How Lie Detectors Work

By: Kevin Bonsor
lie detection machine
lie detection at job interview Peter Dazeley / Getty Images

­For­ more than 15 years, Robert Hanssen led a double life. In one l­ife he was a 25-year veteran with the Federal Bureau of Investigation (FBI) who had access to some of the nation's most-classified information. In his other life, he allegedly was spying for the Russian government. Hanssen's deception was finally discovered, and in February 2001 he was arrested and later pled guilty to 15 espionage-related charges. Spies are probably the world's best liars, because they have to be, but most of us practice deception on some level in our daily lives, even if it's just telling a friend that his horrible haircut "doesn't look that bad."

­People tell lies and deceive others for many reasons. Most often, lying is a defense mechanism used to avoid trouble with the law, bosses or authority figures. Sometimes, you can tell when someone's lying, but other times it may not be so easy. Polygraphs, commonly called "lie detectors," are instruments that monitor a person's physiological reactions. These instruments do not, as their nickname suggests, detect lies. They can only detect whether deceptive behavior is being displayed.


Do you think you can fool a polygraph machine and examiner? In this article, you'll learn how these instruments monitor your vital signs, how a polygraph exam works and about the legalities of polygraph testing.

A polygraph instrument is basically a combination of medical devices that are used to monitor changes occurring in the body. As a person is questioned about a certain event or incident, the examiner looks to see how the person's heart rate, blood pressure, respiratory rate and electro-dermal activity (sweatiness, in this case of the fingers) change in comparison to normal levels. Fluctuations may indicate that person is being deceptive, but exam results are open to interpretation by the examiner.

Polygraph exams are most often associated with criminal investigations, but there are other instances in which they are used. You may one day be subject to a polygraph exam before being hired for a job: Many government entities, and some private-sector employers, will require or ask you to undergo a polygraph exam prior to employment.

Polygraph examinations are designed to look for significant involuntary responses going on in a person's body when that person is subjected to stress, such as the stress associated with deception. The exams are not able to specifically detect if a person is lying, according to polygrapher Dr. Bob Lee, former executive director of operations at Axciton Systems, a manufacturer of polygraph instruments. But there are certain physiological responses that most of us undergo when attempting to deceive another person. By asking questions about a particular issue under investigation and examining a subject's­ physiological reactions to those questions, a polygraph examiner can determine if deceptive behavior is being demonstrated.


The Polygraph Machine

The polygraph instrument has undergone a dramatic change in the last decade. For many years, polygraphs were those instruments that you see in the movies with little needles scribbling lines on a single strip of scrolling paper. These are called analog polygraphs. Today, most polygraph tests are administered with digital equipment. The scrolling paper has been replaced with sophisticated algorithms and computer monitors.

components of a lie detector system
The basic components of a lie detector system

­When you sit down in the chair for a polygraph exam, several tubes and wires are connected to your body in specific locations to monitor your physiological activities. Deceptive behavior is supposed to trigger certain physiological changes that can be detected by a polygraph and a trained examiner, who is sometimes called a forensic psychophysiologist (FP). This examiner is looking for the amount of fluctuation in certain physiological activities. Here's a list of physiological activities that are monitored by the polygraph and how they are monitored:


  • Respiratory rate - Two pneumographs, rubber tubes filled with air, are placed around the test subject's chest and abdomen. When the chest or abdominal muscles expand, the air inside the tubes is displaced. In an analog polygraph, the displaced air acts on a bellows, an accordion-like device that contracts when the tubes expand. This bellows is attached to a mechanical arm, which is connected to an ink-filled pen that makes marks on the scrolling paper when the subject takes a breath. A digital polygraph also uses the pneumographs, but employs transducers to convert the energy of the displaced air into electronic signals.
  • Blood pressure/heart rate - A blood-pressure cuff is placed around the subject's upper arm. Tubing runs from the cuff to the polygraph. As blood pumps through the arm it makes sound; the changes in pressure caused by the sound displace the air in the tubes, which are connected to a bellows, which moves the pen. Again, in digital polygraphs, these signals are converted into electrical signals by transducers.
  • Galvanic skin resistance (GSR) - This is also called electro-dermal activity, and is basically a measure of the sweat on your fingertips. The finger tips are one of the most porous areas on the body and so are a good place to look for sweat. The idea is that we sweat more when we are placed under stress. Fingerplates, called galvanometers, are attached to two of the subject's fingers. These plates measure the skin's ability to conduct electricity. When the skin is hydrated (as with sweat), it conducts electricity much more easily than when it is dry.

