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Police, on the other hand, have found a valid use for polygraphy in their investigations. If a suspect is willing to submit to a test, he may have more than his guilt to hide; he may have useful information. A nervous response to the mention of a specific location, for instance could help police find a murder weapon. 85 percent accuracy sounds pretty good, for this application.

When businesses use the process in pre-employment screening, they can be accused of being unfair, but not of being stupid. They may be willing to forgo 15 percent of potential good employees, provided that they can avoid 85 percent of potential troublemakers. It is a business decision, at least until lawmakers or courts decide that reading emotions during hiring is an illegal invasion of privacy.

Now we come to the true paradox. Lie detector results are inadmissible as evidence for criminal trials in most states. But I have been present at a trial in which the judge instructed the jury that it was their responsibility, not his, to determine the truth of the testimony. To do this, they were told to take into account “the demeanor” of the witness, his directness in answering questions, and anything else that they thought indicated truthfulness. Ironically, scientific tests show that the average person’s probability of catching a lie in this way is only “slightly better than chance,” according to Ekman. Moreover, the jurors who use this approach have the conviction that their accuracy is near 100 percent, despite their knowledge that most witnesses are extensively coached in methods of appearing sympathetic and truthful-in other words, in methods to defeat the system.

Polygraphy is not allowed in courts because 85 percent accuracy is not good enough. Instead courts use a system that is demonstrably worse-which could be a big part of the reason why so many convictions are now being overturned by DNA evidence. Where is the wisdom in that?

Notes 1) Avital Ginton, Netzer Daie, Eitan Elaad, and Gershon Ben-Shakhar,”A method for Evaluating the Use of the Polygraph in a Real-Life Situation”, Journal of Applied Psychology, vol 67 (1982), p. 132. 2) J. Kircher, S. Horowitz, D. Raskin, “Meta-analysis of Mock Crime Studies of the Control Question Polygraph Technique”, Law and Human Behavior vol 12 (1988), pp. 79-90. 3) Paul Ekman, Telling Lies: Clues to Deceit in the Marketplace, Politics, and Marriage, 4 (1985, revised 1991 and 2002), pp. 213-14.

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