Can Polygraph Test Results Be Used in Criminal Cases?

The results of lie detector tests—or polygraph tests—are rarely allowed as evidence in criminal trials. Read on to learn why courts typically distrust lie detector tests and the limited instances in which their results may be permitted.

What Is a Polygraph or Lie Detector Test?

Polygraph machines simultaneously monitor several of a subject's physiological functions—breathing, pulse, blood pressure, perspiration, and skin conductivity—while being questioned. A printout or graph shows exactly when, during the questioning period, physicological responses occurred or changed. The theory underlying a lie detector test is that lying is stressful, and that stress can be measured and recorded on a polygraph machine by monitoring changes to the subject's physiological functions.

How Do Lie Detector Tests Work?

There's more to the polygraph test than the machine itself. The entire procedure can be divided into four basic parts—a pretest, control questions, the test, and a post-test evaluation.

1. Typically, the examiner conducts a pretest a few days or sometimes weeks before the actual test. This pretest allows the examiner to gather information about the subject, test out different questioning techniques, and review the subject's responses.

2. At the start of the actual test, the examiner may use control questions to test the subject's physiological responses and compare those responses to those elicited from relevant test questions.

3. The examiner conducts the polygraph test by asking a series of questions and recording the subject's physiological responses to those questions and the answers.

4. After completing the test, the examiner must evaluate the questions, answers, and physiological responses.

Are Lie Detector Tests Accurate?

It depends on who you ask. The American Polygraph Association cites research results that suggest polygraphs are accurate between 80 to 90% of the time. A different "APA"—the American Psychological Association—argues that polygraph accuracy can't be measured because the science is theoretical. They state there's no scientific proof that stress signals that a person is lying.

Can You Trick a Lie Detector Test?

Again, the answer depends on who you ask. The American Polygraph Association acknowledges that "all things made by man can be defeated." However, they note that new sophisticated software and recording equipment make it very difficult to "beat" the polygraph. On the flip side, a paper on the American Psychological Association website suggests that various methods can be used to alter polygraph results, including adjusting one's physical movements, taking drugs, or trying to alter one's physiological responses.

Can Lie Detector Results Be Used in Criminal Court?

Not usually, and almost never when it comes to using the results as evidence of guilt or innocence. The fact that a defendant took or refused to take a polygraph test also isn't allowed as evidence.

Unreliable Evidence

To be admissible in a case, scientific evidence must be relevant and reliable. Because substantial conflict exists in the scientific community regarding the reliability of polygraph results, most judges won't allow polygraph results to be admitted as evidence. Courts also limit its use because credibility determinations are for a jury to make (not a machine). (U.S. v. Scheffer, 523 U.S. 303 (1998).)

State Bans vs. Case-by-Case Decisions

A few states have outright bans that prohibit the admissibility of polygraph results as evidence. Other states let judges decide on a case-by-case basis. If a party seeks to introduce polygraph evidence for a purpose other than determining the truth of a matter, the judge may allow it. For instance, a judge might allow a subject's answer to a polygraph question for impeachment purposes (to discredit their testimony). Sometimes, a judge may allow the results if both parties agree to it (called a stipulation) or in pretrial or post-trial motions that don't go to a jury.

Talk to a Lawyer

If you're being investigated in a criminal matter, talk to a criminal defense lawyer before agreeing to take a lie detector test. You can't be forced to take a polygraph test. And often it's in your best interests not to take the test.