Normal Topic How Polygraphers Become Deluded About Accuracy (Read 2763 times)
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How Polygraphers Become Deluded About Accuracy
Dec 12th, 2006 at 9:23am
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In A Tremor in the Blood: Uses and Abuses of the Lie Detector (2nd ed., New York, Plenum Trade, 1998), the late professor of psychology David T. Lykken makes some cogent observations that help to explain how many polygraphers become deluded into believing that the polygraph is almost always correct (pp. 68-71):

The Limitations of Expert Opinion

In the standard textbook of polygraphic interrogation, Reid and Inbau assert:

"Our actual case experiences over the years have involved the polygraph examination (either personally or under our direct supervision) of over 100,000 persons suspected or accused of criminal offenses or involved in personnel investigations initiated by their employers. On the basis of that experience, we are confident that the technique, when properly applied by a trained, competent examiner, is very accurate in its indications. The percentage of known errors with the technique used in the laboratories of John E. Reid and Associates is less than 1 percent. [endnote omitted]"

Another highly regarded polygrapher of wide experience, R.O. Arther, similarly claims an accuracy of 99%. [endnote omitted] In 1939, the chairman of the psychology department at Fordham University, the Reverend Walter G. Summers, claimed 100% accuracy on more than 200 criminal cases. [endnote omitted] Tesifying before a committee of the Minnesota state legislature in 1975, a polygrapher from Texas stated that he had given more than 20,000 lie tests in his career and had "never been shown to have made a mistake." David Raskin, a former professor of psychology and primarily responsible for the marriage of the polygraph to the personal computer, reported in 1983 to a federal judge in California that the computer indicated a probability of 100%(!) that John DeLorean was truthful in denying his guilt on a drug charge. Paul Minor, then head of the FBI's polygraph unit, subsequently tested DeLorean and found him to be deceptive. Mr. Minor recently asserted on national television that the lie detector's error rate is only "one to two percent." [endnote omitted]

These are not selected examples. Nearly every experienced polygraphic examiner who has recorded an opinion about the accuracy of tests he has himself administered has chosen an estimate in this range, where 95% is "conservative" and 99% is perhaps typical. And most of these polygraphers are honorable people; it would be absurd to accuse all of them of venal misrepresentation. In many seemingly parallel situations, both in the courtroom and in everyday life, the opinions of such experts, based on their long experience, are taken very seriously.

One must realize, first, that someone who has devoted a career to lie detection, who has given thousands of tests the results of which have seriously affected for good or ill the lives of many people, must inevitably be strongly motivated to believe that these tests have been accurate. Experienced polygraphers would be less than human if they were not quicker to perceive positive than negative evidence of the value of their work. Second, the utility of polygraph testing does not depend solely on the accuracy of the lie test. The polygraph examination acts as a powerful inducer of admissions or confessions and, because of the mystique of the procedure, would do so even if the polygraph were just a stage prop. Examiners who are frequently able to elicit admissions of misconduct or, in criminal cases, admissions of guilty may therefore feel that they control a powerful technique--and "powerful" is easily transmuted into "valid." Moreover, like everyone else, polygraphers are more inclined to remember the good cases than the bad ones and to have a clearer recollection of those instances where their efforts solved some mystery than the ones where they remained in doubt.

These considerations are especially important because, in the vast majority of examinations, polygraphers never know if they were right or wrong. In criminal cases, many crimes are never solved, most suspects never go to trial. How then do we account for the claims of 95% and 100% accuracy? We must attribute them to the inevitable distortion that results when true believers attempt to evaluate the soundness of their own beliefs using "noisy" and inadequate data. [endnote omitted]

How Polygraph-Induced Confessions Mislead Polygraphers

It is standard practice for police polygraphers to interrogate a suspect who has falied the lie test. They tell him that the impartial, scientific polygraph has demonstrated his guilt, that no one now will believe his denials, and that his most sensible action at this  point would be to confess and try to negotiate the best terms that he can. This is strong stuff, and what the examiner says to the suspect is especially convincing and effective because the examiner genuinely believes it himself. Police experience in the United States suggests that as many as 40% of interrogated suspects do actually confess in this situation. And these confessions provide virtually the only feedback of "ground truth" or criterion data that is ever available to a polygraph examiner.

If a suspect passes the polygraph test, he will not be interrogated because the examiner firmly believes he has been truthful. Suspects who are not interrogated do not confess, of course. This means that the only criterion data that are systematically sought--and occasionally obtained--are confessions by people who have failed the polygraph, confessions that are guaranteed to corroborate the tests that elicited those confessions. The examiner almost never discovers that a suspect he diagnosed as truthful was in fact deceptive, because that bad news is excluded by his dependence on immediate confessions for verification. Moreover, these periodic confessions provide a diet of consistently good news that confirms the examiner's belief that the lie test is nearly infallible. Note that the examiner's client or employer also hears about these same confessions and is also protected from learning about most of the polygrapher's mistakes.

Sometimes a confession can verify, not only the test that produced it, but also a previous test that resulted in a diagnosis of truthful. This can happen when there is more than one suspect in the same crime, so that the confession of one person reveals that the alternative suspect must be innocent. Once again, however, the examiner is usually protected from learning when he has made an error. If the suspect who was tested first is diagnosed as deceptive, then the alternative suspect--who might be the guilty one--is seldom tested at all because the examiner believes that the case was solved by that first failed test. This means that only rarely does a confession prove that someone who has already failed his test is actually innocent.

Therefore, when a confession allows us to evaluate the accuracy of the test given to a person cleared by that confession, then once again the news will almost always be good news; that innocent suspect will be found to have passed his lie test, because if the first suspect had not passed the test, the second person would not have been tested and would not have confessed. [endnote omitted]


George W. Maschke
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Re: How Polygraphers Become Deluded About Accuracy
Reply #1 - Dec 12th, 2006 at 5:56pm
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Dr. Lykken's writings are irrefutable and put to rest any discussion regarding the accuracy of polygrapher's opinions of polygraphy.

I note from Dr. Lykken's CV that he wrote the following op-ed article:

Lykken, D.T. (1983). Three big lies about the lie detector. (Invited Editorial) USA Today, (p.7), February 17. Lykken, D

I conducted a Google search for it and was unable to locate it.  Can anyone post a link to said article?
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How Polygraphers Become Deluded About Accuracy

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