POLYGRAPH TESTING

The polygraph is an instrument that measures some of the physiological changes that take place in the human body when a person tells a lie. It is sometimes referred to as a “lie detector”. The term “polygraph” originates from the Greek language and literally means “multi writer”.

This sets it apart from other less accurate devices that only measure one reaction, as opposed to the polygraph’s measuring of multiple reactions, allowing for proper comparison and evaluation.

The attempt to detect deception is as old as mankind itself. Since time immemorial, different methods have been employed to detect deception, for example pulse-taking by the Greeks in 300BC, or the swallowing of a rice cake in China in 220BC. In modern times, more scientific methods have been developed to uncover deception.

The first person to utilise an instrument for the purpose of detecting lies was C Lombroso in 1895. He measured changes in blood pressure and pulse rate with a hydrosphygmograph. J Larson (1921) was the first person to use a continuous method of recording changes in blood pressure and pulse rate and more than one recording to detect deception. In 1926, L Keeler developed a more complex recording instrument, utilising blood pressure changes, pulse rate and respiration. He added the galvanograph in 1936. The first electronic polygraph was introduced in 1974 by Stoelting Co. Today most examiners use computerised polygraph instruments.

In 1952 Reid developed an improved questioning technique by adding control questions. The first polygraph school was opened by L Keeler in 1948. In 1961, Backster developed the Zone Comparison question technique. Several studies on the polygraph have been conducted in the USA in recent years and continuous research and development is taking place in the field.

A polygraph instrument collects physiological data from at least three systems of the human body. Pneumograph tubes are placed over the examinee’s chest and abdominal area to record breathing patterns and finger plates are attached to the fingers to record sweat gland activity. A blood pressure cuff will record cardiovascular activity. The reactions are charted by the polygraph instrument and reflected on charts printed from the recordings.

When a person answers questions during a polygraph examinations the examiner will monitor for a set of physiological reactions which have been correlated to the act of being deceptive on a statistically significant level. These changes can thus accurately indicate the probability of the person answering the questions truthfully based on the data recorded.

The polygraph technique allows for the level of a person’s general nervous tension. Most individuals will be nervous before and during a polygraph examination. However, this only affects the level from which recordings are made and does not affect the result of the examination, i.e. although the level of nervousness will affect the individual’s physiological baseline, it will not affect the variations on the baseline and consequently the result of the examination.