Inside the Lie Detector: Understanding the Pre-Test, Test, and Post-Test Stages

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The polygraph test, or lie detector test, is a device used to measure various physiological responses in an individual. The polygraph test has been in use since the early 20th century and gained popularity in the mid-1900s as a tool for law enforcement in the United States.


Polygraphy is the scientific study of physiological changes in an individual in response to various stimuli. In the context of a lie detector test, or polygraph test, the focus is on detecting changes in physiological responses that are associated with deception.

The accuracy of polygraph is based on the measurement of physiological changes such as:

  • Respiratory system
  • Cardiovascular system
  • Peripheral nervous response


Stage 1: The Pree-Test

The pre-test phase is an important aspect of the polygraph test, as it allows the examiner and examinee to become familiar with one another and establish a level of trust and rapport. This initial period is crucial in creating an atmosphere that will help the examinee feel more comfortable and at ease during the test.

During this period, the examiner will explain the full process of the test and answer any questions or concerns that the examinee may have. It is essential that the examiner explains the process in detail to ensure that the examinee understands how the test works and what is expected of them during the test. This explanation also helps to alleviate any anxiety or concerns that the examinee may have about the test, which can affect the accuracy of the results.

The examiner will also take note of the examinee’s behaviour during this period. The examiner will observe the examinee’s verbal and non-verbal communication and attempt to establish a baseline for their behaviour. This baseline can then be used during the actual test to determine whether there are any significant changes in behaviour or physiological responses.

Stage 2: The Test

The test itself begins with the subject being attached to the testing machines. These machines are designed to monitor the physiological responses of the individual. One of the instruments used is a Pneumograph, which consists of two rubber tubes filled with air that are tied around the subject’s body. One tube is placed across the subject’s chest, and the other is placed around their waist. This device is used to measure respiration and movement when the subject breathes. It records any changes in the air pressure inside the rubber tubes and sends this information to the monitor where it is recorded.

The next instrument used is a Cardiovascular Instrument, which is a blood pressure cuff that measures the changes in the subject’s heart rate and blood pressure. The cuff is wrapped around the subject’s arm and is inflated several times throughout the test to measure these changes accurately. The final instrument used is a Galvanometer, which measures the changes in the subject’s peripheral nervous system, specifically changes in skin conductivity. The Galvanometer records the electrical conductivity of the skin, which is affected by perspiration and emotional arousal.

All of these instruments work together to provide a comprehensive picture of the subject’s physiological responses to the questions asked during the test. As the test progresses, the examiner will ask a series of relevant and irrelevant questions to the subject. The relevant questions are designed to elicit a physiological response if the subject is lying. In contrast, the irrelevant questions are designed to establish a baseline response for the individual.

Stage 3: Post Test

Once the test is complete, the examiner will analyze the results and determine if the subject’s responses indicate deception. In the event that the test results indicate deception, the subject is given an opportunity to explain the results and provide a context for the deception detected by the test. 

It is natural for most people to feel nervous and anxious during a polygraph test, but research has shown that such feelings do not significantly affect the results because the level of anxiety tends to remain stable throughout the test. However, there is still a possibility of human error, particularly if the examiner misinterprets the subject’s responses, although such errors are rare. Inaccuracies can also result if the pre-test phase is not conducted properly, as this can lead to the misinterpretation of data during the actual test.


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