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Lie detector tests – Can you beat a polygraph?

Lie detector test has been used in law enforcement and national security for nearly a century. To detect lies, the machine monitors heart rate, blood pressure, and breathing. While supporters argue polygraphs accurately detect deception, critics suggest the tests are beaten using countermeasures.

Polygraphs measure changes in physiological responses that may indicate lying. The theory is that telling lies causes greater stress than telling the truth, which will produce detectable physical reactions. Polygraphs record cardiovascular activity, respiration, and sweat gland activity during an interview. Trained examiners look for significant fluctuations that may signal deception. Polygraph tests begin with a pre-interview, where the examiner explains procedures and asks basic questions to establish a baseline. During the actual test, the examiner asks diagnostic questions — some irrelevant, some relevant to the investigation. By comparing reactions, the examiner gauges whether the subject was likely truthful or deceitful.

Countermeasures to beat the polygraph

Those motivated to beat the polygraph employ various techniques before or during the lie detector test, aimed at decreasing physiological responses to questioning. Some simple countermeasures include:

– Regulating breathing to alter blood pressure/pulse readings

More sophisticated techniques train subjects to artificially augment reactions to control questions, making the diagnostic questions seem non-deceptive by comparison. Physical countermeasures include placing pins inside a shoe to inflict pain during control questions. Mental countermeasures range from imagining a stressful event after irrelevant questions to silently counting backward to induce mental distraction. Retired FBI agent Jack Trimarco, who now trains people to beat polygraphs, claims specific countermeasure training helps deceptive people pass around 90% of the time.

However, polygraphers are trained to detect these tactics and use follow-up questioning to catch those using them. Examiners look for unnatural breathing patterns, obvious attempts to fidget, and illogical reactions that suggest countermeasures. Admitting countermeasure use is generally interpreted as deception. While countermeasures highlight flaws with polygraph testing, they do not seem to provide a foolproof way for liars to beat the machine. Sufficient motivation, training, and careful implementation may help evade detection, but not without considerable effort.


Given the unreliability of polygraphs, researchers continue seeking alternative ways to accurately detect lies. Possible techniques include functional magnetic resonance imaging (fMRI) to visualize brain activity changes during deception. However, fMRI tests are costly and remain unproven for field investigations.  For now, polygraphs continue being used for their potent psychological deterrent effect during investigations, if not their scientific precision. But those determined to beat the machine still have some options to try. Ultimately, the long quest for an infallible lie detector continues.

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