How Does a Polygraph Work?

o you have heard about polygraph exams, and maybe you are interested in taking one. However, do you know how polygraph works? Although often affectionately referred to as a lie detector, a polygraph does not detect lies! It monitors psychological and physical signs that point to elevates stress and anxiety – a common sign that someone is not quite telling the truth.

Let’s take a closer look at how polygraph exams work.

Why Do People Lie?

Before we go into detail about the polygraph test itself, let’s talk about liars! There are many different reasons why an individual chooses to lie. Some of the most common ones include:

  1. Protection – One of the main reasons people lie is to protect themselves or someone they care about. For example, they may lie to reduce the threat of damage or harm coming to them.
  2. Personal Benefit – Another common reason why people lie is to benefit themselves personally. This could stop them from losing a job, save their reputation, or even get attention.
  3. Personal Grudge – People also commonly lie when they are holding a grudge against someone to get them in trouble or hurt them somehow.

These are by no means the only reason people choose to lie, but they are certainly some of the most common, and we often come across these reasons when conducting lie detector tests.

What Does a Polygraph Measure?

When an individual is lying, there are specific involuntary physiological responses. Lying automatically triggers fear, anxiety, and nervousness. This is what the exam is designed to measure. The exam is designed to analyze the individual’s response to the questions to determine how truthful the answers are. There is a common misconception that if the subject does not feel guilty about their behavior, they will not fail a polygraph, but this is not the case. Guilt is not what triggers the physiological responses that a polygraph looks for – lying is!

A lie detector device has a variety of sensors to measure responses depending. These will include:

  • Blood Pressure Cuff on the arm to measure pulse and blood pressure
  • Rubber Tubing around the chest and abdomen to measure respiration
  • Metal Plates on the fingers to record perspiration

All of these feed into the device, which is connected to a computer to help analyze the recorded data. Lying is usually followed by an increase in heart rate and blood pressure. Breathing may also become irregular, and the subject will sweat more. It is also common for them to move more and exhibit signs of nervous excitement.

Uncover the truth!


Ken Shull served as a Special Agent with the FBI for almost 25 years and was head of the FBI Polygraph program until his retirement in 2001. At that time he set up the Kendall Investigations practice as a private investigator in Knoxville, TN offering Polygraph services, private investigations, and security guards. Ken is a member of the American Polygraph Association and The American Association of Police Polygraphists.

The Truth is Still the Truth Even if No One Believes it, A Lie is Still a Lie Even if Everyone Believes it.