How Lie Detector Works

Lie detectors 

Lie Detector Machine

Lie detector are also called polygraphs, the word polygraph means “many writer”; these devices have this name because they make simultaneous records of several bodily reactions thought to be indicative of emotional arousal.

Physiological changes during lie detection procedures:-

 Most polygraphs measure changes in blood pressure, heart rate, breathing rate and depth, and the resistance of the skin to the flow of a small electric current (the galvanic skin response, or GSR).

Aim of the test:-

  The aim of the test is to help the suspects establish their truthfulness, and describes the polygraph as a scientific instrument for the measurement of various bodily responses.

Process or pattern of lie detection:-

 The examiner obtains biographical information from each suspect and goes over the dozen or so yes/ no questions that will be asked about the suspect.

The examiner may use any one of several questioning techniques. A common one is the control- question technique. In this technique, neutral questions, general questions designed to evoke an emotional response from almost everyone, and specific questions designed to evoke responses only from those with guilty knowledge are used.

The rationale of the control- question technique is that an innocent person who knows nothing of the details of the crime should give a larger response to the control questions than to the specific questions, while the suspect with guilty knowledge should, while also responding to the general questions, give a large response to the specific questions. Thus it is the pattern of polygraph responses on the control and specific questions that is supposed to give a clue to lying or truthfulness.

Example:- in case of stolen, a general question might be “did you ever steal anything before you were 18?” since most of us have, a polygraph response would be expected. A specific question might be “ what happened to the money in the red envelope?” while the difference between the responses to the general and specific questions can be quantified, interpretation of the polygraph record remains somewhat of an art, requiring intuition and judgement on the part of the examiner.

Uses of lie detection machines:-

  These machines are use in trying to discover guilt or innocence, lie detectors are sometimes used for other purposes :-  The police may sometimes use them in an attempt to force confessions from suspects, business and government agencies use lie detector tests to screen applicants for employment.

How good is the polygraph at detecting liars:-

 Psychologists who are skeptical about the accuracy, or validity, of lie detector tests and those whose business it is to give such tests. A number of studies indicate that properly administered tests correctly identify some 70-90 percent of deceivers. However , a very important problem is the large number of false positives ( truthful people who are judged by the polygraph to be lying).

Benjamin Kleinmuntz and julian szucko estimate that false- positive judgements “may label more than 50% of the innocent subjects as guilty”.

Benjamin Kleinmuntz

Other important problems concern the physiological basis of the lie detector test. Not everyone responds to lying with arousal, and the pattern of responses differs from person to person. Some respond with breathing changes , others with heart rate changes and so on.

The examiner may have a difficult time with this. Furthermore, it is possible to “beat the machine”. If a suspect engages in some silent on going mental activity- such as counting backward by 3s- while being examined, responses can be blunted.

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