The term "polygraph" derives from the Greek words poly (many) and graph (writing) "many writings".

The name refers to the manner in which a polygraph machine simultaneously collects, measures and records selected physiological activities. It is this machine that is often referred to as the lie detector.

When used in a polygraph exam, the polygraph machine records the body's involuntary responses to an examiner's questions in order to ascertain deceptive behaviour. The test measures physiological data from three or more systems of the human body. Generally these include the respiratory, cardiovascular, and sweat gland systems.

These systems are useful as evolution has inbuilt humans with a self-defence mechanism that activates whenever a person feels threatened.

The realization that you are in danger or under threat, automatically triggers this self-defence mechanism as your body does its best to protect you.

Typical reactions that automatically occur include the release of a hormone called Adrenaline. This hormone increases a person’s level of alertness. The digestive process slows down as blood is diverted away from all unnecessary functions, sending additional blood to the muscles in case the individual should need to fight or take flight.

The glands in the skin become more active in preparation for a physical response and the persons heart rate and breathing changes.


When a person has done something wrong, such as commit a crime, there is the realization that if caught there will be consequences. These consequences might include imprisonment, embarrassment, loss of a job, or loss of a relationship.

Our instinct for self-preservation makes us want to avoid being punished for what we did. This is usually accomplished by lying. However, a person understands the potential consequences they face if caught in the lie, which causes the body's self-defence mechanism to activate.

The polygraph instrument is a finely tuned diagnostic device which will record the various aspects of a person’s Sympathetic and Parasympathetic Nervous Systems. If an examinee is not being truthful, the machine will detect changes in the examinee’s self-defence mechanism when the examiner asks certain questions. The questions mean the examinee has to act in a mild form of self-defence i.e. tell a lie.


This is a simple overview of one of the ways in which polygraph (lie detector) testing works. However, in reality it is far more complex. It is not a lie detector or truth detector that diagnoses the truth or a lie at the click of a button. A polygraph is a complex psycho-physiological process, which requires integration between a standard polygraph apparatus, a trained polygraph examiner and a complete polygraph examination procedure which is adapted by the examiner to meet the requirements of the case.

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