The science behind polygraph testing: what you need to know





Since its invention and final evolution to its most digital form, the polygraph machine has been the subject for much heated debate. Despite the contestation, however, the science behind polygraph testing stands firm. Polygraph testing involves the monitoring of a subject by a polygraph machine and an interrogation by an examiner trained in forensic psychophysiology. Polygraph tests are requested for a wide variety of reasons, but many people still do not understand the science behind the lie detection method. So, in just a few minutes of reading, find out about the science, both social and technological, behind polygraph testing.


The science of polygraph machines


In most cases, the polygraph machine measures around 4 to 6 different physiological reactions – all of which are recorded by the 3 medical instruments combined in the form of one machine. Where older polygraphs included long strips of paper moving slowly underneath pens that recorded the different physiological responses, newer polygraph technology, such as that offered by Polygraph Truths, uses transducers to convert the relevant information to digital signals.


These signals are then stored and analysed on computers using technology’s most sophisticated mathematical algorithms.


The 3 components of the polygraph machine that make polygraph testing possible


There are 3 main components that you have likely heard are what make up a polygraph machine. It is these 3 pieces of equipment that measure the abovementioned physiological reactions that may occur during polygraph testing.


The cardio-sphygmograph


This component of the polygraph machine is responsible for measuring the blood pressure and heart rate of the person undergoing testing. This is executed through attaching a blood-pressure cuff around the arm of the subject. Throughout questioning, the cuff remains inflated so as to pick up the various physiological responses.


The movement of blood through the subject’s veins is one factor that the cardio-sphygmograph measures. When blood flows through the veins, it generates a sound. When wearing the cuff, this sound is transmitted through the air in the cuff into a bellows, which serves to amplify the sound. The sound’s magnitude is dependent on the blood pressure.


Further, it is the frequency of the changes in the sound that relates to heart rate. This process is how the cardio-sphymograph works, and the relevant physiological reactions recorded all contribute to the polygraph machine’s important recordings.


The pneumograph


Perhaps the most interesting component out of the 3, the pneumograph is responsible for recording the participant’s respiratory rate. This is done through placing one tube around the chest of the subject, and another around their abdomen. The tubes are subsequently filled with air, and when the participant breathes, the air pressure in the tubes is affected.


Then, the results are recorded on the polygraph test.


The galvanograph


This component is responsible for recording the amount of perspiration produced by the subject in specific areas. Electrical sensors, known as galvanometers, are attached to the fingertips of the person being tested. The fingertips are the ideal location for the galvanometers, since the skin of the fingertips comprises a high density of sweat glands.


Through the sweat on the fingertips where the galvanometers are attached, an increase in sweat triggers the decreasing resistance of the electrical current measured. It is these changes that are recorded by the polygraph machine and compared with the other results to form conclusive evidence of deception or truthfulness.


The keymograph


The keymograph is not included in the components that make up the modern polygraph machine. This is because it was the motor responsible for pulling the chart paper underneath the recording pen to record data in older polygraph machine models. Today, the use of computer technology in polygraph testing has made this component unnecessary.




Interestingly, however, when the keymograph was in use, it made these recordings at rates of 6 to 12 inches per minute.


To sum it up so far


Through the 3 key components, the pneumograph, the cardio-sphygmograph and the galvanograph, the physiological reactions of the subject are measured in order to retrieve revealing data that is then analysed by a professional tester. Although most forensic psychophysiologists believe that the pneumograph and cardio-sphygmograph components are significantly more informative than the galvanograph when it comes to polygraph testing, it is still widely believed that an increase in perspiration is indicative of deception – which is why modern polygraph machines still include the component.


The social science: examination conditions


Many experts argue that the environmental conditions under which the polygraph test is conducted is of the utmost importance. For this reason, many polygraph testing companies adhere to the following guidelines when it comes to the examination room within which the test is conducted:


  • The room ought to be well-lit and ventilated.
  • It should be private to a large degree.
  • It needs to be free from outside noise and other distractions.
  • If possible, it should be sound-proof.
  • The space should not contain any decoration, such as paintings, posters, pictures, trinkets or other ornaments.
  • The examination room should be devoid of any unnecessary furniture.
  • Necessary furniture includes: the polygraph desk on which the machine sits as well as the participant’s and tester’s chairs.




