When Lie Detectors Lie or Don't

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So claims that polygraphy is "worse than useless " are too broad; the technology's usefulness depends on the goal. Imagine you need to hire a new employee. You can do this on the basis of a submitted rsum, but I doubt you would trust that alone. You could conduct a telephone interview. Better yet would be a face-to-face discussion, and that's what most employers do. They want to be able to look at demeanor, and perhaps vary the line of questions depending on what they see. Now imagine going one level deeper: asking penetrating questions ("Were you ever arrested?") while listening to the heartbeat and sensing the sweat glands of the subject. Would you forgo that information, if it were easily available?

Only a few of the many reviews of the accuracy of the polygraph system meet rigorous scientific standards, according to the National Academy of Sciences' report. Yet the credible studies show a surprisingly high accuracy rate. In my own informal survey, I was particularly impressed with an Israeli measurement (1) (they tricked their own policemen into cheating, then polygraphed them), a review of prior experiments conducted by Kircher et al. (2), and a review by Paul Ekman, a psychology professor at the University of California, San Francisco (3). I can summarize these results in an oversimplified but helpful way: the polygraph procedure has an accuracy between 80 and 95 percent. Let's call it 85 percent.

If a subject is lying, that figure means that an examination by an experienced polygrapher will detect the deceit about 85 percent of the time. It will go undetected 15 percent of the time. That's why the Russian spy Aldrich Ames could laugh at the test he was given; he was part of the 15 percent. Some people lie easily. Ekman implies that professional card players are particularly immune to exposure. They are probably people who are naturally good at minimizing or hiding emotional response.

It is also true that this accuracy figure implies that only 85 percent of truth-tellers will be exonerated; 15 percent will be falsely accused of lying. That is why the National Academy of Sciences' report came down so hard on the process. The Academy was specifically tasked with examining whether polygraphy should be used for testing current employees of the U.S. Department of Energy. If 10,000 were tested, 1,500 would be incorrectly tagged as liars. Presumably this would include 15 percent of the best and the brightest. The effect on employee morale would be devastating, the Academy's study committee concluded. The Federation of American Scientists' quote about "worse than useless" referred specifically to the fact that despite the harm done to morale, true spies such as Aldrich Ames still slip through.