Deception Detection Tests (DDT) and It’s Evidentiary Value


With the use of additional methods, forensic psychologists understand and assess the internal and external behavioural patterns of suspects, accused, witnesses, and victims to assist in the process of conducting a scientific examination of criminal cases. They aid the investigative authorities in determining a person’s knowledge, involvement, and complicity in a specific case under investigation. These tests are:

(i) Polygraph or Lie-Detection Test

(ii) Brain Mapping or Brain Finger Printing or Brain Electrical Oscillation Signature Profiling Test

(iii) Narco Analysis Test

(iv) Psychological and Geographical Profiling

These tests, such as narco analysis, polygraph, and brain mapping, aid investigators in identifying frauds and, as a result, boost the investigation’s efficiency. They are also known as Deception Detection Tests and have broad moral, scientific, and legal implications in society. The investigating agency insists that while not every information obtained from the accused may be used as evidence in court, it is still a better technique to extract information than employing “Third-degree procedures.”

History Of Deception Detection Test

The first antecedent to the Polygraph test was the study of a suspect’s saliva, mouth, and tongue to determine innocence or guilt. In ancient India, for example, the suspect’s mouth was stuffed with dry rice, whereas in China it was filled with rice powder. Then they were told to spit it out. In several Middle Eastern societies, the accused was forced to momentarily lick hot metal rods. The assumption behind these techniques was that a guilty individual would generate less saliva. As a result, if rice became lodged in their mouths or their tongues became badly burned, they were found guilty.

Lombroso used a blood pressure equipment (hydrosphygmograph) on criminal defendants in 1895 to detect dishonesty.

Various Types of Lie-Detector Tests

The most extensively utilised technology approach for detecting deception is the polygraph. This technology measures psychophysiological reactions (such as heart rate and blood pressure). Examining the pulse rate, blood pressure, and perspiration can help to identify a stress reaction (i.e., heart rate, blood pressure, and sweating).

Polygraphy Tests

The theory initiating this test is that when a subject (person) is lying to the questions asked by the examiner then his body will produce different physiological responses than he produces in normal course which would be detected by examiner by noting measures such as heart rate, blood pressure, respiratory rate, skin conductance and electromyography as few instrument are stick to the subject for recording these measures.

A polygraph test records physiological (bodily) changes in a person when they are asked questions regarding a scenario, incident, or activity that they have been accused of, or about which they are suspected of lying. Some body processes react to fear or threat in specific ways, according to research, experiments, and studies in psychology and the behavioural sciences, and lying may produce this state of worry and anxiety in a person, prompting these functions to react. In a polygraph examination, these reactions are assessed by correctly putting sensors on the parts of the examinee’s body that are related to these responses.

The sensors used in polygraph tests capture breathing, movement of the body, agitation, galvanic skin reaction (hand sweating), blood flow or blood pressure, heart rate or pulse, and body temperature. Sensors are fastened around the torso to monitor breathing, as cuffs or clips around the fingers to measure pulse and perspiration production, and as an arm cuff to measure blood pressure. Due to the established terror responses that lying causes in a person, their replies reveal whether they are replying honestly or deceitfully.

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Narco Analysis

The use of narcoanalysis in criminal investigations has long been debatable. The purpose of the test is to induce a “hypnotic state” in the individual by administering anaesthetic medications in a predetermined dosage. It is believed that once the person’s imagination has been neutralised, he/she would only then share facts that they knows to be accurate. It is seen as a way of getting information from someone when the situation calls for an immediate reaction. It is mandatory that the person being tested has consented to it and knows and understands the entire procedure.

A team consisting of “an anesthesiologist, a psychiatrist, a clinical/forensic psychologist, an audio/videographer, and accompanying nursing professionals” often conducts the test (source). Throughout the session, they keep an eye on the patient’s vital signs, such as blood pressure and pulse rate, and the entire process is documented for the record.

