Origin of 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. A similar device was used by psychologist William Marston 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

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 examination techniques

There are three prominent polygraph examination techniques:

i. The relevant-irrelevant (R-I) technique;

ii. The control question (CQ) technique;

iii. Directed Lie-Control (DLC) technique.

Each of these techniques includes a pre-test interview during which the subject is acquainted with the test procedure and the examiner gathers the information which is needed to finalize the questions that are to be asked. An important objective of this exercise is to mitigate the possibility of a feeling of surprise on part of the subject which could be triggered by unexpected questions. This is significant because an expression of surprise could be mistaken for physiological responses that are similar to those associated with deception.[2]

Needless to say, the polygraph examiner should be familiar with the details of the ongoing investigation. To meet this end, the investigators are required to share copies of documents such as the First Information Report (FIR), Medico-Legal Reports (MLR) and Post-Mortem Reports (PMR) depending on the nature of the facts being investigated.[3]

Most used technique of Polygraph

The control-question (CQ) technique is the most commonly used one. The test consists of control questions and relevant questions. The control questions are irrelevant to the facts being investigated but they are intended to provoke distinct physiological responses, as well as false denials. These responses are compared with the responses triggered by the relevant questions.

Theoretically, a truthful subject will show greater physiological responses to the control questions which he/she has reluctantly answered falsely, than to the relevant questions, which the subject can easily answer truthfully. Conversely, a deceptive subject will show greater physiological responses while giving false answers to relevant questions in comparison to the responses triggered by false answers to control questions.

In other words, a guilty subject is more likely to be concerned with lying about the relevant facts as opposed to lying about other facts in general. An innocent subject will have no trouble in truthfully answering the relevant questions but will have trouble in giving false answers to control questions. The scoring of the tests is done by assigning a numerical value, positive or negative, to each response given by the subject. After accounting for all the numbers, the result is compared to a standard numerical value to indicate the overall level of deception. The net conclusion may indicate truth, deception or uncertainty.

Use of Polygraph Examination in Criminal Justice System

The use of polygraph examinations in the criminal justice system has been contentious. Litigation related to polygraph tests has also involved situations where suspects and defendants in criminal cases have sought reliance on them to demonstrate their innocence. It is also conceivable that witnesses can be compelled to undergo polygraph tests in order to test the credibility of their testimonies or to question their mental capacity or to even attack their character.

Another controversial use of polygraph tests has been on victims of sexual offences for testing the veracity of their allegations. In some jurisdictions polygraph tests have been permitted for the purpose of screening public employees, both at the stage of recruitment and at regular intervals during the service-period. In the U.S.A., the widespread acceptance of polygraph tests for checking the antecedents and monitoring the conduct of public employees has encouraged private employers to resort to the same. In fact, the Employee Polygraph Protection Act, 1998 was designed to restrict their use for employee screening.

Limitations of Polygraph Tests

Polygraph tests have several limitations and therefore a margin for errors. 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. Furthermore, the physical conditions in the polygraph examination room can also create distortions in the recorded responses.

The test is best administered in comfortable surroundings where there are no potential distractions for the subject and complete privacy is maintained. 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.

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 and hence the recording of the physiological responses will not be helpful. 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.

Categorizations of the errors in Polygraph Tests

The errors associated with polygraph tests are broadly grouped into two categories, i.e., `false positives’ and `false negatives’.

A `false positive’ occurs when the results indicate that a person has been deceitful even though he/she answered truthfully.

Conversely a `false negative’ occurs when a set of deceptive responses is reported as truthful. On account of such inherent complexities, the qualifications and competence of the polygraph examiner are of the utmost importance. The examiner needs to be thorough in preparing the questionnaire and must also have the expertise to account for extraneous conditions that could lead to erroneous inferences.

Counter measures

However, the biggest concern about polygraph tests is that an examiner may not be able to recognise deliberate attempts on part of the subject to manipulate the test results. Such `counter measures’ 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 `counter measures’ 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.

