How To Pass A Polygraph Test With Science


Robin Andrews

Science & Policy Writer

3455 How To Pass A Polygraph Test With Science
How good are you at wearing two faces? Ollyy/Shutterstock

Are you a good liar? If so, would you be confident enough to undergo a polygraph test to prove this? Although these devices have appeared in countless films and television shows, their accuracy is in fact highly debatable. The science of lying, and the detection of deception, is far more complex than a few jagged lines on a computer monitor.

The Birth of a Lie


Image credit: Taking biological readings of a subject during a polygraph test. Andrey Burmakin/Shutterstock

Lying is a complex web of biological actions. To start with, a neurological process begins in the brain with the formation of the lie. Blood is sent to the prefrontal cortex (PFC), a part of the brain located just above the eye sockets. The PFC is thought to be associated with planning complex thought processes and making decisions based on new, not old, information, among other things. It is linked to making executive decisions – actions which override all others.

This part of the brain determines what is “good” and “bad” in regards to decision-making, in terms of what may or may not benefit the host. In effect, it is where lying crystallizes. “Lying is a... type of executive response because it involves withholding a truthful response,” said the late Sean Spence, a professor of general adult psychiatry at the University of Sheffield in England, as reported by Scientific American. “When you know the answer to a question, the answer is automatic; but to avoid telling me the true answer requires something more.”

The Detection of Deception


Image credit: Pupil dilation is sometimes measured during polygraph tests. Tomazino/Shutterstock

The biological responses to a lie are involuntary. An increase in heart rate and breathing rate occurs while lying, a classic “fight or flight” response aiming to increase the supply of oxygen to the brain and muscles, preparing to escape trouble or gear up for combat. Your pupil dilates (increases in size) in order to take in more light and see the world around you in more detail.

Your voice patterns also change during lying: for example, rapid speech and suspiciously quick responses are sometimes associated with lying. Finally, you sweat more during lying, a cooling mechanism responding to our increased body temperature due to increased overall blood flow.

The most advanced polygraphs are used to detect all these changes in the human body; simple machines tend to focus on heart or breathing rate only. After establishing the “baseline” measurements of the aforementioned characteristics by asking innocuous questions and observing the subject’s responses, a series of questions related to the crime are then asked by an examiner. Any spikes in activity way above the baseline are supposed to be indications of lying.


A Fatal Flaw

Image credit: There’s a vital difference between deception and anxiety. Filip Obr/Shutterstock

Spence believed that the traditional polygraph test is flawed, precisely because those being interrogated are under constant duress. “[Polygraphs] are not detecting deception but rather the anxiety of being…[accused of deception],” Spence said to Scientific American. Psychopaths – impulsive, poorly-empathetic individuals with abnormal social behavior – tend to mask or not experience anxiety even under pressure, which would allow them to fool a polygraph.

He’s not alone; there has been a huge backlash against polygraph testing for the same reason: It cannot distinguish accurately between anxiety and deception when the subject is under duress, and the results of each test are highly subjective and analyzed by an often biased examiner who intentionally oversells the accuracy of the device. Human emotions and intent cannot be directly read by measuring the changes in heart rate and blood pressure: a classic example of correlation not always equaling causation.


False positives – readings that indicate a person is lying when they actually aren’t – are commonplace; an honest, innocent person may not show any strong reactions at all during the control question part, but will probably exhibit a strong reaction when asked far more serious questions during the actual interrogation.

Most importantly, it also appears that polygraphs can be beaten: Being aware of the test’s mechanisms appears to undermine their ability to detect deception. If a subject understands that spikes in these involuntary bodily characteristics are used to pick up on lying, then they could confuse the examiner by altering the baseline readings. If they appear to be nervous throughout, sweating and have a high heart rate – perhaps by causing themselves a hidden minor injury at the start of the process – then any spikes caused by genuine lies won’t be detected among the already high readings.

As a result of this backlash, polygraph tests are not admissible in many U.S. courts.

The Cartography of a Lie


Image credit: fMRI scans of a human brain. Triff/Shutterstock

Blood flow within the brain can be traced using functional magnetic resonance imaging (fMRI), and Spence and his colleagues demonstrated that when a person lies, different parts of their brain – including the PFC – show actively increased blood flow. The fMRI, Spence said, images the actual processes involved in deception, unlike the traditional polygraph.

According to additional studies by psychologists and psychiatrists at the University of Pennsylvania, it’s not just the prefrontal cortex that becomes active during lying. While being hooked up to an fMRI scanner, participants were given a variant of the Guilty Knowledge Test (GKT), an examination meant to test a subject’s awareness of the specifics of a (real or simulated) crime that would not be known to an innocent person.

Participants were given envelopes with playing cards and asked to lie about which card they had. They were then provided with a series of images of playing cards, including the one they had in their pocket.


They were allowed to press buttons for “yes, that’s my card” and “no, that isn’t my card” for each image that appeared on the screen; when they lied, the fMRI machine picked up the regions of their brain that involuntarily showed the most activity. Using this, the researchers were able to detect lying 78-85 percent of the time.

Hidden Markers

Image credit: Are there neurological markers that always flare when a lie is told? Vitstudio/Shutterstock

This separate fMRI study showed that two other regions – the inferior frontal gyrus and the inferior parietal lobe – are just as active during lying. Collectively, these sections make up what is known as the prefrontal parietal network (PPN), the part of the brain often thought to deal with consciousness; some studies argue that it evolved to provide innovate solutions to complex, novel problems – one of which could be the ability to lie effectively.


These sections of the brain associated with lying appear to be ideal markers for security personnel to look for in real-life suspected criminals during deception detection. However, the studies do point out that being told to intentionally lie is not the same as making the choice to lie independent of instruction. 

So in terms of neurological markers of lying being used in actual criminological interrogations, there’s a long way to go yet before they’re shown to be viable.

Image credit: luckyraccoon/Shutterstock


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  • fMRI,

  • crime,

  • neurological,

  • polygraphs,

  • lie detector,

  • interrogation,

  • legal