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© Copyright 1998, Jim Loy
The traditional lie detector is called the polygraph, because it draws several graphs on paper. It measures several more or less involuntary physical reactions of a person. These reaction are blood pressure, pulse rate, breathing changes, and galvanic skin response. This last is the electrical resistance of the skin, which is heavily influenced by sweating. The theory is that these physical reactions tend to change when a person tells a lie. In general, the theory works.
The results of a polygraph test are mostly not legally acceptable in court. Are there problems with the polygraph? Well, yes, there are some problems.
The polygraph is not 100% reliable. But a jury may treat its results as 100% reliable. A witness can be honestly mistaken, and the polygraph will say that he or she is telling the truth. A jury may then take this as the absolute truth, because of the polygraph. Rarely, a person can lie, and not be caught. Quite a bit more often, a person can tell the truth, and the polygraph can say that he or she is lying. How does this happen? Well, feelings of guilt, or being upset for some other reason, can show up as a lie:
"Did you kill your husband?" "No." But, the lady thought about killing her husband, and feels guilty about that. The polygraph may show that "No" was a lie, even though it was the truth."Did you kill your husband?" "No." But, the lady was out cheating on her husband when he died. She feels guilty, and maybe even feels responsible for his death.
"Did you kill your husband?" "No." But, the lady thinks that some trivial action of hers, like not being able to find her keys, may have caused him to die instead of her.
And there are a few people who feel guilty often, and show up as liars, naturally.
Another kind of lie detector is the psychological stress evaluator, which detects stress in your voice. An example of this was shown in one of the Billy Jack movies, to show that a politician on TV was lying. This device would seem to have the same problems. I have not heard how reliable this device is.
Addendum #1:
Apparently the polygraph is even more unreliable than I thought. There is massive evidence that it is a big scam, a method of intimidating suspects into giving incriminating information. As such, guilty people should demand a polygraph examination, in hopes of conning the police. And organizations that are "protected" by using the polygraph, are not as secure as they think they are; the polygraph breeds a sense of false security.
Another scam is the "honesty test." This is a multiple choice test, which asks questions that tell if you are honest or not. Many of these questions have an obvious correct answer that does show honesty. Let's pretend that you are asked, "If you saw someone drop a dollar bill, would you return it?" If you are half-way honest, you might have to admit, "No, I would keep the dollar." Dishonest people would tend to lie, and say, "Yes, I would return the dollar." So, some of the questions would tend to get the opposite to the desired response.
Addendum #2:
The National Academy of Sciences has recently (Oct. 2002) reported that the polygraph is too flawed to be used for security screening. For example, if the polygraph is used to screen out spies or potential spies, then many perfectly innocent people will test positive for spying, and a few actual spies will test negative and be cleared of spying (as has happened in real life). Also, it would seem that there is still almost no evidence that lying affects blood pressure, pulse rate, breathing changes, or galvanic skin response in any consistent way.
See The National Academies - Polygraph Testing Too Flawed for Security Screening.