What is a polygraph test?

lie detector test

The purpose of a polygraph test or examination (also, and more commonly, known as a lie detector test) is to detect deception; to work out the probability that someone is telling the truth or lie in response to specific questions.

A polygraph test records physiological (bodily) changes that occur in a person when they are asked questions about a situation, event or action that they have been accused of, or it has been suspected that they are lying about. 

Research, experimentation and studies within psychology and the behavioural sciences have identified that some bodily functions react to fear or threat in certain ways and that lying can induce this state of fear and anxiety in a person, causing these functions to react. These responses are measured in a polygraph test through the correct placing of sensors on these areas of the examinee’s body that are relevant to these responses. The sensors record several different physiological responses simultaneously and continuously throughout the lie detector test to determine how the examinee’s body reacts when hearing and answering certain questions. Their reactions indicate whether they are being truthful or deceptive when answering due to the known fear responses that lying triggers within a person.

The sensors used within a polygraph test record blood flow or blood pressure, heart rate or pulse, bodily temperature, galvanic skin response (sweat production of the hands), respiration and, often, movement of the body or fidgeting. Sensors are attached around the torso to measure respiration, as a cuff around the arm to measure blood pressure, and as cuffs or clips around fingers to measure pulse and sweat production.

The bodily responses associated with fear or threat include the increase in blood pressure or blood flow as the body readies itself for conflict or escape, increased body temperature due to the activation of muscles in preparation for this imminent need for activity, increased sweat production in the hands to improve grasping to assist with any conflict or escape need that may be coming and reduced respiration rate as the airways dilate to assist with any immediate need for physical action but no physical action occurs so breathing becomes more effective. Movement of the body or shifting/fidgeting is due to the difficulty keeping still when adrenaline has been released into the body in response to the perception of threat. 

The examination itself is an intrinsic part of identifying deception and polygraph examination does not just involve asking someone questions whilst they are hooked up to a machine. It is far more complex than that. The examination can take different forms as there are different schools of thought within polygraphy and different models of testing employed, but there are many similarities between these models and the differences may not be obvious to the uneducated eye. 

Some polygraph examinations can only address single issues (for example whether or not someone stole some money) whilst some can address several issues or are “multi-faceted” (for example whether or not someone stole some money, whether they had any involvement in the money being stolen at all or if they know who did steal the money). 

The questions that are asked during a polygraph examination are carefully formulated and ordered. The examiner will discuss with the examinee and/or the person requesting the test to find out what issue(s) the test needs to address and what needs to be clarified. They may well ask what questions you would like included in the test. They will then use the information you have supplied to word the questions correctly for a polygraph examination; it is essential that any ambiguity or certainty, or any dual implication is removed from questions to ensure valid and reliable answers and bodily responses are achieved from the examinee. Test models can allow for different numbers of the questions you want to be included in them. These models have to be adhered to if they are to achieve valid and accurate results so it may be that you need to have more than one lie detector test if you have a number of questions or more than one issue that you need to address.

A polygraph test will also include questions that you have not asked for and that you do not require responses to. This is to create the right psychological environment to achieve the greatest clarity from the bodily responses recorded. In other words, this prepares the examinee psychologically and physiologically for when they hear the mention of the specific issue and the questions that are pertinent to this. This is aimed at developing tension within the subject and determining the salience or significance of specific issues to them so that their bodily reactions are heightened and greater differentiation achieved between their responses to the “hot” topic and their baseline functioning. These questions take different forms according to the model of lie detector testing being employed but they generally will include introductory questions (questions that confirm the examinee knows what is going to happen and in which the examinee states whether they intend to answer truthfully or not), irrelevant questions (that are about innocuous, trivial  or insignificant matters such as what day of the week it is, what colour the wall is or if they travelled to the test by car) and comparison questions (which talk about an issue similar to that which the test has been arranged to address but which are exclusionary of that specific issue or are more wide ranging or general). These questions are ordered within a structure that experimentation and study has found best achieve the conditions for the examinee most conducive to detecting deception and it is important that this structure is followed is test results are to be valid and reliable. The questions will also be repeated with more than one set of questioning being used in a polygraph examination to ensure that data is consistent and to allow investigation of any anomaly. 

Before the polygraph examination even starts the examiner will undertake a pre-interview with the subject. This will allow them to gain greater understanding of the issue in question and to listen to the examinee’s narrative and views on this. This enables the examiner to observe the body language and presentation of the subject whilst they tell their side of the story which further assists in deception of detection.

Most polygraph examinations also include a pre-test or practice or acquaintance test which employs a different set of questioning then that which is used in the main lie detector test. This pre-test will allow the examiner to ascertain the normal baseline physiological responses for the individual sitting the test and how these may be affected on the day by nerves or anxiety about the test. The examiner will often include within this pre-test a request for the subject to deliberately and knowingly lie (about something inconsequential) to the examiner so that they can start to observe and record the physiological responses that occur within that individual when they are dishonest which also helps in interpretation of data after the test is concluded.

Once the polygraph examination is completed the examiner will have data that they have collected through the sensors that were attached to the body of the subject and recording physiological response throughout the pre-test and the actual lie detector test.

The examiner will analyse this data through the use of the model that they choose. There are several different models for data analysis, interpretation and scoring in use and again these depend on the model used for the polygraph examination itself and whether the test was single or multi-faceted. Once data analysis or scoring has been undertaken the examiner will be able to advise what response the subject gave to each relevant question asked and whether the examination has found the subject to be honest or dishonest in these responses. 

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