How Does a Polygraph Work in 2023? Experts Solid Answers

The polygraph machine, also known as a lie detector, is commonly used in a variety of settings, such as law enforcement, national security, and employment screening. It is often used as a tool for interrogation as well as a screening method for sensitive positions such as those of law enforcement officers, intelligence personnel, and government officials.

a polygraph test administered to a subject by an investigator.

The machine measures physiological responses such as changes in blood pressure, heart rate, breathing rate, and skin conductivity, among other things. These physiological responses are believed to be indicators of deception, as lying often causes an increase in physiological arousal.

While the use of the polygraph machine has been controversial, it remains a widely used tool for detecting deception. In this article, we will delve deeper into how the polygraph machine works, the science behind it, and how it is used in various settings.


The importance of understanding how a polygraph works

Understanding how a polygraph works is important for a variety of reasons. Whether you are facing a polygraph test yourself or are simply curious about how the technology works, having a clear understanding of the science behind it can help you make informed decisions and interpretations.

One reason why understanding polygraphs is important is that they can have significant consequences for the individuals being tested. False positives, which occur when a truthful person is identified as lying, can result in lost job opportunities, damaged reputations, or even criminal charges. Conversely, false negatives, which occur when a liar is identified as truthful, can allow a guilty individual to avoid consequences and perpetuate harmful behavior.

Furthermore, understanding the limitations of polygraphs can help individuals and organizations make more informed decisions about when to use them. For example, while polygraphs can be useful for certain purposes, such as identifying areas where further investigation is needed, they are not always reliable indicators of truthfulness. By recognizing these limitations, employers and law enforcement agencies can avoid placing undue reliance on the results of polygraph tests.


The theory behind the polygraph’s functioning

An Examiner Interpreting The Polygraph Tests Results

The polygraph is based on the assumption that when people lie, they experience physiological changes that can be measured and recorded. These changes are believed to reflect a heightened state of stress or anxiety that occurs when someone is being deceptive or trying to conceal information.

The polygraph measures several physiological responses to determine whether someone is lying or telling the truth. The most commonly measured responses are blood pressure, heart rate, respiration rate, and skin conductivity. These responses are believed to reflect changes in the autonomic nervous system, which controls bodily functions such as heart rate, blood pressure, and breathing.

During a polygraph examination, the subject is typically asked a series of questions, some of which are designed to elicit a physiological response in the event of deception. The examiner then looks for patterns in the physiological data to determine whether the subject is telling the truth or lying.

The polygraph is based on the idea that the body’s physiological responses are largely involuntary and cannot be easily controlled, even by someone who is attempting to deceive.


What happens during a polygraph test? step-by-step

  • During a typical polygraph test, you will first be asked to complete a pre-test interview with the examiner. This interview helps the examiner establish a baseline for your behavior and get a better understanding of your emotional state. The examiner will also explain the test and the procedure to you, as well as answer any questions you might have.
  • The examiner will then attach sensors to your body, including a blood pressure cuff, two pneumographs (to measure breathing), and electrodes on your fingers to measure your skin’s electrical conductance. These sensors are used to record your physiological responses during the test.
  • Next, the examiner will conduct a “stimulation test” to establish your baseline responses. This involves asking you a series of neutral questions, such as your name and address. The examiner will then intersperse relevant and control questions that are designed to elicit a physiological response if you are lying.
  • Once the test is complete, the examiner and the computer’s algorithm will analyze the results and determine whether you showed signs of deception. The results of the polygraph test are typically presented in a written report but can also be provided verbally.


How the test works in relation to the examiner

An examiner tests an examinee while he is attached to a polygraph machine.

The examiner plays a crucial role in the accuracy and validity of the test results. The examiner is responsible for administering the test, interpreting the results, and reporting the findings to the appropriate parties.

In order to be a qualified polygraph examiner, an individual must undergo specialized training and certification. Many examiners have backgrounds in law enforcement or psychology, and they must adhere to strict ethical guidelines when administering and interpreting tests.

One important aspect of the examiner’s role is to establish a rapport with the examinee. This involves creating a relaxed and non-threatening atmosphere so that the examinee feels comfortable.

During the test, the examiner is responsible for asking the questions and monitoring the physiological responses. The examiner is trained to recognize signs of stress, such as changes in breathing, blood pressure, and sweating, that may indicate deception or anxiety.

It is important to note that the examiner’s role is not to determine guilt or innocence but rather to provide an objective assessment of the test subject’s physiological responses to the questions asked during the test.



The different measurements that are taken during a polygraph test

An examinee attached to the polygraph machine.

During a polygraph exam, several physiological measures are taken:

  • A pneumograph, or two air-filled tubes strapped across the chest, to measure respiratory rate.
  • Two electrodes will be connected to your fingers to measure the level of sweat secretions from your skin and skin conductivity.
  • A blood pressure cuff will be placed on your arm to measure your heart activity and blood pressure levels.
  • A plethysmograph may also be attached to a finger to measure its blood volume.
  • Electronic sensors will be placed on your chest to also monitor your respiratory levels and rate (not often).
  • Motion sensors may also be applied to pick up leg, arm, or anal sphincter movements that may interfere with the test’s data.
  • In some modern environments, a microphone will be placed to measure voice stress.