­Some polygraphs also record arm and leg movements. As the examiner asks questions, signals from the sensors connected to your body are recorded on a single strip of moving paper. You will learn more about the examiner and the test itself later.

Detractors of the polygraph call lie detection a voodoo science, saying that polygraphs are no more accurate at detecting lies than the flip of a coin. "Despite claims of 'lie detector' examiners, there is no machine that can detect lies," reads a statement from the American Civil Liberties Union (ACLU). "The 'lie detector' does not measure truth-telling; it measures changes in blood pressure, breath rate and perspiration rate, but those physiological changes can be triggered by a wide range of emotions."­

Lee, who has been performing ­polygraph exams for 18 years, agrees that polygraphs do not detect lies. "What has happened over the years is that the media has dubbed this lie detection, and that's what's clicked, but from a scientific perspective, absolutely not. There's no such thing as lie detection. I couldn't tell you what a lie looks like."

He does assert that polygraphs can detect deceptive behavior even through the stress brought on by the exam itself. "If the (forensic psychophysiologist) is properly trained and has the experience, he can penetrate that. Through the specific procedure that the FP will employ, anxiety will not penetrate into it."

In the next section, you will learn more about who polygraph examiners are and what makes them qualified to conduct these examinations.


Polygraph Examiners

­­There are only two people in the room during a polygraph exam -- the person conducting the exam and the subject being tested. Today, some polygraph examiners prefer to be called forensic psychophysiologists (FPs). Because polygraph­ examiners are alone in the room with a test subject, his or her behavior greatly influences the results of the exam.

­"It's a very serious factor when someone is being accus­ed of a crime," Lee said. "See, I don't care about the deceptive person. I'm looking for the innocent person. I'm their advocate. I'm totally unbiased and neutral when that person comes walking in. But as soon as I make that assessment that there's no deception indicated, I immediately become their advocate."


The forensic psychophysiologist has several tasks in performing a polygraph exam:

  • Setting up the polygraph and preparing the subject being tested
  • Asking questions
  • Profiling the test subject
  • Analyzing and evaluating test data

How the question is presented can greatly affect the results of a polygraph exam. There are several variables that an FP has to take into consideration, such as cultural and religious beliefs. Some topics may, by their mere mention, cause a specific reaction in the test subject that could be misconstrued as deceptive behavior. The design of the question affects the way the person processes the information and how he or she responds.

Who uses polygraphs?

Polygraphs are limited in their use in the private sector, but they are frequently used by the U.S. government. Here are some entities and occasions that may call for the use of a polygraph:

­There are approximately 3,500 polygraph examiners in the United States, 2,000 of which belong to a professional organization, according to Dr. Frank Horvath, a Michigan State University professor of criminal justice and a member of the American Polygraph Association.

Horvath is concerned about the credentials and qualifications of many polygraph examiners i­n the United States who do not belong to some sort of professional organization. Laws regarding polygraph licensing vary from state to state, and there is no government or private entity that controls polygraph licensing. Horvath also feels that training of polygraph examiners is inadequate.

"I just do not think the field is at the state where we would say that any polygraph examiner is the equal of all other polygraph examiners. That's just not so," Horvath said. "We have a number of standardization problems in terms of examiner qualifications that concern me enormously. You could buy a polygraph [instrument] tomorrow and come to Michigan and you wouldn't be able to practice here because we have a rigorous licensing law, but you could move down to Ohio and open a business tomorrow."

