Lie detector, or polygraph tests, are somewhat misleading because there is no machine capable of definitively determining whether a person is lying. Instead, a polygraph measures a person's vital signs to gauge physiological reactions to a battery of questions. The results are then interpreted against benchmarks, and a decision is made as to the person's truthfulness. Unfortunately, people react differently to questioning and stressful situations, so there is always the risk of a false-positive test result, meaning that someone telling the truth can be accused of lying.
Understand how the test works. Lie detectors monitor respiration, perspiration (sweat), blood pressure and pulse rate. When most people lie, their blood pressure and pulse rate rise briefly and their breath becomes shorter. These changes occur immediately after a person answers a question.
Learn relaxation techniques to control your heart rate and breathing, such as self-guided meditation. Taking a lie detector test can be a stressful and intimidating experience. If you are nervous and sweaty, the polygraph machine may produce results that indicate that you are lying, even if you are not.
Prepare for the test by knowing that a majority of the questions you will be asked are simply "control questions" that have nothing to do with the investigation at hand. A classic control question used early in the testing process is, "Have you ever stolen anything in your life?" Since most people cannot truthfully say they have never stolen something, your response to this question may establish the foundation for all other results of your lie detector test.
Avoid stimulants, such as caffeine, that can make you nervous, irritable or excitable. Some people take a mild sedative to help them relax; others apply a clear antiperspirant on their fingertips and palms to control sweat.
Answer all questions, but do not spend a lot of time thinking about the answers. Typically, a "yes" or "no" response will satisfy a question, so there is no need to spend time elaborating on an answer.
Know your rights. In the United States, only employees of the federal government can be compelled to take a polygraph test. Workers in the private sector are protected by laws that forbid use of a polygraph as a condition for securing or keeping employment.
As of 2009, in 19 states the results of a polygraph test are admissible if both the defense and prosecution agree, and in 18 states the presiding judge may admit a polygraph test as evidence if the test was conducted by a certified examiner. Check the laws in your state, as the legal standards for admissibility of polygraph results are likely to continue to evolve.
Consider speaking to a lawyer before your test. Brief legal counsel may reassure you that the test is nothing serious and the consequences may be meaningless, from a legal standpoint. This will help you relax and feel confident going into the test.