphs aren’t 100% accurate. While the American Polygraph Association (APA) established standards in 2010 that require lie detector tests deployed in specific issue testing to achieve a minimum accuracy score of 90%, multiple factors can cause a polygraph examination to fall below this benchmark.
The best strategy for improving the accuracy of polygraph tests is to have them undertaken by a duly accredited and extensively experienced examiner.
