Can polygraphs be trusted?

Polygraphs can’t be trusted, so why are they still used? Polygraphs, or lie-detector tests, are used by law enforcement and employers to determine whether or not someone is telling the truth. This is how they work. Four sensors keep track of a subject’s breathing, pulse, blood pressure, and perspiration. Sometimes two more sensors are added to track arm and leg movement. The subject is asked three simple questions to establish a base measurement. Then the real questions are asked and the subject’s monitored breathing, pulse, blood pressure and perspiration readings are recorded on moving paper. The examiner can look at the paper at any time to see what result a question may induce. According to Russell Tice of the NSA, the tester also looks at your physical reactions when asking the questions to which they think you will lie.

That sounds simple enough. But why doesn’t it work? Actually, even a well-trained examiner’s interpretation of a lie is completely subjective; and people react differently to lying. Tice says you can set the machine off by lying to certain base questions and biting your tongue at the same time. Did you ever lie to your parents? Have you ever stolen money before? Say ‘yes’ and bite your tongue. The machine will react to the lie and the pain and produce a big reaction. This makes the examiner believe you are easily made nervous when confronted with a question where you have to lie. But, in fact, the examiner is basing that conclusion on exaggerated results. Your reaction to a real lie might look tame by comparison.

Another tactic is calming yourself so you will not set off the machine when you lie. Tice indicated that daydreaming will calm the nerves which will throw off the results. A practiced liar may also be able to fool the machine by faking a cold or tensing certain muscles to make a known truth set off the machine, so truth and lie become indistinguishable.

Based on the fact that polygraph accuracy is fragile, producing both false positives and false negatives, is it time that people stop using polygraphs? At the beginning of my research, I was ready to wipe polygraphs off the map. They were inaccurate and unfair, with false positives and false negatives, hurting the honest and not detecting the dishonest. Several state governments and courts even agreed with me. Many will never allow the use of a polygraph result as evidence. In Federal Court, both sides must agree that the results are accurate before the results are admissible. The Employee Polygraph Protection Act (EPPA) actually prohibits employers in most types of jobs from using a polygraph to hire someone or fire someone. Why would anyone use any scientific tool that is so unreliable?

It turns out that, in many applications, the results of a polygraph don’t matter and aren’t used. Some of the most effective uses for polygraphs actually have nothing to do with their results. Law enforcement uses it to get people to confess. After a polygraph, the tester or interviewer can hint that the results aren’t looking good and advise the defendant to just fess up. Employers use it to keep applicants honest. Employers have found that when an applicant thinks that they will have to take a polygraph, even if they never do, it influences their behavior. Applicants asked to take a polygraph tend to fill out applications more honestly. Also, undesirable employees, with something to hide, don’t apply at all just to avoid taking the polygraph. Like a placebo, the test is used to induce a certain behavior or result. The results are irrelevant to the process. Isn’t it ironic, that the best use of lie-detector tests is to lie to the subjects taking them?



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