How does a lie detector work?

A person undertakes an early version of the polygraph test.
| Photo Credit: US FBI

A: A polygraph, also called a lie-detector test, is generally used during interrogation by police. The instrument works by recording physiological functions such as blood pressure, heartbeat, respiration and perspiration.

A pneumograph tube is fastened around the person’s chest and a blood-pressure-pulse cuff is strapped around the arm. The psychogalvanic skin reflex, an electro-dermal response, and the flow of current between different parts of the body are also measured. Sensitive electrodes are used to pick up the impulses, which are recorded on a moving graph paper. The parameters are recorded when a suspect answers questions put to them by an operator. The data is then used as the basis for deciding whether the person is lying.

When a person lies, the graph deviates from the ‘normal’ shape in one or more of the body functions. Such changes were believed to occur due to the emotional response to telling a lie.

There is consensus among scientists today that polygraphs are ineffective, unreliable, and easily overcome.

The modern polygraph was first constructed in 1921 by John Larson, a medical student at the University of California, along with a police officer. His instrument was capable of continuously recording blood pressure, pulse, and respiration. While the device has been in use since 1924, it has not been accepted as proof of truth-telling in courtrooms.