When Lie Detectors Lie or Don't

Lie detectors have been called "worse than useless." So why are they still used?

Lie detectors are notorious liars. They implicate the innocent and exonerate the guilty. So why are they still used by intelligence agencies? Is that intelligent? Police use them in investigations, yet in most states the results cannot be used in court. Are the police wasting their time? Businesses use them for pre-employment screening. Doesn't such stupidity make them less competitive?

In fact, lie detectors, formally known as polygraph machines, have been getting a bad rap. The National Academy of Sciences, for example, was strongly critical in their recent, widely publicized study. The web site of the prestigious Federation of American Scientists states bluntly, " polygraphs are worse than useless-they are a significant threat to national security.

Let's look a little deeper at the science. The standard method of polygraphy depends on four measurements: heart rate, breathing rate, blood pressure, and skin electrical conductivity. Of these, the last is the most informative, and you can buy a simple device to measure it for under $10. But the true lie detector is not the machine; it is the operator, and that person requires expensive training and extensive experience. In addition to the four measured bodily changes, the operator frequently watches body language and facial expressions, and pays attention to irregularities in verbal response. These are the same indicators we ordinary people use when trying to detect deceit, when sitting on a jury or just talking with a used car dealer.

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Polygraph machines don't actually detect lies. They measure hidden emotional responses. Given that information, the skilled polygrapher can redirect the interview in hopes of making evasiveness or deceit obvious. That makes the examination feel like an interrogation, and that is why subjects dread the process. If you would like to know what it feels like (I've gone through it three times), imagine interviewing for a job while sitting in full view of the interrogator-naked. Some people could do this with equanimity, but most of us would be extremely uncomfortable.

That discomfort is part of what makes polygraph enthusiasts enthusiastic. They themselves experienced the uneasiness of having the polygrapher return, repeatedly, to those very items that made them uncomfortable. They "know" that polygraphy "works." In courtrooms, despite what is seen on the old Perry Mason reruns, the guilty almost never confess. But under a polygraph examination, they often do. They feel that the polygrapher is reading their minds, and they give up all hope of cover.