How does a polygraph test work?

lie detector tests

Polygraph examinations or, as they are more commonly known, lie detector tests, record several different bodily measures to detect changes in response to questions to determine whether someone is being honest or deceptive in their answers.

Sensors are placed on the subject to measure these bodily responses throughout the test and the data they produce can then be analysed according to a number of different measuring or scoring techniques that indicate whether the examinee was lying or telling the truth. In some instances, the data does not produce a statistically significant result, and this renders the test inconclusive and cannot determine whether the subject was honest or deceptive. However, these instances are quite rare.

Lie detector tests are generally considered to be about 87% accurate but accuracy does depend on the examiner, adherence to correct lie detector test principles and methods and the equipment used including the sensors and the analysis software itself.

It is widely accepted in the behavioural sciences that external stimuli such as sound, touch, smells and sights can activate cognitive and emotional response in human beings. Once the brain has processed these stimuli, it sends messages to other areas and systems within the body to activate them in response. The well known “fight or flight” response is an example of this where the brain sends messages to the nervous system and different bodily organs to ready them for physical conflict or a speedy exit.

Researchers in the behavioural sciences have found that the nervous system, the integumentary or skin system, the cardiovascular system and the respiratory system respond in more significant ways to other bodily systems when a threat is detected through stimuli received by the brain. Polygraphy uses these observable physiological responses to determine the probability of whether someone is telling the truth or a lie. The knowledge that one is lying and is undertaking a test to determine whether they are lying, which undoubtedly has some form of personal or social consequence conditional on the result, is perceived as threatening by the brain.

Polygraph testing uses the following physiological measures to detect deception:

Bodily Temperature

The bodily temperature will increase if the brain is activating and contracting muscles in response to threatening situations and the perceived need to potentially “fight or fly” from a stimuli. An externally placed temperature probe is used within polygraph testing to measure this.

Galvanic skin response (sweating)

Sweat gland response on the surface or the hand or fingers is very significant in indicating deception. The stimuli for sweat being produced on the hand is different to that which causes sweating in the armpit or other bodily areas. When threat is perceived sweat is produced in the hand and finger area to increase grasping capacity. Sweat gland activity on the fingers is determined during polygraph testing through use of two electrodes placed on two different fingers to measure conductivity.; when sweat levels increase, conductivity increases.

Pulse/Heart Rate:

When the brain perceives a threat either through cognitive or emotional processing, the sympathetic nervous system is activated to increase blood pressure to enable increased transportation of oxygen and nutrients to the muscles that are likely to be required to respond to the threat through fight or flight action. The need for increased blood pressure necessitates an increase in heart rate to pump the blood around the body at a greater rate and force. Polygraph testing measures pulse by a sensor attached to the finger.

Blood Pressure:

As stated above, when the brain perceives a threat either through cognitive or emotional processing, the sympathetic nervous system is activated to increase blood pressure to enable increased transportation of oxygen and nutrients to the muscles that are likely to be required to respond to the threat through fight or flight action. Blood pressure is measured in lie detector testing through either a cuff attached to the arm, or a sensor attached to the hand.

Respiration/Breathing rate:

In a normal situation where threat is perceived the muscle contraction and movement that follows would typically necessitate a dramatic increase in respiration. This would occur to obtain the greater levels of oxygen and nutrients to the areas of the body that need them (such as the muscles) and reduce the levels of carbon dioxide that can build up to become toxic.

In response to threat and the expectation of this imminent need to act, norepinephrine is released to the muscles which dilate the airway which results in a reduction in resistance and an increased air flow to the lungs. However, due to a lie detector test requiring a person to remain still and static and the muscle physiology not increasing as would usually be expected in fight or flight response, the dilated airway means that a person will have adequate airflow with a reduced number of respirations or breaths.

This causes a reduction in breathing rate to be observed and this is measured by means of a sensor being attached to the torso (around the chest area) of the subject in a polygraph examination. 

Body movement/Fidgeting:

The increased arousal in the body through perceived threat and the activation of systems stated above can cause difficulty for a person to sit still during a lie detector examination. This can be measured during the test through use a seat sensor or through visual observation of the subject by the examiner. Any prolonged fidgeting or inability to sit still once reminded by the examiner will cause the examiner to be unable to undertake testing or to render the test inconclusive.

Following a polygraph test having been sat and data collected form recording the above physiological responses, the charts that this data produces are assessed to ensure that adequate data has been collected and that this data is reliable.

The data is then analysed through use of the scoring system chosen by the examiner to be most applicable and relevant to the testing model utilised during the polygraph examination.

This score, in combination with the observations made by the examiner during the testing process, will enable a decision to be made as to whether the subject was being truthful or lying and a result produced.

Where the data is not reliable or sufficient, for whatever reason, this will not achieve a statistically significant result and the test will be judged inconclusive meaning that it is not possible to say whether the subject was lying or telling the truth.

Another test could be sat with some changes to variables such as ensuring the subject sits still, making adaptations to the language used within the questions, the examiner gaining greater clarity over the issue at heart, increasing the level of saliency promoted by changing comparison and irrelevant questioning or making changes to the physical environment to be more conducive to a conclusive test.

Where a subject is found to deliberately not co-operate with a test by, for example fidgeting a great deal, not answering questions in the correct format, repeatedly interrupting or stopping the test or not focusing on the test and the examiner’s questions this will be perceived as indicating dishonesty and, although it will not be possible to determine a conclusive result the examiner will provide their professional view that there was some reason for the subject being avoidant and evasive during testing and the obvious reason for this would be that they intended to be deceptive.

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