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Polygraph Examination

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)

Limitations of polygraph tests

Polygraph tests have several limitations and, therefore, a margin for errors:

  1. The premise behind these tests is questionable, because the measured changes in physiological responses are not necessarily triggered by lying or deception. Instead, they could be triggered by nervousness, anxiety, fear, confusion or other emotions.
  2. Furthermore, the physical conditions in the polygraph examination room can also create distortions in the recorded responses.
  3. The test is best administered in comfortable surroundings where there are no potential distractions for the subject and complete privacy is maintained.
  4. The mental state of the subject is also vital since a person in a state of depression or hyperactivity is likely to offer highly disparate physiological responses which could mislead the examiner.
  5. In some cases, the subject may have suffered from loss of memory in the intervening time-period between the relevant act and the conduct of the test. When the subject does not remember the facts in question, there will be no self-awareness of truth or deception. Hence, the recording of the physiological responses will not be helpful.
  6. Errors may also result from ‘memory-hardening’, i.e. a process by which the subject has created and consolidated false memories about a particular incident. This commonly occurs in respect of recollections of traumatic events and the subject may not be aware of the fact that he/she is lying.

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)

About the Author

author Sunil Sharma is an advocate; editor and compiler of legal commentaries, having authored more than 40 books.

See: Laboratory Procedure Manual - Polygraph Examination (Directorate of Forensic Science, Ministry of Home Affairs, Government of India, New Delhi - 2005)
See: Selvi vs State of Karnataka, AIR 2010 SC 1974

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