Do Lie Detector Tests Really Work?

The modern polygraph test is widely used, but is it accurate?

Posted Jan 14, 2020

In February of 1994, the FBI arrested Aldrich Ames, who had been a CIA employee for 31 years. Ames was arrested and charged with espionage. He was a Russian spy. For nine years, he had been passing secrets to the Russians in exchange for over $1.3 million. His spying activities had compromised dozens of CIA and FBI operations. Worse yet, his treacherous crimes had led to the deaths of several CIA spies and the imprisonment of many more.

During the time that Aldrich Ames was operating as a Russian spy, the CIA had twice given him a lie detector test. Despite having no special training in how to defeat a lie detector test, Aldrich passed both times.  

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Source: wikimedia

The modern polygraph, better known as the “lie detector test,” is a fascinating little instrument with a long and controversial history. The earliest version a polygraph instrument was developed in 1921 when John Larson cobbled together previously developed measures of respiration, heart rate, and blood pressure that had individually shown promise as a measure of lying. Technological developments continued, and the modern polygraph is now an integrated, state-of-the-art, computerized system that continuously monitors blood pressure, heart rate, respiration, and perspiration.

The theory behind the polygraph is that when people are lying, they experience a different emotional state than when they are telling the truth. Specifically, it is thought that when people are lying, especially in high stakes scenarios such as police interrogations, they are anxious or afraid of being caught in a lie. When guilty people are asked questions that would reveal their guilt (e.g., Where were you last Tuesday?), and they lie, the fear of being detected causes increased activation of their sympathetic nervous system. This activation leads to an increase in heart rate, blood pressure, respiration, and perspiration. These changes are part of the fight-or-flight system that initiates whenever was are scared. You have probably felt your heart pounding or your palms sweating when faced with danger, be it a vicious dog, an angry boss, or an upcoming exam.

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Source: wikimedia

The polygraph is designed to detect those subtle changes in a person’s physiological responses when they lie. The general idea is that when a person is being honest, their physiological responses remain stable under questioning, whereas a guilty person’s heart will race. One of the most common polygraph procedures is called the comparison question test (also called the control question test). The examinee is asked relatively benign questions such as “Where do you live.” They are also asked questions that are not relevant to the crime, but which would likely trigger an emotional reaction such as, "Have you ever told a lie?” They are then asked questions about the alleged crime such as, “Did you steal the documents?” The premise of the comparison question test is that a guilty person will have a much stronger physiological reaction to the crime question, whereas an innocent person will not.

The polygraph is used in criminal investigations, although it is generally not admissible as evidence in a trial. It is also used as a pre-employment and continuing employment screening tool for many federal employees who work in sensitive positions, such as CIA agents and FBI agents. Private businesses, however, cannot force their employees to submit to a polygraph test.

So, does the polygraph actually work? Are the results accurate? It does work much of the time. Typically, when someone is lying, a well-trained polygraph examiner can tell. It is not 100% accurate though. The American Polygraph Association is the world's leading association dedicated to the use of evidence-based scientific methods for credibility assessment. It is an organization whose members are largely polygraph examiners. They estimate the accuracy of the polygraph to be 87%. That is, in 87 out of 100 cases, the polygraph can accurately determine if someone is lying or telling the truth.

That sounds pretty impressive, but it is important to keep in mind that the polygraph is failing 13% of the time. The federal government sought an unbiased evaluation of the polygraph, so they tasked the National Academy of Sciences with a full investigation of the polygraph’s accuracy. In 2003, this large team of notable scientists came to the conclusion that the polygraph was far less accurate than the polygraph examiners had claimed. Some scientists have claimed that the accuracy may be closer to 75%. This lackluster performance is the reason why polygraphs are not used as evidence in criminal trials. They just cannot be trusted.

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Source: wikimedia

For more clear evidence that the polygraph is unreliable, just look back to the Alrich Ames case mentioned at the top of this article. Ames lied during his polygraph examinations at the CIA, and he passed each time. In this case, the lie detector test failed. When asked how he passed the polygraph test, Ames said that he followed the advice of his Russian handlers. They told him, "Just relax, don't worry, you have nothing to fear." The Russians knew that the polygraph was flawed. They knew that it was only accurate if the examinee was worried and anxious. They knew that if Ames could just relax, he would pass.