How Does a Modern Lie Detector Work?

Although not admissible in court, the polygraph is a vital tool in the legal field.

Lie detector tests, also known as polygraphs, monitor various bodily functions to determine if an individual is lying. This technique works to help a detective or other law officer to determine if the individual is lying about a detail and might cause the person to admit to a lie or crime. This technique is used by lawyers, police investigators and probation officers to determine the truth.

Components

A modern polygraph, or lie detector, has four primary components. The pneumograph measures the respiration speed and depth of the person being tested; this is typically a band that goes around the person's chest. The galvanometer measures the person's electrical output; this is done through a small electrode attached the person's pointer or ring finger. The blood pressure cuff or a cardiograph measures blood pressure and heart rate. The machine takes and registers all of this information into a chart format, allowing the results to be constantly monitored.

Control Question

The tester will ask several questions that have clear yes or no answers. The tester will ask the tested individual to reply yes or no to these questions, with some of being true and others deliberately false. This will provide the tester with a comparison basis upon which to gauge the truth of falsehood of the rest of the questions.

Questions

The questions for the tested individual must be done in a specific manner. This includes references to the crime or action at least once every three questions, which must have a yes or no answer. This is called relevance questioning and is used to trigger subliminal changes in a person's breathing, heart rate and blood pressure. Any additional wording in the answer can cause problems with the machinery and the tester's ability to read the results.

Detecting the Lie

The theory behind the lie detector test is that the subject under examination will subconsciously change the way he is breathing, his heart rate and might even sweat when comforted with a truth that he must deny. This works by promoting a fear of the lie and of getting caught in the lie. Although the machine is not able to determine if the statement was a lie, the tester can compare the results from previous questions to see a change in vital signs. This is not a full-proof method, and mistakes do occur.

Non-Admissible Evidence

As of early 2010, the results of the polygraph, or lie detector, are not admissible in the court of law. This means that despite use of the machine, there can be no action taken against a person because of the results, and the courts will not hear evidence regarding the results. The exception to this is if the subject made a confession while undergoing the lie detector test.

References

• University of Mary Washington: Modern Lie Detector
• Anti-Polygraph: How to beat the lie detector
• Illumin: Lie detection