This is a supplement to the main article: The Polygraph or Lie Detector
Here is the textbook explanation that Department of Defense Polygraph Institute-trained polygraphers provide to their subjects:
You may be a little nervous, especially if you have not had a PDD [“psychophysiological detection of deception,” a more scientific sounding term for “lie detection”] examination before. This is expected and is quite normal. To help put you at ease, I will explain what the instrument is and how it works.
The polygraph is a diagnostic tool that is used to determine if a person is telling the truth. It simply records physiological changes that take place in your body when you are asked questions. Today, changes in your respiration, sweat gland activity, and blood pressure will be recorded. Please notice the two rubber tubes on the desk. One will be placed across your chest and the other will be placed around your abdominal area. They will be used to record your breathing. There are two metal fingerplates next to the rubber tubes. These plates will be attached to two of your fingers and will record your sweat gland activity. Finally, there is a blood pressure cuff on the desk. It is the same type of cuff a doctor uses to measure blood pressure. It will be placed on your arm and will monitor changes in your cardiovascular activity.
These physiological changes are a result of an automatic response system in your body. It is a response system over which you have no control. For example, visualize yourself walking down a dark alley late at night. Suddenly you hear a loud noise. You will instantaneously decide either to remain where you are and investigate the source of the noise, or to flee the area, sensing danger to your well being. Regardless of the choice you make, your body automatically adjusts itself to meet the needs of the situation; your heart may beat faster, your breathing may change and you may break out in a cold sweat.
When you were growing up, if you are like most people, you were raised to know the difference between right and wrong. Quite probably, all of the adults you came in contact with—your parents, grandparents, relatives, teachers, church officials—taught you that lying, cheating, and stealing were wrong. Ever since you were a young child, you have been programmed to know that lying is wrong. Think about the first time you lied and got caught. Remember how your body felt during that confrontation. Your heart may have been racing or you may have been sweating.
However, the responses were automatic; your body adjusted to the stress of the situation. People are not always 100% honest. Sometimes it is kinder and more socially acceptable to lie than to be honest - such as telling someone you like their clothes when you really think the clothes are awful. It is important for you to understand that even though a lie might be socially acceptable or only a small lie, or a lie by omission, your body still responds. The recording on the polygraph will show only the physiological responses. It cannot know what kind of lie you are telling. Therefore, it is extremely important that you be totally honest…