Patrick B. McGinnis, PhD, LMHC

How a Polygraph Works

You hear about lie detectors all the time in police investigations, and sometimes a person applying for a job will have to undergo a polygraph test (for example, certain government jobs with the FBI or CIA require polygraph tests). The goal of a lie detector is to see if the person is telling the truth or lying when answering certain questions.

When a person takes a polygraph test, four to six sensors are attached to the person. A polygraph is a machine in which the multiple ("poly") signals from the sensors are recorded on a single strip of moving paper ("graph"). The sensors usually record:

Sometimes a polygraph will also record things like arm and leg movement.

When the polygraph test starts, the questioner asks three or four simple questions to establish the norms for the person's signals. Then the real questions being tested by the polygraph are asked. Throughout questioning, all of the person's signals are recorded on the moving paper.

Both during and after the test, a polygraph examiner can look at the graphs and can see whether the vital signs changed significantly on any of the questions. In general, a significant change (such as a faster heart rate, higher blood pressure, increased perspiration) indicates that the person is lying.

When a well-trained examiner uses a polygraph, he or she can detect lying with high accuracy.

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.

Man vs. Machine
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. 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.

Polygraph Instrument
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.

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 triggers certain physiological changes that can be detected by a polygraph and a trained examiner 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:

Some polygraphs also record arm, leg and other physical movements. As the examiner asks questions, signals from the sensors connected to your body are recorded on a single strip of moving paper.

Voodoo or Valid
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 .., ."

Of course, polygraphs do not detect lies. Over the years the media has dubbed this lie detection, but from a scientific perspective, absolutely not. There's no such thing as lie detection. Polygraphs can detect deceptive behavior even through the stress brought on by the exam itself. The instrument is attuned to the physiology of the examinee before the actual tests start, through using what is called an acquaintance test.

Polygraph Examiners
There are only two people in the room during a polygraph exam -- the person conducting the exam and the subject being tested. Other people in the room during a test could greatly influence the results of the exam.

My role as a polygraph examiner is that of a seeker of truth. I'm looking for the truth. I am totally unbiased and neutral when a person walks in. As soon as I make an assessment that there's no deception indicated, I immediately become that person's advocate.

The polygraph examiner has several tasks in performing a polygraph exam:

How the question is presented can greatly affect the results of a polygraph exam. There are several variables that a polygraph examiner 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's a Qualified Examiner?
There are approximately 3,500 polygraph examiners in the United States, 2,000 of which belong to a professional organization such as the American Polygraph Association.

We should all be 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. Florida, for instance, does not license polygraph examiners

Today, most 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 International Academy of Polygraph, which was attended by Dr McGinnis. The school is accredited by the American Polygraph Association.

Here are the steps that students at the International Academy of Polygraph must complete before becoming polygraph examiners:

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.

Going on the Box
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 is divided into several stages. Here's how a typical exam might work:

Countermeasures
Often, people who are being given a polygraph exam will try to employ countermeasures in an attempt to beat the instrument. The idea of countermeasures is to cause (or curtail) a certain reaction that will skew the test's result. Sometimes attempts at countermeasures have the opposite effect and result in an invalid test (indicating countermeasures were employed - or the opposite of the desired result, a failed test. The examiner is trained to look for signs of such attempts and may employ certain sensors designed for this.

The Legalities of Polygraphs
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 who you talk to about it, Both sides of the argument have the same research to look at, but they come to very different conclusions. Generally, research indicates that polygraph testing is around 90 percent accurate.

Polygraphs generally are always admissible when part of a psychological assessment where other tests are administered.

Federal Cases
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:

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.

Who Uses Polygraphs?

Polygraphs are frequently used by the U.S. government. Here are some entities and occasions that may call for the use of a polygraph:

Related Links:

Last modified: 10/12/09