How does a lie detector work?

A lie detector, often referred to as a polygraph machine, is a combination of three or four medical devices that are used to monitor changes in the body. The body functions are heart rate, blood pressure, breathing rate (respiratory), and galvanic skin resistance (sweatiness).

The examiner is looking for changes in comparison to normal levels. The concept here is that a person who is being deceptive will have significant involuntary responses occurring when subjected to stress.

Several tubes and wires are connected to the subject. Two air filled rubber tubes (pneumographs) are placed around the person’s chest. These tubes are measuring breathing rate. The tubes are connected to a bellows. The bellows are connected to mechanical arm, which is connected to an ink-filled pen that makes marks on a scrolling paper. As the proposed liar’s chest is expanding and contracting, the pressure of the air in the tubes is changing.

That’s the analog kind you see in movies. Today’s digital machines use a transducer to convert the amount of displaced air into an electronic signal.

A blood pressure cuff is attached to the subject’s upper arm. Tubes go from the cuff to the machine. Changes in blood pressure cause changes in the amount of air in the tubes, connected to bellows, now a transducer, and moves a pen on the machine. The systolic (high number) and diastolic (low number) blood pressure are being monitored, same as the sphygmomanometer the doctor uses at the clinic or hospital.

Of course, this pressure cuff is keeping track of heart rate at the same time. Heart rate is the number of beats per minute, usually around 72.

The galvanic skin resistance is measuring the sweat on one’s fingers. The fingers are the most porous places on the human body. Electrodes are placed on the fingers. The machine measures the skin’s ability to conduct electricity. Skin conducts electricity much better when it is wet (sweat) compared to being dry. We sweat more when under stress. Some machines detect arm and leg movement.

How good are lie detectors? Not very good at all. Imagine you are asked to take a lie detector test. Think how intimidating that can be. All these tubes and wires hooked up and the examiner starts asking questions. A normal person would be afraid the machine will interpret a truthful response as a lie. That is called a false positive.

Trained liars, like spies, can easily fool a lie detector. They can use tacks placed in shoes, biting the tongue or lip, use sedatives, or put antiperspirants on their fingertips.

How good is the examiner? Lots of subjective judgment involved in looking at the results of any lie detector test.  Lie detectors are seldom admissible in court, especially in local and state courts. At the federal level, it is usually up the judge involved.

Robert Hanssen was one of the FBI’s most trusted agents. For 20 years, from roughly 1980 to 2000, Hanssen was a mole and sold the Russians our most vital secrets, in what has been described as “the worst intelligence disaster in U.S. history”.  Hanssen took a lie detector test and passed it. The 2007 movie “Breach” does a pretty good job of how he was caught. It does not go deeply into the tremendous damage he caused. A 2002 made-for-television movie Master Spy: The Robert Hanssen Story is excellent. If interested, “google” in Robert Hanssen and see why he was given a 150-year sentence and sits in solitary in a federal prison in Colorado.



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