A few checks will be made to ensure that the sensors are correctly connected and the chart recording system is running. Once everything has been set up properly, the examination will begin.

Your physiological reactions will be recorded from three major systems in the body, all of which are controlled by the autonomic nervous system.

These physiological reactions are collected from:

  • Cardiovascular System: heart rate, relative blood pressure, and blood volume
  • Respiratory System: breathing rate or shallow breathing
  • Electrodermal System: sweat gland activity


Blood pressure and heart rate data

This data is collected by the arm-encircling cuff, or heart monitor, placed on the upper arm. The cuff is connected to the polygraph and filled with air through tubes.

Changes in the blood pressure modulate the air pressure in the cuff and are recorded.

Respiratory system data

The breathing pattern is measured by two devices that record thoracic movements, or volume, that change during respiration.

One of the tubes is wrapped around your chest, while the other is wrapped around your abdomen. The air pressure inside these tubes will fluctuate as you breathe in and out, and the device will record this.

Electrodermal or perspiratory data

This data will measure the sweat or galvanic skin response using a two-piece galvanometer attached to two of your fingertips.

The galvanometer works by sending a small electric current into the skin of one finger and recording how much current passes through the other finger.

If you are sweating, the water and salt will encourage the current more than someone whose skin is dry. The amount of electric current is recorded and reflects the amount of sweat produced at the fingertips.



Types of plygraph test questions

Based on your pre-test responses, a predetermined series of questions will be asked. They will be presented a minimum of three times, while data is continuously collected as you respond. All questions should be discussed and reviewed with you; this gives you an opportunity to make adjustments.

There will be four types of questions, asked in different combinations: relevant, irrelevant, control, and concealed (or guilty) knowledge questions.

  • Relevant Questions: These are questions that are relevant to the subject of the interview. It will be your physical responses to these questions that will be of most interest to your examiner. For example, if you are under investigation for theft, a relevant question would be asked by directly inquiring if you stole the item in question. However, if your exam is part of a pre-employment check, a relevant question may relate to your entire job history and your stated qualifications.
  • Control Questions: The purpose of these questions is to elicit the same emotional state that you will potentially be in when lying, and therefore they are supposed to be difficult to answer truthfully. For example, when asked, “Have you ever stolen anything?” most people won’t admit this in the environment of a polygraph, but the chances are that they have stolen even the smallest thing during their lifetime. These questions will be used to provide a base for comparison when analyzing your reactions to relevant questions and are usually unrelated to the subject of the interview.
  • Irrelevant questions: Irrelevant questions will be completely unrelated to the subject in question. They can be as simple as “what is the day today?” or “what country are we in?” These types of questions are meant to cause you as little stress as possible and provide a baseline to judge other questions. During the exam, irrelevant questions are interspersed with relevant or control questions.
  • Concealed Information Questions: These questions are only inserted when trying to gather information about a crime of which you are suspected. For example, if the crime in question is theft, questions may be asked about the possible locations of the theft’s occurrence. Examiners will be looking for certain reactions when different locations are suggested, along with a strong physiological reaction expected to indicate that that particular suggested location may be the correct one.


How long does a polygraph test take?

The length of a polygraph exam can vary depending on the complexity of the issue being investigated and the number of questions that need to be asked. Typically, a polygraph exam can last anywhere from 1 to 3 hours.

Each question set may last only a few minutes, but you may find that you answer these sets several times, and you’ll be provided with short breaks, which will add to the overall time.

Collecting data from the polygraph equipment typically involves periods of 10–15 minutes at a time.

After the test is complete, the examiner will review the physiological data and analyze the results. This can take anywhere from a few minutes to several hours, depending on the complexity of the issue being investigated and the amount of data collected during the test.

There is always the chance of an examination being cut short due to confession or extended due to inconsistencies in the machine readings.


Factors that can influence the length of the test

An examinee is attached to the polygraph machine while the examiner observes the results.

When it comes to the length of a polygraph examination, there are a variety of factors that can come into play.

  1. One of the biggest factors is the complexity of the questions being asked. If you’re being asked about a simple matter, such as whether or not you stole a piece of candy from a store, the exam might only take 30 minutes or so. However, if the questions are more complex and require a more in-depth examination, the exam could take several hours.
  2. Another factor that can influence the length of a polygraph exam is your own physical condition. If you’re feeling anxious or stressed, it may take longer to get accurate readings from the physiological measures being taken during the exam. Additionally, if you have certain medical conditions, such as heart problems, it may take longer to get accurate readings, which could result in a longer exam.
  3. The experience and skill of the polygraph examiner can also play a role in how long the exam takes. A more experienced examiner may be able to get accurate readings more quickly, resulting in a shorter exam. On the other hand, a less experienced examiner may need to spend more time to ensure they are getting accurate readings, resulting in a longer exam.
  4. Finally, the number of questions being asked can also affect the length of the exam. The more questions there are, the longer the exam will take. It’s important to note that the number of questions asked during a polygraph exam can vary widely depending on the purpose of the exam and the examiner’s approach.