About the polygraph tester


It goes without saying that the polygraph tester is an impartial figure who is an expert in reading and analysing the machine’s recordings, as well as comporting themselves appropriately throughout the questioning and answering process. The tester will have knowledge of both the instrument’s abilities and limitations.


Further, the tester will have been selected due to their high levels of integrity and morality. All professional polygraph testers know that their primary obligation is to be an impartial quester for truth. They will also be obligated to make sure that subjects are mentally and physically fit to endure the test, as well as guard them from technological error that could impact them negatively or positively.


About the test subject


Should you be questioning whether or not to sit, or send someone for, a polygraph examination, you should be aware that there are specific criteria that must be adhered to and acknowledged in the case of every subject. For example, the following permanent physical illnesses will disqualify someone from undergoing a polygraph examination:


  • Breathing disorder
  • Narcotic drug addiction
  • Certain heart conditions
  • Mental derangement of any kind


The following temporary illnesses also make an individual unfit for polygraph testing:


  • Mental or external fatigue
  • Fever
  • Colds
  • Allergies
  • Injury
  • Pain
  • Physical discomfort
  • Influence via sedatives
  • Influence via liquor


These instances will require a postponement of a polygraph test until the participant is well again.


The 3 phases of a polygraph examination


There are 3 phases that make up a proper polygraph test;


  1. The pre-test interview
  2. The interrogation and recording through the instruments
  3. The post-test interview


All 3 phases must be executed in order for the polygraph test to be considered successful in any way.


The pre-test interview


Before the test actually begins, the examiner will spend some time speaking with the subject. There are a whole host of forensic psychophisyologists who believe that this leg of the process is completely necessary and of the utmost importance. Through talking to the subject, the examiner is able to gather a baseline on the emotional state of the participant, and is then able to develop the questions that will be asked during the actual testing phase.


Although the test questions are formulated by the investigator, they are based off of information that has been provided by the investigator, or individuals concerned. In general, sketchy facts, theories and suspicions are not enough to go on when it comes to polygraph testing.


Questions need to be formulated with as much detailed and accurate information at the helm. Questions in polygraph tests are only answerable by either “yes” or “no”. All questions are supposed to be clear with reference to one, and only one, element of an accusation or offense each.


There are 3 types of questions;


Irrelevant: which have nothing to do with the investigation, but rather have to do with the test subject’s life. For example, their marital status or occupation. The tester asks these questions to establish a baseline or pattern of how a subject answers normally.


Relevant: which pertain to the concerns under investigation.


Control: which are unrelated to the issue under investigation but are of a similar nature.


Before the test, the polygraph examiner will go through the questions with the subject so that they are aware as to what exactly to expect. The examiner will also thoroughly explain the purpose of the polygraph examination.


Once this phase is completed, the examiner will attach the machine components to the subject and begin the interrogation.


The interrogation


It is during this phase that the examiner will pose the pre-composed questions to the subject and hear their yes or no responses. The examiner will both make note of their own findings, that are based off of their forensic psychophysiological training, as well as the recordings of the polygraph machine.


The post-test interrogation


This last leg of the journey serves as an opportunity to clarify the findings. The examiner may also wish to question the participant as to whether there were reasons for a specific answer to a relevant question other than the knowledge of a crime or transgression. This is also the time to obtain any additional information that relates to law enforcement purposes if the polygraph test concerns criminal activity.


The scientific theory behind polygraph testing


As you may have guessed by now, there is an underlying theory of polygraph testing; that being that when individuals lie, they become considerably nervous about the deception. This could manifest in an increased heartbeat, increased blood pressure, increased sweating or a change in the rhythm of breathing.


The fact that each subject has unique physiological reactions, to an extent, is undeniable, however. But this is where the baseline comes in, and the pre-test interrogation. As previously mentioned, a baseline which allows the tester to get a sense for the participant’s normal reactions is established before the commencement of the actual interrogation.


This decreases the likelihood for irrelevant nervousness to impact the test where they are not applicable. It is the deviation from the baseline of what the tester may consider as truthfulness that is indicative of a sign of deception.




Polygraph Truths, backed by science


We at Polygraph Truths acknowledge and adhere to both the social and technological science that govern the effective use of polygraph testing. As a result, we offer the highest standard of lie detection services, ensuring a high level of accuracy in your endevours pertaining to polygraph services. Please do not hesitate to contact us for more information!