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Brain Mapping techniques

The P-300 test is another name for the Brain Mapping Test. To determine whether the person is hiding any information during this Brain Mapping test, he/she is first questioned and interviewed. Presenting the individuals with a list of phrases causes the brain’s related memory to become activated. This test consists of examining and measuring ‘event-related potentials’ (ERP) i.e. electrical waveforms emitted by the brain after it has absorbed an external event.

This entire test is based on the theory that whenever the subject is exposed to images, videos, and words that contain both relevant and irrelevant stimuli (stimuli that are related to a crime scene or in any other way to a case), the examiner can record how the subject’s brain responds to those stimuli. To prevent inconsistencies arising from weather conditions, the test must be conducted in an insulated, air-conditioned space.

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Geographical Profiling

In order to identify the most likely site of the offender’s residence, investigators use a technique called geographic profiling, which examines the locations of a connected sequence of crimes. By combining qualitative and quantitative methodologies, it helps to comprehend an offender’s spatial behaviour and narrow the scope of the inquiry to a specific region of the community. The approach aids police investigators in prioritising information in large-scale serious crime investigations that can involve hundreds or thousands of suspects and tips. It is typically employed in situations of serial murder or rape (but also arson, bombing, robbery, terrorism, and other crimes).

Evidentiary Value of Deception Detection Tests

The Indian Evidence Act of 1872 has no formal provision for the acceptance of evidence obtained using such tests; yet, the Indian judiciary has repeatedly expressed both negative and favourable opinions on the subject. Sections 24 to 27 of the Act imply that remarks obtained by Narco-analysis would be inadmissible as evidence. Furthermore, if there is any indication of coercion, intimidation, or any other type of influence, such tests are considered null and void under Section 24(8) of the Act.

Concerning legal grounds, one concern that emerges is whether these tests, if performed, infringe one’s right to privacy by invading on their thoughts and body. The Supreme Court reviewed the legitimacy of these tests at length in Selvi v. State of Karnataka, AIR 2010 SC 340, and laid forth the premise for conducting them. The legal issues in this decision include the involuntary administration of specific scientific procedures, especially narco-analysis, polygraph, and Brain Mapping tests, for the aim of strengthening criminal investigative efforts.

The Supreme Court ruled in the Selvi case that no individual should be forced to use any of the procedures in question, whether in the context of criminal inquiry or otherwise. This would be an unjustified interference into personal liberty. The following are key takeaways:

Voluntary drug administration with safeguards: We do, however, allow opportunity for voluntary drug administration in the framework of criminal justice, provided that appropriate precautions are in place.

Its evidential value is not enhanced by voluntary administration: Even if the subject has consented to any of these tests, the test findings cannot be admissible as evidence since the subject does not have conscious control over the replies throughout the test.

Information subsequently discovered permissible under Section 27 of the IEA: However, any information or material uncovered as a consequence of willingly administered test results might be entered under Section 27 of the Evidence Act of 1872.

Guidelines from the National Human Rights Commission: In 2000, the National Human Rights Commission issued “Guidelines for the Administration of Polygraph Test (Lie Detector Test) on an Accused.” These instructions must be properly followed, and equivalent precautions must be used while doing the ‘Narcoanalysis method’ and the ‘Brain Electrical Activation Profile’ test.

International practice with regard to DDT


Narco-analysis tests in India now have some validity but are not totally accepted. The conditions under which this evidence was gathered are considered by Indian courts while determining admissibility.

United States of America

Various investigations have been undertaken in the United States of America by the American Medical Association and the American Psychiatric Association, following which narco-analysis has been deemed untrustworthy. It is thought that under the influence of truth serums, a person may offer deceptive replies. Furthermore, it has been suggested that narco-analysis might potentially violate the Fifth Amendment.


Canada lacks any rules governing the admission of scientific evidence. The Supreme Court of Canada favours determining admissibility based on the merits of each unique case and has not established any permanent standards in this regard.

United Kingdom

In the United Kingdom, narco-analysis test results are not deemed acceptable. They are not regarded as trustworthy and have little credibility in English courts.

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