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Lie Detectors


Techniques Interrogation > Lie Detectors

Description | Example | Discussion | See also



There are several lie detection machines, all of which are based on the principle of detecting anxiety. When we are stressed, our bodies show physiological signs, which can be measured by various devices.


The polygraph, by far the most well-known device, is a machine that electronically measures changes in respiratory rate and volume, heart rate and blood pressure plus sweat gland activity, that indicate arousal of the autonomic (involuntary) nervous system.

indicate stress that can be associated with lying.

The idea is that the person is asked a series of questions and that when they tell lies, their body will unconsciously change its state and the machine will detect this change.


The Computerized Voice Stress Analyzer (CVSA), a later version of the psychological stress evaluator (PSE), measures vocal micro-tremors which tend to be associated with experienced stress.


Pupillometrics is a method of measuring blinking rates and pupil dilation or constriction, which can be done with video recordings and careful later analysis.


A person who believes themself innocent of a crime agrees to a lie-detector test in the hope that this will convince the interrogators of this.

Listen. We can use the polygraph which will sort things out or you can just tell me now.


The term 'lie detector' is a common term but not really an accurate. All the machines do is measure body response. The real skill is in the questioning and interpretation of results.

It is very easy for the other person to show signs of stress when being questioned, which is why the lie-detector outputs are calibrated first by asking the person a series of questions to which the truth or falsehood of answers is known.

Fake lie detectors have also been used, including one case of putting the person's hand on a photocopier and telling the person it was a 'truth machine'. For the interrogator, the bottom line is 'whatever works'.

Polygraph evidence is admissible in court, as long as it can be demonstrated that it was used with care and by a trained operator. There is, however, much criticism of this and many do not believe that polygraphs provide sufficiently valid data.

The human being is actually a good lie detector in itself, although we are also susceptible to having the wool pulled over our eyes. A trained person can detect many lies by observing body language. It is possible to cover up some signs, but nobody can control all of the muscles in the body (including 90 in the face!).

See also

Lying, Deceptive body language


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