Lie detector tests, also referred to as polygraphs, are used to try to determine whether a person is or is not telling the truth based on measuring his physiological responses as he answers a series of questions. Examples of the physiological responses that are measured by polygraphs include a person’s blood pressure, his heart rate and pulse, his breathing patterns and his perspiration rate. While lie detector tests are commonly used to interrogate possible suspects in a crime, whether or not the results can be used in court will be up to the judge overseeing the case.
One of the reasons that whether or not the results of a polygraph will be admissible in court is that the results of these tests are not always reliable. While people can “cheat” the test by practicing being comfortable with lying (which means that their physiological responses won’t change when they lie), alternately, people who are already nervous about taking a lie detector test will display signs of anxiety and agitation during the entire test (which means that there would be no difference in the person’s physiological response when he is telling the truth versus lying and that the results of the test would be inconclusive).
When deciding whether or not to admit the results of a polygraph into evidence, as with other scientific tests, the party who wants these results to be admitted (i.e., the prosecutor or the defense lawyer) has to be able to prove that the theory backing the test has been tested, peer reviewed and subject to publication. Additionally, the party will also have to verify that the test has a known rate of error and that this type of testing is widely accepted by within the scientific arena. Although people who support use of polygraphs claim that these tests have about a 90 percent rate of validity (i.e., a 10 percent error rate), the National Research Council has not uncovered any evidence that these tests are effective or reliable, and such testing consistently has proven to have poor results when it comes to interrogating sex offenders. Perhaps even more damning than this is a ruling issued by the Supreme Court in 1998 that stated that, “there is simply no consensus that polygraph evidence is reliable… a polygraph expert can supply the jury only with another opinion.”
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