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How Lie Detectors Work

The Polygraph Machine

The polygraph instrument has undergone a dramatic change in the last decade. For many years, polygraphs were those instruments that you see in the movies with little needles scribbling lines on a single strip of scrolling paper. These are called analog polygraphs. Today, most polygraph tests are administered with digital equipment. The scrolling paper has been replaced with sophisticated algorithms and computer monitors. ­

Digital Lie Detector­
The basic components of a lie detector system

­When you sit down in the chair for a polygraph exam, several tubes and wires are connected to your body in specific locations to monitor your physiological activities. Deceptive behavior is supposed to trigger certain physiological changes that can be detected by a polygraph and a trained examiner, who is sometimes called a forensic psychophysiologist (FP). This examiner is looking for the amount of fluctuation in certain physiological activities. Here's a list of physiological activities that are monitored by the polygraph and how they are monitored:

  • Respiratory rate - Two pneumographs, rubber tubes filled with air, are placed around the test subject's chest and abdomen. When the chest or abdominal muscles expand, the air inside the tubes is displaced. In an analog polygraph, the displaced air acts on a bellows, an accordion-like device that contracts when the tubes expand. This bellows is attached to a mechanical arm, which is connected to an ink-filled pen that makes marks on the scrolling paper when the subject takes a breath. A digital polygraph also uses the pneumographs, but employs transducers to convert the energy of the displaced air into electronic signals.

  • Blood pressure/heart rate - A blood-pressure cuff is placed around the subject's upper arm. Tubing runs from the cuff to the polygraph. As blood pumps through the arm it makes sound; the changes in pressure caused by the sound displace the air in the tubes, which are connected to a bellows, which moves the pen. Again, in digital polygraphs, these signals are converted into electrical signals by transducers.

  • Galvanic skin resistance (GSR) - This is also called electro-dermal activity, and is basically a measure of the sweat on your fingertips. The finger tips are one of the most porous areas on the body and so are a good place to look for sweat. The idea is that we sweat more when we are placed under stress. Fingerplates, called galvanometers, are attached to two of the subject's fingers. These plates measure the skin's ability to conduct electricity. When the skin is hydrated (as with sweat), it conducts electricity much more easily than when it is dry.
Lie Detector Results­

­Some polygraphs also record arm and leg movements. As the examiner asks questions, signals from the sensors connected to your body are recorded on a single strip of moving paper. You will learn more about the examiner and the test itself later.

Detractors of the polygraph call lie detection a voodoo science, saying that polygraphs are no more accurate at detecting lies than the flip of a coin. "Despite claims of 'lie detector' examiners, there is no machine that can detect lies," reads a statement from the American Civil Liberties Union (ACLU). "The 'lie detector' does not measure truth-telling; it measures changes in blood pressure, breath rate and perspiration rate, but those physiological changes can be triggered by a wide range of emotions."­

Lee, who has been performing ­polygraph exams for 18 years, agrees that polygraphs do not detect lies. "What has happened over the years is that the media has dubbed this lie detection, and that's what's clicked, but from a scientific perspective, absolutely not. There's no such thing as lie detection. I couldn't tell you what a lie looks like."

He does assert that polygraphs can detect deceptive behavior even through the stress brought on by the exam itself. "If the (forensic psychophysiologist) is properly trained and has the experience, he can penetrate that. Through the specific procedure that the FP will employ, anxiety will not penetrate into it."

In the next section, you will learn more about who polygraph examiners are and what makes them qualified to conduct these examinations.