Admissibility in Law

An unresolved question is whether the results obtained through polygraph examination is of a testimonial nature. In this test, inferences are drawn from the physiological responses of the subject and no direct reliance is placed on verbal responses. In some forms of polygraph examination, the subject may be required to offer verbal answers such as `Yes’ or `No’, but the results are based on the measurement of changes in several physiological characteristics rather than these verbal responses.

In India, the Supreme court, considered the question of its admissibility in detail in the case of ‘Selvi v. State of Karnataka, (2010)’.

While concluding the judgment, the court held-

  • The compulsory administration of the test violates the `right against self- incrimination’. This is because the underlying rationale of the said right is to ensure the reliability as well as voluntariness of statements that are admitted as evidence. Supreme Court has recognised that the protective scope of Article 20(3) extends to the investigative stage in criminal cases and when read with Section 161(2) of the Code of Criminal Procedure, 1973 it protects accused persons, suspects as well as witnesses who are examined during an investigation.

The test results cannot be admitted in evidence if they have been obtained through the use of compulsion.

  • Article 20(3) protects an individual’s choice between speaking and remaining silent, irrespective of whether the subsequent testimony proves to be inculpatory or exculpatory.
  • Forcing an individual to undergo the test violates the standard of `substantive due process’ which is required for restraining personal liberty. Such a violation will occur irrespective of whether these techniques are forcibly administered during the course of an investigation or for any other purpose since the test results could also expose a person to adverse consequences of a non-penal nature.
  • The compulsory administration of technique is an unjustified intrusion into the mental privacy of an individual. It would also amount to `cruel, inhuman or degrading treatment’ with regard to the language of evolving international human rights norms.
  • Furthermore, placing reliance on the results gathered from this test comes into conflict with the `right to fair trial’. Invocations of a compelling public interest cannot justify the dilution of constitutional rights such as the `right against self-incrimination’.
  • No individual should be forcibly subjected to test in question, whether in the context of investigation in criminal cases or otherwise. Doing so would amount to an unwarranted intrusion into personal liberty.
  • However, the court left room for the voluntary administration of the test in the context of criminal justice, provided that certain safeguards are in place.
  • Even when the subject has given consent to undergo test, the test results by themselves cannot be admitted as evidence because the subject does not exercise conscious control over the responses during the administration of the test.
  • However, any information or material that is subsequently discovered with the help of voluntary administered test results can be admitted, in accordance with Section 27 of the Evidence Act, 1872.
  • THE NATIONAL HUMAN RIGHTS COMMISSION HAD PUBLISHED `GUIDELINES FOR THE ADMINISTRATION OF POLYGRAPH TEST (LIE DETECTOR TEST) ON AN ACCUSED’ IN 2000. These guidelines should be strictly adhered to and similar safeguards should be adopted for conducting the `Polygraph technique’.

The text of these guidelines has been reproduced below:

(i) No Lie Detector Tests should be administered except on the basis of consent of the accused. An option should be given to the accused whether he wishes to avail such test.

(ii) If the accused volunteers for a Lie Detector Test, he should be given access to a lawyer and the physical, emotional and legal implication of such a test should be explained to him by the police and his lawyer.

(iii) The consent should be recorded before a Judicial Magistrate.

(iv) During the hearing before the Magistrate, the person alleged to have agreed should be duly represented by a lawyer.

(v) At the hearing, the person in question should also be told in clear terms that the statement that is made shall not be a `confessional’ statement to the Magistrate but will have the status of a statement made to the police.

(vi) The Magistrate shall consider all factors relating to the detention including the length of detention and the nature of the interrogation.

(vii) The actual recording of the Lie Detector Test shall be done by an independent agency (such as a hospital) and conducted in the presence of a lawyer.

(viii) A full medical and factual narration of the manner of the information received must be taken on record.


Selvi v. State of Karnataka, (2010)

(Reference in the article, as quoted by court in the judgment)

[1] Laboratory Procedure Manual – Polygraph Examination (Directorate of Forensic Science, Ministry of Home Affairs, Government of India, New Delhi – 2005) (referred by court in selvi’s case)

[2] David Gallai, `Polygraph evidence in federal courts: Should it be admissible?’ 36 American Criminal Law Review 87-116 (Winter 1999) at p. 91

[3] Selvi v. State of Karnataka, (2010)