Today, some polygraph examiners take classes and work an internship in order to become an accredited examiner with national associations. Some states also require examiners to be trained. There are many schools around the United States that have been set up to train people to conduct polygraph exams. One of these schools is the Axciton International Academy, which was started by Lee. The school is accredited by the American Polygraph Association and certified by the American Association of Police Polygraphists.

Here are the steps that students at the Axciton Academy must complete before becoming licensed forensic pyschophysiologists:

  • Prior to enrolling in the school, students must possess a baccalaureate degree or have five years of investigative experience and an associates degree.
  • Students must attend and pass a 10-week intensive course. Curriculum includes psychology, physiology, ethics, history, question construction, psychological analysis of speech, chart analysis and test-data analysis.
  • Students must enter an internship program and conduct a minimum of 25 exams for actual cases. These exams are faculty reviewed. This internship can take anywhere from eight months to one year.

Following the completion of these requirements, the student becomes a polygrapher and may obtain a license in his or her state if that state requires one. There is no standardized test that all polygraph examiners must pass in order to practice.


Lie Detector Test

­ Undergoing a lie detector test can be an intimidating experience that can challenge the nerves of even the most stoic person. You are sitting there with wires and tubes attached to and wrapped around your body. Even if you have nothing to hide, you could be afraid that the metal-box instrument sitting next to you will say otherwise. Fittingly, undergoing the uncomfortable experience of a polygraph test is often referred to as "going on the box."

A polygraph exam is a long process that can be divided up into several stages. Here's how a typical exam might work:


  • Pretest - This consists of an interview between the examiner and examinee, where the two individuals get to learn about each other. This may last about one hour. At this point, the examiner gets the examinee's side of the story concerning the events under investigation. While the subject is sitting there answering questions, the examiner also profiles the examinee. The examiner wants to see how the subject responds to questions and processes information.
  • Design questions - The examiner designs questions that are specific to the issue under investigation and reviews these questions with the subject.
  • In-test - The actual exam is given. The examiner asks 10 or 11 questions, only three of four of which are relevant to the issue or crime being investigated. The other questions are control questions. A control question is a very general question, such as "Have you ever stolen anything in your life?" -- a type of question that is so broad that almost no one can honestly respond with a "no." If the person answers "no," the examiner can get an idea of the reaction that the examinee demonstrates when being deceptive.
  • Post-test - The examiner analyzes the data of physiological responses and makes a determination regarding whether the person has been deceptive. If there are significant fluctuations that show up in the results, this may signal that the subject has been deceptive, especially if the person displayed similar responses to a question that was asked repeatedly.

There are times when a polygraph examiner misinterprets a person's reaction to a particular question. The human factor of a polygraph exam and the subjective nature of the test are two reasons why polygraph exam results are seldom admissible in court. Here are the two ways that a response can be misinterpreted:

  • False positive - The response of a truthful person is determined to be deceptive.
  • False negative - The response of a deceptive person is determined to be truthful.

"If we look at laboratory-based studies, false-positive errors occur somewhat more often than false-negative errors," Horvath said.

Critics of polygraph exams say that even more false-positive errors occur in real-world scenarios, which biases the system against the truthful person. These errors are likely to occur if the examiner has not prepared the examinee properly or if the examiner misreads the data following an exam.­


Countermeasures and Legalities

Often, people who are being given a polygraph exam will employ certain countermeasures in an attempt to beat the instrument. There are Web sites and books that instruct you on how to fool the polygraph. Here are just a few examples of how people try to trick the device:

  • Sedatives
  • Antiperspirant on fingertips
  • Tacks placed in the shoe
  • Biting tongue, lip or cheek

The idea of countermeasures is to cause (or curtail) a certain reaction that will skew the test's result. A subject may attempt to have the same reaction to every question so that the examiner cannot pick out the deceptive responses. For example, some people will place a tack in their shoe and press their foot down on the tack after each question is asked. The idea is that the physiological response to the tack may overpower the physiological response to the question, causing the response to each question to seem identical.


