The origins of polygraph examination have been traced back to the efforts of Lombroso, a criminologist who experimented with a machine that measured blood pressure and pulse to assess the honesty of persons suspected of criminal conduct. His device was called a Hydrosphygmograph. Psychologist William Marston used a similar device during World War-I in espionage cases, which proved to be a precursor to its use in the criminal justice system. In 1921, John Larson incorporated the measurement of respiration rate and by 1939; Leonard Keeler added skin conductance and an amplifier to the parameters examined by a polygraph machine.
The theory behind polygraph tests is that when a subject is lying in response to a question, he/she will produce physiological responses that are different from those that arise in the normal course. During the polygraph examination, several instruments are attached to the subject for measuring and recording the physiological responses. The examiner then reads these results, analyzes them and proceeds to gauge the credibility of the subject’s answers.
Instruments such as cardiographs, pneumographs, cardio-cuffs and sensitive electrodes are used in the course of polygraph examinations. They measure changes in aspects such as respiration, blood pressure, blood flow, pulse and galvanic skin resistance. The truthfulness or falsity on part of the subject is assessed by relying on the records of the physiological responses.1)
Polygraph tests have several limitations and, therefore, a margin for errors:
However, the biggest concern about polygraph tests is that an examiner may not be able to recognize deliberate attempts on part of the subject to manipulate the test results. Such countermeasures are techniques, which are deliberately used by the subject to create certain physiological responses in order to deceive the examiner. The intention is that by deliberately enhancing one’s reaction to the control questions, the examiner will incorrectly score the test in favour of truthfulness rather than deception. The most commonly used countermeasures are those of creating a false sense of mental anxiety and stress at the time of the interview, so that the responses triggered by lying cannot be readily distinguished.2)
Sunil Sharma is an advocate; editor and compiler of legal commentaries, having authored more than 40 books.