Polygraph test

The Polygraph Test is commonly known as a “lie detector test.” It is a procedure that detects several physiological indicators, including pulse, blood pressure, respiration, and skin conductivity, which indicate if a person is being untruthful when asked a series of questions. The principle behind the polygraph test is that any untruthful responses produce different physiological responses than those associated with non-deceptive answers.

ConductScience offers the Polygraph Test Device.


The polygraph test or “lie detector test” is used to detect physiological indicators that someone is being deceptive while asking a series of questions. These indicators include:

  • Blood pressure
  • Pulse
  • Respiration (upper and lower chest respiration)
  • Skin conductivity (skin temperature)
Methods to detect deception

Depending on the purpose, subject, and examiners, a polygraph test can take several hours. Examinations are usually split into three components:

  •  Pretest interview: Where information about the interview is provided to the subject, physiological and phycological circumstances are addressed, and consent procedures are initiated. Measuring devices, including pneumographs (to measure respiratory behaviors), sphygmomanometer (blood pressure monitor), and electrodes (for heart rate monitoring) are placed on the subject.
  • Question procedure: Before questions are asked, baseline values for heart rate, respiratory rate, and blood pressure rates are taken. Four kinds of questions can be asked:
    • Questions relevant to the investigation 
    • Control questions compare the subject’s reactions and responses
    • Irrelevant questions for baseline value comparison
    • Concealed information questions 
  • Post-test: To establish if the subject’s responses indicate deception or truthfulness

The polygraph system includes the following:

  • Sensor: Components used to measure physiological indicators for respiration, blood pressure, and heart rate
  • Main unit
  • Microprocessor: for test preparation, control, and data analysis
  • Test software

The Polygraph Test is commonly known as a “lie detector test.” It is a procedure that detects several physiological indicators, including pulse, blood pressure, respiration, and skin conductivity, which indicate if a person is being untruthful when asked a series of questions. The principle behind the polygraph test is that any untruthful responses produce different physiological responses than those associated with non-deceptive answers.

During a polygraph test, three medical instruments (pneumograph, galvanograph, and cardiosphygmograph) combined in one machine are attached to the person. The instruments include sensors to record four to six physiological responses when the person is asked questions. A data acquisition system collects the data from the equipment and translates it to analyzable information using sophisticated mathematical algorithms. The examiner first asks a few simple questions to create a standard for the digital signals and then moves on to the actual questions. The examiner analyzes the data during or after the test to identify any significant changes in the person’s vital signs to identify if they are lying. 

Polygraph tests are used as an interrogation tool for various purposes, such as criminal suspects and screening new employees for employment in sensitive public or private sectors. However, it is important to consider that people can react differently when lying, or the examiner’s interpretation may not be accurate depending on their expertise. Therefore, a polygraph test is not always perfect. 

Brief History of the Polygraph Test

In the late nineteenth century, Lombroso, an Italian criminologist, noted that a person’s blood pressure and heart rate increase when a person is lying. He recorded these changes on a graph. Lie detection technology was further developed during World War 1 by William M. Marston, a lawyer/psychologist. He also found out that a person’s blood pressure increased when a person was lying and used the technology to question prisoners at war. However, the first actual polygraph was developed by John Larson in 1921, which measured and recorded blood pressure and respiration. However, perspiration, which is the third physiological channel of the modern polygraph machine, was added in the 1920s by Leonarde Keeler.

Components of a Polygraph Machine 
  • Pneumograph 

The Pneumograph component of the polygraph machine is used to record the subject’s respiratory rate. It consists of two 10 inches of convoluted tubes. One tube is fastened around the subject’s chest, and the other is placed around the subject’s abdomen. The tube expands and contracts when the subject breathes. Any changes in the subject breathing rate are recorded on the polygraph.

  • Galvanograph 

The Galvanograph measures a person’s perspiration. It comprises electrical sensors known as galvanometers, which are attached to the person’s fingertips. Fingertips are a good location for measuring perspiration since they contain many sweat glands. The polygraph records any decrease in electrical current resistance that occurs when the amount of sweat touching the galvanometer increases.

  • Cardiosphygmograph 

The Cardiosphygmograph measures the subject’s blood pressure and heart rate. It consists of a blood pressure cuff, a rubber pump, and a sphygmomanometer. When the blood pressure cuff is wrapped around the subject’s right arm, its rubber portion is placed over the brachial artery, ensuring a more accurate reading. The cuff remains inflated during questioning. The sound of blood flowing through the subject’s veins is sent by the air in the cuff to the bellows, which amplifies it. The magnitude of the sound corresponds to blood pressure, while the frequency of the sound changes corresponds to heart rate.

  • Data Acquistion System 

The Data Acquisition System (DAS) is the system that collects data from the equipment and converts it into analyzable information. A polygraph examiner can then read and evaluate this data.

The Polygraph Test Procedure 
  • The Pre-Test 

The Pre-Test comprises a preliminary interview between the subject and the examiner before the actual questions begin. During the pre-test, the examiner explains the procedure of the polygraph test and analyzes the subject’s responses and behaviors when asked questions. These questions allow the examiner to identify the subject’s normal physiological responses to provide a basis for any irregularities in their physiological responses during the actual test. 

  • The Actual Test 

The subject is connected to the polygraph machine and seated in a room alone with the examiner. The examiner usually asks around 10 to 11 questions, of which only 3 or 4 are relevant while the others serve as controls. The control questions provide non-lying physiological data, which are used as reference points and compared with the data to the relevant questions. 

  • The Post Test 

The examiner analyzes the physiological data collected to look for any irregularities in each question. Irregularities during the relevant questions indicate that a person is lying. However, the physiological data recorded is often not very well-defined. 


The polygraph test is used in various applications, including 

  • Criminal asset location 
  • Police applicant screening 
  • Pre-employment screening 
  • Evidentiary polygraphy 
  • Counterintelligence screening 
  • Sex offender management 
  • Pre-trial stipulation 
  • Counter-terrorism programs
  • Political asylum validation 
  • Counter-narcotics programs

The polygraph test only records responses that the subject thinks are true and does not determine fact. It does not actually detect lying but detects physiological responses that are often associated with lying. The accuracy of the polygraph test can be affected by the competence of the examiner. 

  • The polygraph test is used to identify whether a person answers questions truthfully or deceitfully. 
  • It comprises a machine with several medical instruments attached to measure the subject’s pulse, blood pressure, respiration, and skin conductivity. 
  • The polygraph’s data acquisition system converts the physiological data retrieved from the instruments into analyzable information, which the examiner uses to identify honest or dishonest answers. 
  • The polygraph testing procedure comprises a pre-test, an actual test, and a post-test.
  • The polygraph test is used for various applications, including for the screening of new employees and for interrogation in criminal investigations. 

Synnott, J., Dietzel, D., & Ioannou, M. (2015). A review of the polygraph: history, methodology and current status. Crime Psychology Review1(1), 59-83. doi:10.1080/23744006.2015.1060080

Rajan, P. B. (2019). Polygraph Tests-Benefits and Challenges. Academicus International Scientific Journal10(19), 146-155.

Lewis, J. A., & Cuppari, M. (2009). The polygraph: The truth lies within. The journal of psychiatry & law37(1), 85-92. doi:10.1177/009318530903700107 

Katherine To for Illumin. Fall 2003. Lie Detection: The Science and Development of the Polygraph

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