Polygraph: The Science

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1.0 Introduction

1.1 Legal Definition of Polygraph

A polygraph, more commonly referred to as a lie detector test, measures and records physiological indicators such as blood pressure, pulse, respiration and skin conductivity while the subject is asked and subsequently answers a series of questions. The belief underpinning the use of the polygraph is that deceptive and misleading answers will produce physiological responses that can be differentiated from those associated with non-deceptive or honest answers [1]. 

Polygraph examinations have been used for a wide variety of purposes, including criminal and civil matters. These include criminal issues relating to homicide, sexual abuse and assault, terrorism, arson, kidnapping, theft, insider trading, threats, fraud, breach of trust, and sabotage as well as civil issues relating to insurance fraud, employee theft, leak of information, industrial espionage, workplace disputes, loyalty and reliability issues, and pre-employment screening [2].

2.0 The Science

2.1 The Polygraph Instrument

The polygraph instrument measures and records physiological phenomena while the examinee is asked and responds to a series of questions. During a polygraph examination, the examiner places various components on the examinee’s body, connecting him or her to the polygraph instrument. The components contain sensors that detect physiological responses and changes from the body during examination [3]. These responses are simultaneously displayed via ink writing pens onto a chart or via a computer’s display screen.

Using the polygraph instrument, physiological data is collected from three major systems, including (a) the cardiovascular system – heart rate, blood pressure, and blood volume; (b) the respiratory system – movement of the chest cavity; and (c) the endocrine system – sweat gland activity. The polygraph instrument may also detect other responses like arm and leg movement [4]

A polygraph instrument, complete with sensors and computer display. The adjoining polygraph chart shows recordings of responses from the cardiovascular, respiratory, and endocrine systems during a polygraph examination.


Images courtesy of www.howstuffworks.com

2.2 Physiological Responses

Polygraph practitioners believe that the physiological responses and changes measured by the polygraph instrument reveal deception. The polygraph does not measure deception directly, but rather it is said to detect physiological responses that are believed to be stronger during periods of dishonesty than they are at other times. According to polygraph theorists, a deceptive response to a question causes a reaction – such as stress, fear of detection, or psychological arousal – that affects the body and increases respiration rate, heart rate, blood pressure, and skin conductance (e.g. sweaty fingertips). A pattern of strong, physiological responses to a series of questions is believed to indicate that the examinee may be deceptive or answering dishonestly [5]. 

Respiratory patterns are measured by pneumographs, which are devices that record thoracic movements or volume change during respiration. One of the pneumograph tubes is strapped around the chest of the examinee and the other is placed around his or her abdomen. Each pneumograph is connected to the polygraph machine by an air-filled rubber tube. When the examinee breathes in and out, the air pressure in the tubing changes to reflect his or her breathing rate and frequency. The polygraph instrument records these responses. Respiratory responses that may point toward deception include shortness of breath and laboured breathing [6]. 

An arm-encircling cuff that is placed around the upper arm collects blood pressure and heart rate data. The cuff is filled with air and connected to the polygraph machine by air-filled tubes. As blood pumps through the examinee’s arm, changes in blood pressure will alter the amount of air pressure in the cuff; these changes are recorded by the machine. Physiological responses such as a faster heart rate and an increase in blood pressure may suggest deception [7]. 

The measurement of electro-dermal activity – or sweat – is conducted by fingerplates that are attached to two of the examinee’s fingers. These plates measure the skin’s ability to conduct electricity. Dry skin is a poor conductor of electricity. If the examinee perspires, however, then the water and salt from the sweat will reduce resistance and allow a larger amount of electric current to travel along the surface of the skin. This increase in current reflects the amount of sweat produced in the examinee’s fingertips. Polygraph practitioners believe that an individual sweats more when placed under stress, and so sweaty fingertips may point toward deception [8]. 

2.3 Examination Procedures and Techniques

Before a person undergoes a polygraph, he or she will sit down for an interview with the examiner. This is known as the pre-exam interview. During this phase, the examiner will:

  • inform the examinee of the specific purpose of the examination;
  • advise the examinee that the examination must be taken voluntarily or not at all;
  • determine if the examinee is mentally and physically suitable to be examined;
  • provide the examinee with a detailed explanation of the procedure and instruments;
  • obtain the examinee’s version of the facts regarding all of the issues to be examined;
  • review with the examinee all of the questions that will be asked during the polygraph exam; and
  • advise the examinee of his or her constitutional rights and right to retain legal counsel [9]. 