How the results are interpreted by the examiner

One common method used by examiners to interpret the results is called the comparison question technique (CQT). In this technique, the examiner will compare the physiological responses to a series of control questions (CQ) to the physiological responses to relevant questions (RQ) related to the specific issue being investigated.

If the physiological response to a relevant question is greater than the physiological response to the control questions, the examiner may determine that the examinee is being deceptive.

Another technique used to interpret the results of a polygraph exam is the peak of tension technique (POT). This technique is based on the assumption that when a person lies, there will be a momentary peak of tension during the deception that will be reflected in the physiological responses. The examiner will analyze the physiological data for any peaks that occur during the exam, which may indicate deception.

This may seem straightforward, but the examiners analyzing the spikes and dips have had substantial training to look for abnormalities. They search for abnormally increased physical reactions while also looking for reduced physical reactions and reactions that are uncorrelated in a reasonable timeframe with the questions asked.

The interpretation of the chart and its spikes and dips will be guided by a computer algorithm and a protocol. The protocol is based on three principles:

  • Reliability
  • Accuracy
  • Validity


Reliability is generally used to indicate how many times this procedure has been repeated across different times, places, and subjects. Test-retest reliability measures standard procedure against specific situations.

Accuracy and validity go together when interpreting exam results. This refers to how well the exam indicated deception and non deception.

It’s important to note that the interpretation of polygraph results is not a simple “lie or truth” determination. Instead, the examiner will analyze the results and provide an opinion as to the likelihood that the person being tested was being deceptive or truthful. The examiner will take into consideration the physiological responses, the type of questions asked, and other relevant factors.

It is also important to note that the interpretation of the results of a polygraph exam is subjective and can vary among examiners.


The types of conclusions that can be drawn from a polygraph test

A letter claiming that the polygraph was "passed".

There are four possible results from a polygraph examination:

  • Successful: “Deception Not Indicated” (DNI) means that the examiner has determined that the subject was not being deceptive in their responses to any of the questions asked during the exam. This can be based on a lack of significant physiological responses or consistent responses to both relevant and control questions.
  • Failed: A result of “deception indicated” (DI) means that the examiner has determined that the subject was being deceptive in their responses to one or more of the questions asked during the exam. This can be based on a number of factors, such as increased physiological responses when answering certain questions or inconsistent responses between relevant and control questions.
  • Inconclusive: An inconclusive result means that the examiner was not able to definitively determine whether the subject was being deceptive or truthful in their responses. This can be due to a number of factors, such as inconsistent physiological responses or an inability to establish a baseline for the subject’s responses.
  • Purposely Non-Cooperative (PNC): You were caught trying to use countermeasures during the test or did not comply with the given instructions.


Test results are usually provided verbally or via USPS Priority Mail. The average response time is between 2 and 3 days following the exam, but this could be longer depending on the amount of review time. In certain industries, you will receive the results right after the test is finished.

According to various state licensing laws and the American Polygraph Association’s Standards and Principles of Practice, the results of a polygraph can only be released to authorized personnel.

Those who can receive the results are the examinee and anyone else who is specifically designated in writing by the examinee, firm, corporation, or government agency that requested the exam, along with some others who might be required by law.


Is the examiner’s decision final?

Simply put, the examiner’s decision is not final. The results are not deemed final until there has been a quality control review. All polygraph examiners are held to strict guidelines to provide each applicant with fair and accurate examinations.

Quality control consists of senior-level federal polygraph examiners. Under the federal procedure, the polygraph results are submitted to the quality control team for final review and determination of the end exam result.


Test errors

The polygraph technique is not foolproof, and errors do occur, as with any test.

One type of error is a false positive. This occurs when the polygraph indicates that a person is lying when they are, in fact, telling the truth. False positives can occur when the polygraph operator misinterprets physiological responses or when a person experiences anxiety or nervousness for reasons unrelated to deception, such as being falsely accused.

Conversely, a false negative occurs when the polygraph indicates that a person is telling the truth when they are, in fact, lying. False negatives can occur due to the examiner’s misreading of your physiological data. It is also possible for him to do everything correctly and still have the test result in an error.

If you think there were mistakes in the results, you might be able to appeal them. To assess the outcomes, you might need to speak with a specialist such as the Axeligence team, which is well-versed and experienced in appealing failed results.

Share and Support Us in Reaching the Right People:
Accelerating Solid Intelligence, One Person at a Time.

Welcome to Axeligence, your trusted partner for intelligence, investigation, and polygraph consulting services.

Accelerating Solid Intelligence, One Person at a Time.

*Please be patient as we are in the process of translating all of our content.


We use cookies to ensure you get the best experience on our website.