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How a polygraph test works

The polygraph, a combination of medical instruments, is designed to measure involuntary responses, such as changes in heart rate, breathing, blood pressure and perspiration, as a person is questioned. Deceptive behavior produces changes in the body in response to stress. Examiners say they can determine whether a subject is using deceptive behavior by asking questions and examining his or her reaction.

How people beat the test

Here are a few methods people use to avoid reactions that will trigger a positive polygraph test result:

+ Sedatives can calm the subject.

+ Antiperspirant can prevent the fingertip from sweating.

+ Tacks placed in the shoe can cause the physiological response to each question to be the same, in response to pain rather than the question.

+ Biting the lip, tongue or cheek during each question will cause a similar reaction.

The examiner, called a forensic psychophysiologist, connects sensors to different parts of the body. The sensors are connected to a laptop computer that graphs the results.

Arm and leg movement is sometimes measured.

A blood pressure cuff around the arm measures the change in the sound the blood makes as it moves through the arm.

Pneumographs are bands placed around the chest and stomach that measure breath rate. As air inside the tubes is displaced with each breath, the frequency and amount is converted into a digital signal.

Galvanometers are metal plates that measure galvanic skin resistance, the skin's ability to conduct electricity, which increases as it becomes moist with perspiration.