It is necessary for the examiner to assess the individual’s mental and physical suitability prior to testing, as factors like pregnancy, health problems, use of anti-depressants, and substance abuse can affect test results. The examiner will also use the pre-exam interview to gauge the individual’s behaviour and response to information. Understanding how the individual reacts to questions gives the examiner an idea of that person’s normal physiological responses. This helps the examiner to detect irregularities that may indicate deception or dishonesty during the test [10]. 

The polygraph exam begins when the examinee is connected to the machine and asked a series of questions that require a ‘Yes’ or ‘No’ answer. One technique sometimes used by examiners is the relevant-irrelevant test. A relevant question relates to the crime of which the examinee is suspected and can include asking whether the examinee perpetrated the crime or knows who did. An irrelevant question is intended to provoke no emotion (e.g. “Is today Monday?”). Guilty examinees are expected to show stronger physiological responses to relevant questions than to irrelevant ones, whereas innocent examinees are expected to react similarly to both question types. However, this manner of testing is rarely used today, as the procedure can vary wildly from one examination to the next. The lack of standardization for administration and scoring makes the relevant-irrelevant test unsuitable for scientific use [11]. 

A more commonly used technique is the control question test. The examinee is asked a series of questions, some of which are relevant (e.g. “Did you murder Mr. Smith?”), and some of which are irrelevant but are designed to elicit physiological reactions from innocent examinees (e.g. “Have you ever stolen anything?”). The irrelevant questions – or control questions – are designed and intended to induce a dishonest response from the examinee. The expectation is that innocent examinees will experience concern about their answers to control questions that will show in their physiological responses. These responses are then compared to their physiological responses to relevant questions. It is expected that an innocent examinee will react more strongly to control questions than to relevant questions, and that a guilty examinee will react strongly to both question types but show heightened physiological responses to relevant questions [12]. 

Another commonly used technique during polygraph testing is the guilty knowledge test. The examiner presents the examinee with a set of questions, some of which contain details of a case under investigation that have not been made public. It is assumed that the true answer would be known only to investigators and those present at the incident. For example, a question might involve the type of implement used to kill someone. The examiner would monitor an examinee’s reaction to a question such as, “Was Mrs. Smith killed with a hammer (pause), or a screwdriver (pause), or a knife (pause)?” Several similar questions would be asked [13].  Guilty examinees are expected to reveal their guilty knowledge by responding more strongly to the true item than to the others. It is important to note, however, that the guilty knowledge test is applicable only when there is a specific incident and when there are relevant details that are known only to investigators and those present at the incident. The test will not be applicable to generic events [14]. 

Similar to the above test is the peak-of-tension test. Using this technique, the examiner will ask questions in a recognizable order (e.g. “Was the amount of money stolen $1,000? $2,000? $3,000? etc.”). A guilty examinee is expected to show a pattern of responsiveness that increases as the correct answer approaches and decreases once it has passed. Because the examiner knows the correct answer, he or she can evaluate the examinee’s pattern of physiological responses for involvement in the incident [15]. 

Once the polygraph exam is complete, the examiner will conduct a post-exam interview. During this final phase, the examiner will analyze the physiological data collected during the test and inform and discuss the results with the examinee. The results will be one of the following:

  • No Deception Indicated (NDI) – the examinee is telling the truth;
  • Deception Indicated (DI) – the examinee is not telling the truth;
  • Inconclusive – no opinion can be rendered; or
  • Incomplete – insufficient data has been collected [16]. 


1 National Research Council of the National Academies, The Polygraph and Lie Detection (Washington: The National Academies Press, 2003) at 11-12 [NRC].

2 Polygraphia, The Services, online: Polygraphia: Polygraph Examination Services [Polygraphia].


4 Anthony Gale, The Polygraph Test: Lies, Truth and Science (London: Sage Publications, 1988) at 11-12 [Gale].

5 NRC, supra note 1 at 13.

 6 Katherine To, Lie Detection: The Science and Development of the Polygraph, online: University of Southern California [To].


 8 Ibid.

9 Polygraphia, supra note 2.

10 To, supra note 6.

11 NRC, supra note 1 at 254.

12 Gale, supra note 4 at 14.

1Ibid at 15.

14 NRC, supra note 1 at 257.

15 bid at 258.

16 Polygraphia, supra note 2.