The lie-detector test does not hold evidentiary value in a court, says Devesh K. Pandey

The art of deception is indeed inherent to the unscrupulous. The more a person is adept at preserving secrets and telling lies without any qualms, the more there is a possibility of evading confession. But modern science has thrown open the gateways to a guilty mind through use of interrogation techniques that promise to dig out the truth.

Lie-detectors, brain-mapping and narco-analysis tests are three such modern tools of interrogation that, according to many police officers, can help enforcement agencies crack complex cases of crime. The basic premise on which these techniques operate is the age-old belief that the offender always lives with a guilty conscience. This sense of guilt, if not disclosed voluntarily, has to be extracted somehow and that requires the use of such modern techniques.

The lie-detector is a device that records various physiological parameters like respiration, muscle movements, pulse and blood pressure of the person who is subjected to the test. During this test, the subject is initially asked normal questions such as his name and address, the replies to which are generally expected to be consistent and true. The second round of questioning concerns issues on which the subject is tempted to skirt the truth. These falsified answers are manifested as changes in the meticulously recorded physiological parameters. The person is then asked questions pertaining directly to the case and then the physiological recordings are analysed to determine the degree of deception.

Although this technique of interrogation is considered very effective in zeroing in on an offender, the lie-detector test does not hold evidentiary value in a court. “As per the law, the guilt of a person has to be proved beyond reasonable doubt. This objective standard of jurisprudence is not satisfied by the lie-detector test as it is not considered a perfect scientific tool,” says eminent lawyer Surat Singh.

According to Dr. Singh, the lie-detector test interprets the nervousness of the subject as guilt. “A person can be nervous because of various reasons. The very fact that there can be multiple causes of nervousness suggests that the test is not absolutely reliable. According to the law, if there are two possible interpretations of an event, the one which is consistent with the innocence of a person is preferred,” says Dr. Singh.

Brain mapping is another tool that is employed to find out if a suspect is indeed linked to the crime. In this test, the subject is shown photographs and words related to the crime and his/her electrical brain responses are recorded. The test helps confirm if the subject has information about the offence, but on several occasions it may not lead to a conclusive result.

Many experts believe that the narco-analysis test -- in which the suspects are injected with a “truth serum” so that they are deprived of reasoning capability and brought to a mental state where they readily answer relevant questions -- is also not a perfect tool because subjects who have been administered the drug are also prone to delusions and fantasies.

Although the reliability of modern interrogation techniques in throwing up clinching evidence is a matter of debate, there is a general consensus over their use in strengthening circumstantial evidence. Police officers believe that these tools can be of immense help in gathering vital clues. All it requires is efficient handling.