Whether you pass or fail a polygraph exam will often have very little legal ramification. Often, defense lawyers brag that their client has passed a polygraph. Of course, you will rarely hear of a defendant taking a polygraph if he or she failed it.

Polygraphs are rarely admissible in court. New Mexico is the only state in the United States that allows for open admissibility of polygraph exam results. Every other state requires some type of stipulation to be met prior to admitting polygraph exams into record. In most cases, both sides of a legal case have to agree prior to the trial that they will allow polygraphs to be admitted. On the federal level, the admissibility criteria are much more vague and admission typically depends on the approval of the judge.

The main argument over the admissibility of polygraph tests is based on their accuracy, or inaccuracy, depending on how you want to view it. The level of accuracy of a lie detector depends on whom you talk to about it, Horvath said. Both sides of the argument have the same research to look at, but they come to very different conclusions.

Employee Polygraph Protection Act of 1988

Employees in the private sector are not subjected to polygraph exams like employees of the federal government. The U.S. federal government is the largest consumer of polygraph exams. Private sector employees are protected by the Employee Polygraph Protection Act of 1988 (EPPA). This law only affects commercial businesses. It does not apply to schools, prisons, other public agencies or some businesses under contract with the federal government.­

EPPA provides that a business cannot require a pre-employment polygraph and cannot subject current employees to polygraph exams. A business is allowed to request an exam, but cannot force anyone to undergo a test. If an employee refuses a suggested exam, the business is not allowed discipline or discharge that employee based on his or her refusal.

­"We have people in the scientific community who look at the same research that I look at and they reach a conclusion that is quite different," Horvath said. "From their point of view, they allege that polygraph testing is probably only around 70 percent accurate, and it has a great bias against truthful people. Then, what the proponents say, looking at the same research, they reach a quite different conclusion, and that is that polygraph testing is around 90 percent accurate."

At the federal level, there have been specific legal cases that have shaped the admissibility of polygraphs. The results of these cases are mixed: There have been some federal circuits that have admitted polygraph results, while others have flatly denied them. Here are just a few of the legal cases that have shaped how polygraphs are viewed by the U.S. courts:

  • Frye v. United States (1923) - U.S. Court of Appeals of District of Columbia - This is the original decision dealing with scientific evidence and its admissibility in court. Frye was accused of murdering a doctor. At the time, he took a unigraph, a precursor to the polygraph. The unigraph measured only the cardiovascular activities of the body. The examiner reported Frye to be truthful, and Frye moved to have that evidence admitted in court. The court ruled that before any scientific evidence could be admitted into the court of law it must first be accepted by the scientific community. At that time, there were no studies done on unigraphs or polygraphs, so the evidence was not admitted.
  • United States v. Piccinonna (1989) - U.S. Court of Appeals, 11th Circuit - This decision allowed for polygraph results to be admitted in court, but only if one of two requirements is met: Either the two parties in the case agree to allow it, or the judge decides to allow it based on criteria established by the 11th Circuit.
  • Daubert v. Merrell Dow Pharmaceuticals (1993) - U.S. Supreme Court - The court opened the door for scientific evidence, and gave judges broader discretion as to whether or not to admit polygraphs. This applies to all federal courts but does not apply in state courts, although particular states do accept this ruling.
  • United States v. Scheffer (1998) - U.S. Supreme Court - Moving beyond the broader topic of scientific evidence, this military case directly involved polygraphs. The court ruled that the U.S. president has the prerogative to deny polygraph results in military tribunals because polygraph testing is so controversial.

It seems clear that no final decision has been made on the federal level. At the state level, polygraph admissibility is generally handled on a case-by-case basis. The courts' ambiguity stems from the questionable validity of polygraph exams. Interestingly, the biggest opponent to polygraph admission in court is the U.S. federal government, which happens to be the largest consumer of polygraphs exams.

There are still many questions that must be answered before polygraphs are accepted by the courts and the public at large. Of course, we may never see this broad acceptance. No matter if you agree or disagree with the use of polygraphs, thousands of people undergo these tests every year, and many people's lives are changed forever by their results.