Criminal Law

Are Lie Detector Tests Admissible in Court?

One of the greatest challenges for the judge and jury in the courtroom is determining which witnesses are telling the truth. This makes witness believability vitally important at a trial. While witnesses must promise to tell the truth before taking the witness stand, judges and juries often hear conflicting testimony that indicates someone is lying. So, a lie detector test could theoretically help the jury determine the truth, but only if the polygraph test is both reliable can be used as evidence in court. It turns out that neither is true: Polygraph tests have questionable reliability and are generally not admissible as evidence in court, although they can be used in investigations and in applying to some federal employment positions.

Are Lie Detectors Accurate?

It might seem like the technology of the polygraph, also known as the lie detector test, could take the guesswork out of determining who is lying and who is telling the truth. That's how many television crime dramas made them look. However, polygraph tests are not always accurate. The accuracy of lie detector test results can vary depending on the person administering the test, the machine used, and the person taking the test. As such, polygraph results are generally not admissible in criminal cases unless both parties agree to it. Even then, some states do not allow polygraph tests to be used as evidence, and often a trial judge has discretion to allow (or not) a lie detector test. Those who are opposed to allowing the results of lie detector tests to be used in court argue that jurors might accept the test results without considering the test's accuracy. Jurisdictions that allow the results of lie detector tests in court also allow each party to present evidence as to why the test is or is not reliable.

What States Allow Polygraph Tests in Court?

The United States Supreme Court leaves the question of the admissibility of lie detector test evidence up to individual jurisdictions. Some courts allow lie detector evidence in certain proceedings or only when both parties agree to its admissibility. Other jurisdictions do not allow any lie detector evidence. The states that sometimes allow polygraph tests as evidence in criminal case include:
  • Alabama
  • Arizona
  • Arkansas
  • California
  • Delaware
  • Georgia
  • Idaho
  • Indiana
  • Iowa
  • Kansas
  • Nevada
  • New Jersey
  • New Mexico
  • North Dakota
  • Ohio
  • Utah
  • Washington
  • Wyoming

The Argument for Lie Detector Admissibility

Those who think polygraph evidence should be used in court say that the tests are reliable most of the time and, therefore, useful information. However, many experts disagree with the assumption that lie detector tests are reliable in most situations. Accordingly, in leaving the decision up to individual jurisdictions, the Supreme Court commented that there is no reliable scientific evidence about the accuracy of lie detector tests. Still, the Supreme Court has not forbidden it outright.

How a Lie Detector Works

Essentially, the polygraph machine is attached to the person's body, and the examiner asks the person a series of questions. The polygraph test measures the person's physiological responses, such as rising blood pressure, to the questions to determine if they are telling the truth. How can this lead to a wrong result? As one example, a person could be nervous and worry about the results of the test even if they are telling the truth, leading to higher blood pressure readings and causing the machine to think they are lying.

Should You Take a Polygraph Test?

Although lie detector results are not always admissible in court, many attorneys and law enforcement officers continue to use the technology when questioning witnesses and suspects. If law enforcement administers a polygraph test to a suspect, they also need to issue the Miranda warnings before conducting the test. You should never agree to take a lie detector test without first consulting with a criminal defense attorney if police want to question you about a crime. Even if police cannot use the test results as evidence against you, they may use it in building a case. Just like you should always take advantage of your right to remain silent and right to an attorney, you can refuse to take this test.