A polygraph test is also known as lie detector test that is used to  prove truthfulness of a subject by measuring and recording of several physiological parameters such as blood pressure, pulse rate, respiration, skin conductivity.

Detection of deception via physiological   interaction is common all over the world. In addition to the police and defense department, lie detector test is being used by the many industries, bank, insurance companies and other organizations.

PRINCIPLE OF LIE DETECTOR TEST : the basic principle behind this is that when a person faces a problem while answering the questions relating to the crime scene in which he is involved.  He has to decide what answer he should give. If he gives truthful answer, he would get punished. So he tries to hide the truth by giving false answers. Thus, this mental conflicts bring in certain visible and invisible physiological disturbances. The invisible ones are detected through instruments, recorded in the form of graph.

The defense mechanism in polygraph involves following aspects:

  1. The individual is asked about the crime. He knows that if he answers truthfully he would be punished for the same.
  2.  The fear of detection and punishment induces a subject to tell lie.
  3. The mental phenomenon causes excitation. Psychologists name this process as ‘defense mechanism’.
  4. The defense mechanism stimulates adrenal glands. The stimulated adrenal glands secretes the hormone adrenaline that interacts with the body and cause physiological changes in the form of increased blood pressure, increased heart beat, lower galvanic skin response of the skin.
  5. The examiner records this change on the computer along with the responses to the neutral questions that was asked by the questionnaire.
  6. The psychologist who carried out the interrogation using polygraph evaluates the responses.

Around 1920 Larson and Keeler developed an instrument known as polygraph. The instrument polygraph could record an individual’s blood pressure, pulse rate and respiration during interrogation. Keeler further added galvanic skin resistance of the skin and give interpretation in the form of graph known as Poly gram.


To verify the veracity of the statement of suspect, witness and complainant in various types of crime.

To replace the  examination method of interrogation in a  very   scientific manner.

To corroborate the findings of the investigation.

To examine the white collar crime, economic offender, cyber criminal and other offenders.

INTERROGATION: The detection of deception or lie detector test involves various instruments to carries out successful interrogation. The other elements are:

EXAMINER:  the examiner should have at least master’s degree in the forensic psychology. The PhD scholar gain preferences. He must have received training in this field or above all  integrity of the examiner must have above board.

SUBJECT:  the subject is one upon whom interrogation takes place.

  • The subject should be in a good mental  and physical health  at the time of examination.
  • He is not under the influence of drugs or liquor.
  • He has given informed consent.


  • Proper interrogation room: the set up of interrogation room plays a major role in achieving success in the interrogation. The room should be carried out in a closed room with instruments such as a lie detector device, audio – video recording devices and a pen and paper.
  • Background knowledge of the case: the examiner must have basic knowledge of the case before starting an interrogation. Such as : postmortem report of a victim, statement of  an accused,  victim and the witnesses, FIR and a proper of sketch of the crime scene and most importantly informed consent of an accused.
  • Pre-interrogation interview of the subject: this step is important as to:
    • Establish a rapport with the subject to gain trust.
    • If he is nervous. Stabilize the subject with the room, equipment.
    • Inform the subject about the process of an interrogation, its results so that the fear  and nervousness of subject vanishes.
    • Assess the subject for any abnormalities.
  • Equipment:
  • Pneumograph: it is a rubber tube, fastened around the subject’s chest and another around the abdomen to measure respiratory changes.
  • Sphygmograph: is an inflated cuff. It is attached to the subject’s upper arm to measure cardio-vascular changes.
  • Electrodes /galvanometer: it is attached to the subject’s palm or index or ring finger to measure the galvanic skin resistance of the body.
  • Plethysmograph: it is a transducer, attached to the thumb to measure the blood volume reflecting pulse rate.
  • Questioning technique:
  • Relevant or irrelevant technique:

The examiner should ask first irrelevant questions such as “Is today Thursday?” Is your name XYZ?

And also relevant questions such as “Did you shoot the victim with a revolver?” .

The difference in the physiological changes observed in relevant and irrelevant questions indicates the person is telling truth or not.

  • Peak of tension technique: In this technique, the emotionally charge questions after several  irrelevant questions Is asked. The irrelevant questions build the tension that is released after the relevant question has been asked. The technique is based on the fact that the guilty  person posses knowledge that no innocent person could posses.
  • Controlled question technique:  these questions relate to the associative evidences already  knows the examiner. Only the culprit shows a reaction to questions, as he alone knows them.

INTERPRETATION : the most difficult step of lie detector test is the interpretation of the poly gram.

  1. Simultaneously occurrence of suppression in respiration and increase in blood pressure immediately before the subject’s reply.
  2. Decrease in blood pressure after the reply.
  3. Slowing down of subject’ pulse rate after the questioning is over and the subject is told that no more questions would be asked of him.
  4. Heavier breathing immediately after the reply.
  5. Variation in blood pressure curve during the test.

6. If there is a general upward curve during the test indicates increase in tension sues to fear of guilt and deception or if the curve is downward then there is a release of tension.

7. Each questionnaire is run three times.

8. Interpretation of GSR is based on upward movement of the curve due to changes in the electrical resistance of the skin the subject.

LEGAL ASPECT: article 20(3) states: “no person, accused of an offence, can be compelled to be a witness against himself”.

No one is compelled to give evidence against himself. He (subject) goes not give information. The information is extracted by scientific procedure by the scientist. In fact, the consent of subject is taken before he is subjected to lie –detector test.

In short, the polygraph test does not infringe article 20(3) of the Indian constitution

Get Daily Updates Modern Forensic

You May Also Like

NARCO – ANALYSIS TEST : 4 basic steps of interview

The term “Narco” was coined by Horsley, and it derived from Greek…