Lying has been a source of frustration, bewilderment, and legal problems for as long as there have been people. Science of People reports more than 82% of lies go undetected and that most people can only spot a lie with 54% accuracy. These are some startling statistics that can make you question almost every conversation.

Fortunately science has been at work trying to improve the ways that a person can detect lies for just as long as people have been lying. Both spotting lies in person and lie detectors—or polygraph machines—work based on a principle of comparing body functions and responses when someone is telling the truth to when they are asked a question to which they might lie.  If you’ve ever wondered how a lie detector can really tell if someone is lying, here is a breakdown of what polygraphs do and how they can tell if someone is lying.


The First Polygraph

From the beginning of the 20th century there were many attempts to make a modern device that could accurately tell is someone was being honest. Most failed, but the one that seemed to get the closest was invented by a man named John Larson in Berkeley, Califonia in 1921. Larson was a medical student who was trying to improve police interrogation procedures and build upon earlier models of lie detectors that monitored blood pressure while people were being asked questions.

Larson’s design wasn’t perfect, but a man named Leonarde Keeler took it soon after, improved upon it, and began championing its accuracy and usefulness in both law enforcement and many other areas of everyday life.


How Does A Polygraph Work?

A polygraph still performs the same basic function as those earlier devices—monitoring blood pressure. But, it also looks at perspiration and heart rate. As Illumin states in their overview of the Polygraph, “Despite it’s name, the lie detector, also known as the polygraph, is not capable of detecting lies.” Instead, what the machine does is produce a reading of different those three different physiological sings while someone is answering questions.

A skilled polygrapher constructs a series of questions that will test a person in many different ways. They then examine the body’s responses in order to gain insight into their state of mind while they answer. This is done through control questions which are designed to force the person being tested to tell the truth an then more ambiguous questions where they will then compare the person’s bodily reactions to the control questions—giving them an idea of how stressed they are as they answer.

The technology is far from being able to determine lie from truth every time right off the bat, but it’s the best that anyone has been able to come up with yet. The polygraph continues to advance with modern computerized technology, making it harder and harder for people to hide the truth.