Do Lie Detectors Really Work?

We’ve all seen the movies and the TV shows where the suspect is hooked up to the lie detector machine, asked a series of questions, and the needles either remain still in the event of honesty, or scribble furiously across the page when a lie is told. It makes for great television drama, but if we were to hook a polygraph up to another machine and ask it “Do lie detectors really work?”, would it give a truthful answer? Do lie detectors really work?

Telling Tall Tales

First, as a matter of definition, lie detector tests (and the accompanying machines) don’t test for lies – they measure a subject’s blood pressure, respiration and pulse, assuming that someone who is attempting to lie undergoes physiological reactions (nervousness, tension, anxiety, stress). Someone telling the truth, on the other hand, would not experience such reactions.

Failing The Test

However, as far back as 1991, psychologists threw out the idea that a person’s honesty (or lack thereof) could be gauged by changes in their body. Conclusions about deception have to be inferred by those administering the test, putting the burden of proof (and impartiality) as much on the expertise of the examiner and/or the questioner, as much as the subject of the test.

Testing Under Pressure

It doesn’t require a polygraph test to know that even people with nothing to hide can experience a stress reaction when they are strapped into a chair, connected to a machine, and asked a barrage of questions. Conversely, a pathological liar or a spy can lie so smoothly and convincingly that the machine picks up no undue reactions – no quickening pulse, no increased blood pressure, normal rate of respiration, etc.

Poor Test Results

Faith in polygraph machines has never been high. In 2002, the National Academy of Sciences said that accuracy levels of lie detectors are “well below perfection”, and that after a hundred years of investigation, the nature of polygraph machines rest on “weak scientific underpinnings”.

Not helping matters is that lie detector machines tend to create a lot of false positives – claiming a statement is a lie when a person is telling the truth.

Legal Tests

Today, evidence obtained from polygraph tests is considered so unreliable that most courts of law will not admit them, unless both parties (such as a Minnesota criminal defense lawyer) agree to their use. Even in cases where the polygraph is still used, critics compare it to other pseudo-sciences that have been attempted in criminal investigations, such as handwriting analysis, voice-stress analysis and even using MRI machines to determine brain activity during questioning.

Do lie detectors really work? They don’t. Honestly.

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