A lie detector test is a device that measures the involuntary physiological changes of a subject’s body as the subject responds to a question or statement. Polygraphs are the most popular lie detector tests used in the United States. Polygraphs measure physiological arousal factors, including heart rate, blood pressure, respiration, perspiration, and skin conductivity. The theory of the lie detector test is that these physiological responses will be different when the subject is truthful versus when the subject lies. Therefore, the term ‘lie detector test’ is a misnomer of sorts, since the test does not speak to the truth of the answer or statement, but rather to the fear, stress, and anxiety of the subject as they answer.
The administrator of the test first differentiates between the subject’s physiological response to answering a question honestly and answering dishonestly, and then looks for those responses as a series of questions are asked. Different examiners utilize different questioning techniques. Usually, the subject of the test will be asked direct questions and control questions and their response will be observed as they answer. Alternatively, the subject’s response may be observed as they hear and react to statements or information that are read out loud to them.
The use of lie detector tests such as polygraphs is controversial as their validity can be questionable. Notably, the test is incapable of differentiating between the physiological factors that often accompany dishonesty, and those factors that may simply be a result of the subject’s anxiety at undergoing questioning. Because there is no uniform psychological and physiological response to dishonesty, someone lying could be very calm while they answer, while another person answering truthfully could be very anxious. Moreover, countermeasures exist that may be employed by subjects to help them ‘beat’ the test.
Due to the unreliable nature of lie detector tests, the results from these tests and the inferences of the examiner are generally inadmissible as evidence during a trial. Most state and federal jurisdictions have per se rules against the admittance of lie detector evidence. Some states allow admittance of lie detector evidence only if there is a stipulation between the parties agreeing to the admittance. Although the Federal Rules of Evidence do not explicitly bar lie detector evidence in federal courts, such evidence is nonetheless blocked in most circuits by Rule 702, as the science behind lie detector tests is unreliable and prone to error. The Supreme Court decided in United States v. Scheffer that the exclusion of lie detector evidence does not violate a defendant’s right to a fair trial, even if the defendant wishes the evidence to be admitted.
Although the results are usually inadmissible as evidence, lie detector tests can nonetheless be useful in interrogations during criminal investigations. Many confessions have been made after a criminal suspect fails a lie detector test. However, any results of a lie detector test conducted on a criminal defendant that become publicized can be very influential on public opinion of the case, especially if picked up by the news media.
Federal and most state law bans employers in the private sector from requiring employees and prospective employees to submit to a lie detector test. If an employee decides to submit to a test, these laws also prohibit an employer from taking adverse employment action against an employee based upon the results. Some states allow exceptions for embezzlement concerns, in which case the testing procedure is heavily regulated and often monitored. The government may use polygraphs as part of personnel screening for employment, especially for positions dealing with national security.
[Last updated in June of 2020 by the Wex Definitions Team]