Polygraph Test: Everything You Need to Know About How a Session Unfolds
- Caroline Hébert
- Sep 29
- 8 min read
The polygraph, often mistakenly called a “lie detector”, fascinates as much as it intrigues.
It is commonly imagined as a mysterious tool reserved for police dramas and FBI special agents.
Yet in reality, the polygraph test is a rigorous, regulated, and much more humane process than one might think.
In this article, you are invited to take a behind-the-scenes look at a polygraph session, step by step, to understand how it actually unfolds.
What is a Polygraph?
Before diving into the details, let’s clarify what a polygraph is. It is a sophisticated device that measures and records various physiological reactions of the human body: chest movements, sweating, heart rate, blood pressure, and blood flow.
The fundamental concept is simple: the body does not lie. When lying, the body feels stress and perceives a threat, triggering automatic and involuntary physiological reactions. Imagine finding a snake in your bathroom. Or someone suddenly braking in front of you on the road. Or an intruder entering your home. Your body will react instantly, whether you want it to or not: your heart rate will increase, your hands will sweat, and the blood flow will concentrate around your vital organs… This is called the “fight or flight” reaction. When someone lies, the brain perceives a similar threat: the risk of being exposed.
Of course, the biological reaction triggered by telling a lie is much less intense than the one caused by coming face to face with a bear in the forest. But the principle and biological result are the same, and polygraph instruments can detect these reactions, whether intense or extremely subtle.
So, the polygraph does not detect lies per se, but rather the physiological reactions associated with deceiving the interrogator and thereby, the risk of being uncovered.
Forget images of needles scratching long paper tapes like in old spy movies. Nowadays, everything is digital. The sensors are discreet and non-invasive, data is analyzed by computer, and the professional administering the test—the polygraph examiner—is trained to interpret the results with rigor and precision.
Preparation of the Test by the Polygraph Examiner
A polygraph test is not just about hooking someone up to a device and asking questions. Significant analysis and preparation take place before the session.
The first step is an in-depth discussion with the client, whether the person to be tested or the individual or organization requesting the test. The context varies: it could be screening for a sensitive job, a criminal case, a civil or family dispute, an infidelity case, or a sexual misconduct file.
Defining the Objective of the Test
In a human resources context, the aim might be to ensure a candidate has not falsified their resume, or to check for any history of fraud or inappropriate sexual behavior.
In an infidelity case, questions will be more direct and focused on a specific event or a general allegation of adultery.
In a criminal matter, it will be about verifying the subject’s involvement in a given crime, validating or refuting an alibi, or confirming testimony.
Developing the Questions
This is the most delicate stage. Questions must be clear and precise, relating to concrete behaviors, leaving no room for interpretation.
For example, in a bank robbery accusation, one question might be: “Did you take money from ABC Bank?” But what if the tested person did not physically take money from the teller but instead acted as a lookout at the entrance? Or their role was to drive the getaway car? Answering “no” to the question “Did you take money from ABC Bank?” could let the suspect pass the test, even though they actively took part in the robbery. They would have truthfully answered the question asked. That is why questions must be clear and precise. A better question in this context would be: “Were you involved in any way in the robbery of ABC Bank on January 12, 2023?”
Another, more complex example: imagine the tested person is a man suspected of murdering a young woman named Marie Tremblay. The seemingly simple question would be: “Did you kill Marie Tremblay?” However, if the actual events were that the man drugged Ms. Tremblay intending to sexually assault her, left while she was still alive but unconscious, and she died an hour later from an overdose, answering “no” could let the suspect pass. For him, he did not kill her; he "only" drugged and assaulted her. This would be a “false negative.” In this case, a more appropriate question would be: “Did you cause the death of Marie Tremblay?”
To add complexity, suppose a man is suspected in the disappearance of his wife, who was found dead two weeks later at the bottom of a river inside her car. If asked, as before, “Did you cause your wife’s death?” this situation could arise: If the couple argued the night of the disappearance, the woman left the house angry and had an accident causing her death, the suspect might fail the polygraph, even though he had nothing to do with her death, simply because he feels guilty about the argument and letting her leave angry. In his mind, he may feel he did “cause” her death, believing if he had acted differently, she would still be alive. This would be a “false positive.”
What polygraphy seeks to evaluate is not the unfortunate sequence of events or the subject’s perception of guilt, but the concrete actions taken. In this case, an appropriate question would be: “Did you physically cause your wife’s death?”
As you can see, formulating polygraph questions is crucial because any ambiguity can significantly distort results. Sometimes the polygraph examiner may spend nearly an hour developing the three or four right questions for a test, even if they seem simple and obvious to a layperson.
Often, clients arrive with a list of questions they want asked, hoping the polygraph examiner will use them. You now understand that this approach is not feasible. The polygraph examiner is trained to create the most relevant and suitable questions to obtain reliable and concrete answers to the issues the client really wants to investigate.
The polygraph examiner will gather as much information as possible about the person and situation by consulting with the client and reviewing documents and evidence. This work combines investigation, listening, and analysis.
Conducting the Polygraph Test
A polygraph appointment typically lasts between two and four hours and is divided into essential stages: the pre-test interview, the test itself, result analysis, and the post-test interview.
