A polygraph test, more commonly known as a lie detector test can be used in a criminal defense case to help exonerate the defendant. When someone who has been accused of a crime willingly submits to a lie detector test and passes it helps to establish their innocence and can be a powerful tool in getting a case dismissed.

How Does A Lie Detector Test Work?

A polygraph is an electrical machine that a person is hooked up to. The device is capable of measuring biological changes in a person depending on how they answer a question. It will observe blood pressure, pulse, respiration, and skin conductivity. When a person lies there is a physiological blimp in their system which the machine will take note of. If they are being truthful about the answers they are giving the machine, in theory, will show this.

In the event of a criminal defense case the examiner would ask the defendant specific questions about the crime committed, particularly if they had committed the crime. If the answer is no and the test shows truthfulness, then a criminal defense lawyer can submit these results to the prosecutor in the hopes of getting the case dismissed.

If the defendant’s answers show them as untruthful the defense lawyer would not use the results as a defense strategy.

Why Not Always Use A Polygraph In Defense Cases?

If polygraph machines can find lies then it could be suggested, why not just hook up every person charged with a crime to a polygraph to see if they are telling the truth and determine guilt that way? Although polygraph machines are capable of tracking the physiological changes in a person that are congruent with lying they are not 100% reliable. There are false positives or ways to ‘cheat’ the machine. As such, these tests are not admissible as evidence in a jury trial. A prosecutor must agree with the defense attorney to allow the results to be admitted as evidence.

When To Use A Polygraph Test in A Criminal Case

There are three distinct uses for a polygraph test in a criminal defense case.

To Get A Case Dismissed - If a defense lawyer is looking to use polygraph test results to get a case dismissed they would want to have the lie detector test conducted as early on in the case as possible, typically during the pretrial process.

To Convince The Prosecutor - There is a high probability that polygraph test results from one test will not be enough to persuade a prosecutor to drop charges, however if necessary a defense lawyer can press the issue offering to have a second or even third test conducted and with the polygrapher of the prosecutor’s choosing. If a defendant is found truthful with the use of a state’s polygrapher the chances of their case being dismissed becomes much greater.

To Get A Plea Bargain - If a lie detector test is conducted and the defendant ‘fails’ and it implies that they are guilty of the crime in question it can be a valid strategy to use these results to plead guilty but request a reduced charge. This could help a defendant from going to jail or being found guilty of even greater charges.

Can You Fail A Lie Detector Even Though You Are Telling The Truth?

Unfortunately, it is quite possible to fail a polygraph test even if a person is innocent and telling the truth. Because the polygraph machine is designed to measure changes in a person’s biology such as blood pressure or perspiration it is possible for it to misread someone’s body changes as a lie when they are actually anxious. Sitting hooked up to a lie detector machine can be a stressful experience for anyone and this stressed situation can cause the machine to find a false positive of guilt even though the person could be innocent. This is why the results of these tests do not automatically have to be used in a case if the outcome is not what the defense lawyer hoped they would be.

The decision to use a polygraph test in a criminal defense case is a strategic one. Defense lawyers may choose this route to aim for case dismissal, convince the prosecutor, or negotiate a plea bargain. When opting for a lie detector test, timing is crucial, with pretrial examinations being the most effective. Furthermore, it may be necessary to conduct multiple tests with the participation of the prosecutor's chosen polygrapher to bolster the credibility of the results. While polygraph’s can be valuable tools in certain circumstances, the potential for inaccuracies and misinterpretation underscores the need for caution. As a result, lie detector tests are just one of many tools at the disposal of defense attorneys, to be used judiciously and with an understanding of their limitations.