For more than a century, people have used polygraphs, sometimes known as lie detector tests. It’s been hailed as a technical wonder that makes it possible for investigators to discern with precision when a subject is telling the truth or lying. I provided an overview of the results of the polygraph’s accuracy in a prior post. I’ll give a quick rundown of the polygraph’s operation here.

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The creation of the Polygraph

William Marston, John Larson, and Leonarde Keeler created the polygraph in a series of incremental additions and improvements during the early 1920s. They combined elements that monitor blood pressure, respiration, heart rate, and skin electrical conductivity (to gauge perspiration) into a single device. These days, a polygraph is a computerized, integrated system.

The Polygraph’s Workings

The foundation of the polygraph is the idea that physiological changes in the body are a reliable result of psychological states. More accurately, when individuals lie, they frequently feel dread, shame, anxiety, guilt, and worry—especially when it comes to high-stakes criminal investigations or job applications.

The polygraph can then identify the physiological alterations in the body brought on by certain emotional states. The sympathetic nervous system is responsible for the physiological alterations, which are often known as the “fight or flight” response. People’s bodies readied themselves to deal with threats in their surroundings by either running away or facing them. The body is primed for action by the sympathetic nervous system. The skin starts to perspire, the heartbeat quickens, blood pressure spikes, and deep, fast breathing occurs.

When faced with a dangerous animal, when we barely miss a car accident on the highway, when we have to speak in front of a large group of people, or when we are afraid of being discovered lying, our fight-or-flight reaction kicks in. Even the most minute physiological changes brought on by lying are picked up by the polygraph.

Polygraph Methods

A baseline can be established if we assess someone’s physiological reactions when they are being honest and are probably not anxious or afraid. When someone is telling the truth, we can then see what their blood pressure, heart rate, breathing rate, and perspiration look like. Then, we may contrast that initial response with the one they provide in response to inquiries about which they might be lying.

The control question test is a popular polygraph technique (CQT). Relevant inquiries are posed to the examinee, such as “Did you murder your wife?” Moreover, they are subjected to a series of control inquiries (such as “Have you ever intentionally said harmful things to people close to you?”) that are likely to induce psychological distress and bodily reactions. The CQT process is predicated on the idea that guilty individuals will react more strongly to the relevant questions than the control questions, whereas innocent people will react more strongly to the control questions than the relevant ones.

The Guilty Knowledge Test is an additional polygraph method (GKT). The GKT process is based on the theory that when presented with information they are aware is connected to a crime, people would react physiologically strongly. For example, if someone is charged with killing their neighbor, the examiner may inquire, “Did your neighbor die from a knife wound? A gun? “A rope? Poison?” The theory is that the murder weapon, as opposed to the false ones, will cause a larger physiological reaction in the suspect, indicating that they had unique knowledge of the crime.

The Polygraph’s Limitations

Polygraphs are not without their detractors, despite being regularly employed for more than a century in the US and other countries. The validity and reliability of the polygraph are typically the targets of criticism. The issue with dependability arises from the fact that when numerous polygraph examiners assess the same examination findings, there is sometimes disagreement over what constitutes proof of deceit, what constitutes no evidence of deception, and what constitutes inconclusive results.

A legitimate critique is the other main one. In other words, does the polygraph truly measure dishonesty? There’s no proof that lying causes a different set of physiological responses. Heart rates can spike for a variety of causes in people. Similarly, when questioned, some liars may have no physiological responses at all.

Finally, a wealth of research suggests that polygraph accuracy often hovers around 90%. Even though 90% accuracy can sound amazing, an innocent individual who receives false polygraph results and is therefore perceived as dishonest may see things differently. Although the polygraph is a clever piece of contemporary technology, it is by no means a perfect lie detector.