A polygraph, popularly referred to as a lie detector, is an instrument that measures and records several physiological quantities such as blood pressure, pulse, respiration, breathing rhythms/ ratios, and skin conductivity while the subject answers a series of questions, in the belief that deceptive answers will produce physiological responses that are different from those associated with non-deceptive answers.

It is an examination based on an assumption that there is an interaction between the mind and body. It is conducted by various components or the sensors of a polygraph machine, which are attached to the body of the person who is interrogated by the expert. Lying by a suspect is said to be accompanied by specific, perceptible physiological and behavioral changes and the sensors and a wave pattern in the graph expose this.

There are three basic approaches to the polygraph test:

  • The Control Question Test (CQT). This test compares the physiological response to relevant questions about the crime with the response to questions relating to possible prior misdeeds. This test is used to determine whether certain criminal suspects are involved in the crime.
  • The Directed Lie Test (DLT). This test tries to detect lying by comparing physiological responses when the subject is told to deliberately lie to responses when they tell the truth.
  • The Guilty Knowledge Test (GKT). This test compares physiological responses to multiple- choice type questions about the crime, one choice of which contains information only the crime investigators and the criminal would know about. The questions are in multiple choices and the suspect is rated on how he reacts to the correct answer. If he reacts strongly to the guilty information, then proponents of the test believe that it is likely that he knows facts relevant to the case.


There are two main criticisms against Polygraph Test:

  1. Violates fundamental rights: The main provision regarding crime investigation and trial in the Indian Constitution is Art. 20(3). According to Art 20(3) of the Constitution, no person accused of any offence shall be compelled to be a witness against himself. It deals with the privilege against self-incrimination, referred to as the Right to Silence. The characteristic features of this principle are:
    • The accused is presumed to be innocent until proven guilty.
    • That it is for the prosecution to establish his guilt.
    • That the accused need not make any statement against his will, known as the Right to Silence.

The Section 161 (2) in the Code of Criminal Procedure (CrPC) also protects the Right to Silence.

If a polygraph test is performed on a suspect without his consent, it may violate his Right to Silence. Also, prolonged polygraph examinations may be used as a tool by which confessions are extracted from a suspect.

  1. Validity: Polygraphy has little credibility among scientists despite claims of 90-95% validity by polygraph advocates. Criticisms have been given regarding the validity of the administration of the Comparative Questions test (CQT). The CQT may be vulnerable to being conducted in an interrogation-like fashion. This kind of interrogation style would elicit a nervous response from innocent and guilty suspects alike.


Voice Stress Analysis (VSA) is a lie detection technology that uses “micro tremors” in the human voice to detect lies. VSA technology is said to record psychophysiological stress responses called microtremors that are present in a human voice when a person suffers psychological stress in response to a stimulus (question) and where the consequences of lying may be dire for the subject being tested.

The technique’s accuracy remains debated. There are independent research studies that support the use of VSA as a reliable lie detection technology, whilst there are other studies that dispute its reliability. It is said that there is no known scientific basis for the underlying theory of “micro tremors”. Some have described it as pseudoscientific.

VSA is distinct from Layered Voice Analysis (LVA). LVA is used to measure many different components of the voice, but is not reliable in the detection of ‘deceptive stress’. LVA measures a wide range of emotions, including excitement, confusion, attention, and more.

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