You might have seen your favorite television detective use a polygraph examination on a murder suspect and wondered exactly how that machine works. How can a lie detector tell if someone is lying? The truth is that it can’t. Polygraphs don’t display any information about lying or truth to the polygrapher; instead they display information about the person’s physical responses to an interrogation.


Lying And Body Language

For a very long time, scientists have known that there are clues in our body language that should point out to us if someone is lying. There are numerous lists of the telltale signs of deception created by experts in nonverbal communication. Many of the easy to spot signs involve erratic or nervous movements, touching the face excessively, or sustaining unnatural eye contact. Experts also advise people trying to detect lies to look for micro expressions; these are the brief moments where you might see a look of extreme anxiety or discomfort on someone’s face before they begin constructing a lie.


The Polygraph

There are other physical signs of the anxiety produced by deception that mostly involve increased heart rate, blood pressure, and perspiration levels. These are of course nearly impossible to spot when talking with someone, and can vary for many other reasons. That’s why John A Larson invented the polygraph machine in 1921. He was a psychologist working on ways to improve police procedures with regards to suspects and realized that making it easy to see these physiological elements would help police to more accurately determine if someone was lying.


How It Works

As How Stuff Works explains, the polygrapher asks a series of questions, some of them “relevant” and some of them “control”. The control questions are simple facts about the person’s life, giving the polygrapher an idea of how their body responds to questions when they are telling the truth. Then, when asking relevant questions, such as did you murder your wife? Did you rob the bank? The polygrapher has a reference point for determining if their response was reasonably the truth or not.

That’s why a polygraph doesn’t give a simple, or necessarily 100 percent accurate reading on whether something is truth or lie. Rather, it is a complex way of looking at the body’s responses to questions. Whether you believe this response give away deception or not is up for you to decide and debate with the experts who have drawn these conclusions. No matter what you think, the polygraph remains one of the few tools available for trying to find the truth.