1. The Pre-Test Interview
The session starts by assessing the candidate’s physical and mental ability to undergo the test that day. Certain conditions might require postponement. We check for anything that could prevent the candidate from remaining still during the test (e.g., severe pain), interfere with sensor accuracy (e.g., frequent coughing or sneezing), or impair reasoning (e.g., intoxication).
It is important to note that stress is not an obstacle to polygraph testing and does not influence pass or fail chances. Everyone is nervous before a polygraph. The polygraph examiner is trained to distinguish general nervousness from physiological reactions linked to deception. This is part of their daily work.
Documents such as consent forms are completed. Polygraph tests are always voluntary. In North America, no one can be forced to take a polygraph, and the examinee can leave at any time.
The interview itself begins before the person is seated in the polygraph chair. The pre-test interview is often overlooked in popular imagination but is in fact the longest and most crucial part of the appointment. The polygraph examiner welcomes the person, explains the entire process calmly and respectfully, with no judgment or accusation. The goal is to find the truth, whatever it may be, not to trap or condemn the person tested. The candidate is reassured: this is a safe space where vulnerability is accepted, and there will be no surprises or manipulative tactics.
The sensors are shown to the candidate, the measurements explained, and all questions answered.
The candidate and polygraph examiner discuss the situation to be evaluated, the circumstances leading to the test, and different versions of events. Then, the test questions are presented. As mentioned, there are no surprises during a polygraph test. The person is informed of all questions beforehand. The questions are even practiced orally, without connecting the sensors, to ensure understanding and comfort. This is also the time to clarify anything that could cause confusion. The polygraph examiner takes notes, observes reactions, listens carefully, and adjusts the approach. They remain neutral, without influencing or judging.
The examiner is not there to accuse or suspect. His goal is to obtain answers, not to confirm a preconceived hypothesis. The examinee's credibility is never questioned at this stage. If doubts remain, they will be addressed later in the post-test interview.
2. The Polygraph Test Itself
This is the core of the session but usually lasts only about twenty minutes. The person is seated comfortably, and sensors are placed on the arm, chest, and fingers, then the test begins.
Before the real questions, a practice test is conducted with simple questions whose answers are known by both the candidate and polygraph examiner, to verify that equipment works well and establish baseline physiological reactions. This also allows the candidate to experience the polygraph process and feel comfortable before the real test.
Questions in both practice and actual tests are standardized and answered only with yes or no. Each question is asked multiple times, in different sequences and randomized order, to observe reactions over time rather than relying on a single isolated response.
The person must remain still, as any movement (fidgeting, coughing, scratching) can distort the measurements. The polygraph examiner watches carefully and notes anything that might influence results.
3. Interpretation of Results
After the test, the polygraph examiner analyzes the data. This is both a technical and analytical step. They examine all physiological reactions per question, looking for anomalies or deliberate attempts to skew the test. “Contaminants” (noises, coughs, involuntary movements, or countermeasure attempts) are identified and excluded to keep only relevant reactions.
Using specialized software, the polygraph examiner obtains an overall score that classifies the result into one of these categories: pass (no sign of lying), fail (reactions consistent with lying), or inconclusive (insufficient or ambiguous data).
4. The Post-Test Interview
This final step varies depending on the test result. Generally, the polygraph examiner immediately shares the result with the examinee. If passed, all is well, the interview ends positively, and the person leaves with peace of mind.
If failed, the approach differs. The examiner seeks to understand the reasons for the result. They attempt to obtain a confession and understand the truth, but without pressure or accusation. The tone remains empathetic and open. Even if the person admits nothing verbally, their body has given clues. The polygraph examiner then offers support and damage control to understand and, if possible, repair consequences of the lie.
In inconclusive cases, a discussion occurs about the results and possible causes (physical issues, difficulty following instructions, etc.) The candidate might be asked to repeat the test immediately or return another day when in a better physical or mental condition.
In summary, the polygraph test is much more than a simple lie detector. It is a complex tool relying as much on technology as on human factors. It combines interrogation techniques, biological and psychological knowledge, analysis, and strategic interviewing. Its effectiveness depends on thorough preparation, the examiner's neutrality, their ability to create a climate of trust, and careful formulation of clear, context-appropriate questions.
Preparing for Your Polygraph Appointment: Tips for the Day Before
Inform and discuss any medical conditions (respiratory or heart issues, pregnancy, chronic pain, allergies/colds/flu, etc.) with your polygraph examiner well before the appointment to ensure you are fit to take the test.
Allocate at least 3 hours for the appointment and avoid scheduling other commitments immediately after to avoid time-related stress.
Do not consume alcohol, drugs, or energy drinks on the day of the test.
Try to get a good night’s sleep before the test.
Maintain your usual diet and prescription medication routines.
Dress comfortably, preferably wearing a short-sleeved shirt to allow proper placement of the blood pressure cuff, and bring a jacket for comfort during pre- and post-test interviews. Avoid dresses or short skirts, as crossing legs is not allowed during the test.
Wear socks with your shoes (or bring a pair if wearing sandals), as you will likely be asked to remove shoes during the test.
Bring a photo ID to your appointment.
Némésis offers criminal profiling services, polygraph testing, investigation, and investigative analysis to Quebec police forces, legal professionals, as well as any entity or citizen requiring specialized expertise in private investigation.