How Does a Lie Detector Work?

While the term "lie detector" may conjure up images of a device that's capable of knowing beyond a doubt whether or not a person is lying, that is not really an accurate picture. Instead, a lie detector, or polygraph machine, measures a person's biological responses to a series of questions. These biological responses are then used to determine whether or not the person is telling the truth. Studies show that when a person lies, his body involuntarily reacts as his heart rate increases, blood pressure rises and perspiration becomes visible. The lie detector analyzes these things to make an educated guess as to whether or not the person is telling the truth.

The Biological Responses

When a person takes a lie detector test, he is hooked up to a machine. Typically, this includes the placement of a blood pressure cuff as well as two specialized devices that are placed on the fingertips and around the chest and abdomen. The device placed on the fingertips is designed to measure sweat. Sensors are placed around the person's chest and abdomen, which is used to measure his breathing rate. Once everything is in place, the test's administrator will begin questioning the person and evaluating his responses for any deceptive behaviors.

Before the actual questioning begins, the person administering the lie detector will meet with the individual she is examining. The two will talk one-on-one, and the test administrator will evaluate the person's normal responses. This first step is called a pre-test and generally lasts about an hour. Once the pre-test is complete, the questioning begins.
Usually, the test consists of about 10 questions, of which only a few are actually relevant to the situation at hand. The relevant questions are called design questions; the remaining questions are control questions. Control questions are very broad questions such as "have you ever lied to anyone in your life?" Because almost no one can honestly answer no to questions such as these, the examiner can get an idea of the individual's deceptive behaviors if he does lie. Once questioning is complete, the examiner will analyze the individual's responses to both the control and design questions. Using data such as changes in blood pressure and pulse rates, the administrator then makes a determination as to whether or not the individual is lying.