EVERYONE HAS lied and been lied to in their lives. How to detect deceit, however, is a different question. Humans naturally develop a sense to identify a lie; for instance, people perceive a suspect who sweats or avoids eye contact while testifying as someone untrustworthy. Nonetheless, these clues are not enough to prove one’s dishonesty—a more validated method is needed to do so. Following centuries of research and development, a polygraph is the closest invention to a lie detector. One may be familiar with a polygraph through Vanity Fair’s Lie Detector video series, but not with its mechanisms and methodologies.

A brief history of polygraphs

   Throughout history, various techniques—some ridiculous, some cruel—have been tested to confront lies. Out of these many theories, the one agreeable assumption was that liars display an internal reaction that is detectable under investigation. Daniel Defoe, a renowned British novelist, theorized that this internal reaction is the rise in pulse rate in his essay published in 1730[1]. Later, Cesare Lombroso, an Italian criminologist, investigated the changes in pulses and blood pressure of testifying criminals to determine whether they were lying or telling the truth. Using his findings, Lombroso modified an existing scientific instrument, called the hydrosphygmograph[2], for the Italian police to measure the changes in blood pressure and pulse when questioning suspects[3].

   Today, the modern polygraph originates from the Keeler polygraph, invented by Leonarde Keeler in 1924. Keeler’s instrument measured multiple physiological responses—pulse, blood pressure, respiration, and perspiration—of a subject while they answered yes-no questions[4]. The modern polygraph assesses the same indicators; only now, the measurements are recorded on digital graphs instead of scrolling papers with inked needles.

     How polygraphs work

   Humans are empathic creatures and lying is inherently stressful. When people lie, they experience a range of negative emotions such as fear, shame, and anxiety. Especially when antagonized, the risk of getting caught red-handed comes as a threat. This triggers the sympathetic nervous system, a survival instinct that prepares the body to either fight against or flee from a threat. The heart races, blood pressure soars, breath hastens, and hands sweat. These reactions are what a polygraph evaluates using a combination of medical devices[5].

   A polygraph assesses pulse and blood pressure, more scientifically called cardiovascular activities, utilizing a blood-pressure cuff. The cuff—connected to the polygraph—is strapped around the subject's upper arm. Blood pumping through the arm generates sound, which changes pressure and displaces air in the tubes. These changes in pressure are then detected and converted into electrical signals for polygraphy software to display on computer monitors[6].

   Respiratory activity is measured using two pneumographs—rubber tubes filled with air that are wrapped around the test subject's chest and abdomen. When the chest or abdominal muscles expand during inhalation, the pressure change shifts the air inside these tubes. Similar to cardiovascular activities, this pressure change is detected using a pressure transducer that transfers the information to computer graphs[6].

   Galvanic skin resistance (GSR), also known as electrodermal activity, is the moisture level on the fingertips, in other words, how sweaty the hands are. The GSR is measured using      galvanometers through finger plates that are attached to two of the subject's fingers. These plates gauge the skin's capacity to conduct electricity, with hydrated skin exhibiting higher conductivity compared to dry skin[6].

   In addition to the three mentioned channels, various compatible tools can also be used. One commonly used type is activity sensors, which include devices like headsets, seat pads, and foot-rest pads. These sensors are incorporated to detect any movement made by the subject, whether significant or subtle. Polygraph examiners monitor body movements because it can interfere with the accuracy of the subject’s physical responses. It is within the examiner’s capacity to interpret the results[7].

Questioning techniques

   Polygraph testing employs two primary methodologies: the Comparison Question Test (CQT) and the Concealed Information Test (CIT). Each has its own distinct approach and set of advantages and limitations.

   The CQT is the more traditional and widely used method. The test begins with the subject attached to polygraph instruments and asked simple questions to establish normal signals. The questioner then asks relevant questions—that directly pertain to the crime—and control questions—that are not directly related to the crime but about moral character—to compare the signals. Truthful individuals are expected to show greater fear of control questions, as these questions evoke concerns about past honesty. A pattern of stronger physiological responses to relevant questions than control questions generally indicates deception. If there is no difference, the result is inconclusive[7].

   Advantages of the CQT method include its simplicity and ease of administration, making it a common choice for law enforcement and employers. It is also generally well-understood by both examiners and test subjects. However, critics argue that the CQT relies on the assumption that deceptive individuals will exhibit greater physiological responses to relevant questions. This assumption is not foolproof, as some innocent individuals may also experience anxiety during questioning. False positives and false negatives can occur, leading to potential inaccuracies in the test results[7].

   The CIT operates on a different principle; rather than comparing truth and deception, the CIT focuses on detecting specific knowledge that only a guilty individual would possess. In this method, the examiner presents a series of questions with multiple-choice answers, including one correct option and several incorrect ones. The questions are related to the crime or event under investigation. The idea is that a guilty person will have a heightened physiological response when presented with the correct option, as their body recognizes the relevant information[7].

   The CIT offers some advantages over the CQT. It is less prone to countermeasures because it does not rely on generalized physiological responses to truth or deception. It is also considered more accurate in some cases, particularly when the examiner has access to relevant and reliable information. However, it may not be suitable for cases where the relevant information is not known to the examiner or is ambiguous[7].

The accuracy of polygraphs

   The accuracy and reliability of polygraph testing remain a topic of debate. According to a 2020 study, the estimated accuracy stands at 87%, leaving room for both false positives and false negatives[8]. Despite not being accepted as evidence in court, polygraphs continue to be applied in law enforcement to interrogate suspects, with the Federal Bureau of Investigation (FBI) utilizing them for job applicants. The reason for this can be attributed to the perception of polygraphs’ effectiveness rather than their actual reliability. As long as people believe in their ability to detect lies, polygraph tests can elicit responses, such as anxiety or confessions[9]. However, the future efficacy of polygraphs hinges on whether public perception continues to support their use.

[1] "An Effectual Scheme for the Immediate Preventing of Street Robberies and Suppressing All Other Disorders of the Night"

[2] Hydrosphygmograph: Medical apparatus to measure pulse pressure using a water-filled cylinder and rubber-sealed tubes

[3] European Polygraph Academy

[4] “To Tell the Truth: A Short History of the Polygraph”

[5] Big Think

[6] HowStuffWorks

[7] A review of the polygraph: history, methodology and current status

[8] American Polygraph Association

[9] Vox