A lie detector test, often called a polygraph, measures a person's physiological reactions when asked a question. The tests attempt to show when a person gives a deceptive or false response. Employers, criminal investigators, as well as attorneys often use these tests for different purposes.
There are two main groups who ask people to take lie detector tests: criminal investigators and employers. If you are ever asked to take a lie detector test, regardless of who is asking, you need to consider the circumstances of your situation and make your decision only after consulting with a qualified attorney.
The typical polygraph test measures a person's breathing rate, pulse, blood pressure, perspiration, and possibly other physiological phenomena. A tester usually administers the test in a room where only the tester and the subject are present.
The tester will often begin the process by asking the subject a series of questions before connecting the subject to the polygraph machine. After connecting the machine, the tester will then ask the subject a series of questions. The tester might also explain the conditions of the test and collect basic information about the subject's physiological responses. As the tester asks the subject questions, the polygraph machine records the physiological changes the subject exhibits.
The tester analyzes the subject's responses as questions are answered. The theory behind these tests is that a person who is lying will exhibit typical physiological behaviors, such as an increased pulse and perspiration, while a truthful person will not. But we all know that nervousness is not confined to the guilty, and many times, the results of lie detector tests are inaccurate, in both directions.
In any criminal investigation, the state must be able to produce enough evidence to convince either a court or jury that the accused has, beyond a reasonable doubt, committed a crime. In order to prove its case the state—through its police, prosecutors, and investigators—has to gather evidence. Evidence typically comes in the form of testimony, photographs, video recordings, forensic evidence, and sometimes from a polygraph test.
If criminal investigators ask you to take a polygraph test, it's safe to assume they are trying to gather evidence, usually against you. Occasionally, a suspect will ask to take a test in order to establish his innocence.
You are never under any legal obligation to take a lie detector test in a criminal investigation. Even if the police tell you the test is mandatory or threaten to arrest you if you refuse, you don't have to. And volunteering for a test to prove your innocence can be risky, because the results of the test are not guaranteed to be accurate.
Unless both parties agree to allow in the evidence, the actual results of the polygraph tests (your physiological responses, and the inferences that the tester will draw from them) are not usually admissible in a criminal case. Their admission depends on where the case is brought (which state, or which federal district if it's a federal case). The states and the federal courts use different legal tests to determine whether polygraph results are admissible. Most courts have found polygraph evidence to be too unreliable to be admitted in a case. Some states have per se rules against their admissibility.
But, the important point to remember about lie detector tests is that they are processes designed to gather evidence against you. Whether that evidence comes from the pre-test questions, the post-test interview, or the test results themselves, all of it is designed to gather enough evidence so the state can pursue its investigation, charge the case, and convict you of a crime. Even if a court refuses to allow the results of the test as evidence in a trial, the prosecutor can use the results and statements you make in the pre or post-test interview to assist him in developing the case against you.
For example, a polygraph tester might ask a subject a series of questions while the subject is connected to the machine, but also after the machine has been disconnected and turned off. Subjects often feel much more comfortable after they believe the test to be over and will sometimes, during this post-test interview, make statements or omissions that the investigator can then later use in court.
Some employers give employees lie detector tests if, for example, those employees have access to sensitive or classified information. Other employers might ask employees to submit to a lie detector test if someone has stolen money from the company, or as a condition of employment during the hiring process.
In general, employers cannot use polygraph tests to screen potential employees or to fish for information from current employees, but they can use the tests if they have a good, work-related reason and the tests are narrowly focused.
For more information on workplace testing, see Workplace Testing: What Your Employer May Require and State Laws on Polygraphs and Lie Detector Tests
There is no clear answer when it comes to deciding whether to take a polygraph test, whether you've been asked by the prosecutor or would like to take a test to establish your innocence. Either way, it's vital you speak to an experienced criminal defense attorney as soon as possible. If, after consulting with your attorney, you decide that you want to take a polygraph, it's always better to hire your own test administrator instead. You need to speak to a criminal defense attorney as soon as you are questioned by criminal investigators, even if you don't believe you have done anything wrong.