Is a Polygraph Test Admissible as Evidence?


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Have you ever wondered why, in a system of justice that relies so heavily upon people telling the truth, every witness is not strapped to a polygraph machine (i.e., a lie detector)? It is a logical question that leads to others about how interrogations and investigations are conducted when polygraphs are used. So, is a polygraph test admissible as evidence?

In virtually every jurisdiction, the answer is a resounding no. Though called lie detectors, the reality is that a polygraph machine does not have any capacity for detecting the truth or falsity of a statement. Rather, it measures a person's biological processes to determine if they are experiencing a physiological event, such as an increase in blood pressure or heart rate. These conditions are considered to be indicators that someone may be lying, as the increased stress of telling a falsehood creates a subtle, but measurable change in one's vital readings.

Unfortunately, dozens of other factors can also affect the readings detected by a polygraph machine. For instance, nervousness of any kind could read with the same increase in activity as a lie. This nervousness could simply be caused by being concerned about the test giving a false positive, creating a self-fulfilling prophecy. In fact, some polygraph administrators, particularly in law enforcement, are given training on how to induce a false positive response or to ask questions in a fashion that makes them difficult to answer with a yes or no. On the other hand, those taking the test have reported being able to evade the detection of a lie by using various techniques such as faking a cold, squeezing the muscles of one's posterior, and so forth. These techniques would cause a reading on the polygraph that would show increased physiological activity even when the person is not lying, making it difficult to detect any variation when the person does tell a falsehood.

As a result, polygraphs have been successfully challenged on several occasions in various jurisdictions on the basis of their scientific uncertainty. Because the results of a polygraph test can mean many things and are so unreliable in detecting actual lies, they do not rise to the level of reliability required for scientific evidence in a courtroom and polygraph test results are usually inadmissible as evidence.

This does not stop many from asking that someone undergo a lie detector test though, often as part of a criminal investigation. A little known trick of interrogators when administering a polygraph is to convince the person undergoing the examination that the lie detector has shown results that do not bode well for the subject, and that he or she should simply come clean. This induces a large number of people to confess, even though the lie detector test may have shown no falsehoods or been inconclusive.

Of course, as with most things in a courtroom, every rule has its exception. In the rare instance that both parties agree that the results of a polygraph exam should be admissible for some reason, the court could allow it as evidence. Polygraphs are also commonly used as part of the screening process for certain types of jobs, such as law enforcement and some high level security positions. Nevertheless, for purposes of court procedures, absent a stipulation of the parties, the results of a lie detector test are likely never admissible. If you are involved in a case in which the results of a lie detector test are being used against you, you should immediately contact an attorney to assist you.

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Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.

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