The polygraph lie detector works on the same principles as detecting behavioral betrayals of deceit, and it is vulnerable to the same problems. The polygraph exam does not detect lies, just signs of emotion.
How does a lie detector work?
Wires from the polygraph are attached to the suspect to measure changes in sweating, respiration, and blood pressure. Increases in blood pressure or sweating are not in themselves signs of deceit. Hands get clammy and hearts beat faster when emotion is aroused.
Before giving the polygraph test, most polygraph operators try to convince the suspect that the polygraph never fails to catch a liar, giving what is known as a “stimulation,” or “stim,” test. The most common technique is to demonstrate to the suspect that the machine will be able to tell which card the suspect picks from a deck. After the suspect has picked a card and returned it to the deck, he is asked to say no each time the polygraph operator asks him if it is a particular card.
Some of those using this technique make no mistakes, because they don’t trust the polygraph record to catch the lie but use a marked set of cards. They justify deceiving the suspect on two grounds. If he is innocent, it is important that he thinks the machine will make no mistake; otherwise, he might show fear of being disbelieved. If he is guilty, it is important to make him afraid of being caught; otherwise, the machine really won’t work. Most polygraph operators don’t engage in this deceit but rely upon the the polygraph record to spot which card was taken.
Where Polygraph Exams Fall Short
The suspect must believe in the ability of the lie catcher. Signs of fear would be ambiguous unless matters can be arranged so that only the liar, not the truth teller, will be afraid. The polygraph exams fail not only because some innocents still fear being falsely accused or for other reasons are upset when tested, but also because some criminals don’t believe in the magic of the machine. They know they can get away with it, and if they know it, they are more likely to be able to do so.
Some polygraph operators attempt to extract a confession by convincing their suspects that they can’t beat the machine. When a suspect does not confess, some polygraph operators will browbeat the suspect, telling the suspect that the machine has shown that the suspect is not telling the truth. By increasing detection apprehension, the hope is to make the guilty confess. The innocent suffer the false accusations but supposedly will be vindicated. Unfortunately, under such pressures some innocents will confess in order to obtain relief.
Learn How to Detect Lies through Micro Expressions
Anyone can learn how to spot deception. Read more about micro expressions, the facial expressions revealing someone’s true emotions.