Polygraphs: the truth

Lie detector tests are becoming more widespread in the USA. But do they work?

Howard Fienberg

Topics Politics

Following the arrest on 25 February 2001 of Robert Philip Hanssen, who was accused of spying on the USA for Russia, US attorney general John Ashcroft and Federal Bureau of Investigation (FBI) director Louis Freeh declared a tightening of national security measures.

The FBI plans to require polygraph testing of many more agents with access to important secrets, as is the policy in the Central Intelligence Agency, the Department of Defense and the National Security Agency. Polygraphs are also used to varying degrees for screening job candidates in the public and private sectors, and in both private and criminal investigations.

Polygraphs are such familiar instruments that few question the validity or efficacy of polygraph testing. But how well does the device really detect lies?

The US Department of Energy administered it to American physicist Wen Ho Lee when first accusing him of leaking nuclear secrets to the Chinese. He passed the test, but the FBI subsequently reviewed the same test results and declared Lee a liar. Convicted spy Aldrich Ames passed numerous polygraph examinations over the years, while he was also passing CIA secrets to Russia.

The American Polygraph Association in 2000 cited 12 studies demonstrating an average accuracy of 98 percent (1). On the other hand, University of Minnesota psychologist William Iacono accuses polygraph examiners of ‘wishful thinking’ – ‘almost no published peer-reviewed scientific papers exist that bear out the accuracy of current polygraph techniques’ (2).

That may be why the American Medical Association and the American Psychological Association contend that polygraphs yield little more than a 50/50 chance of success. Data is hard to come by. The false positive rate could be anywhere from one percent to 20 percent. The National Research Council recently convened a panel on polygraphs to try to settle the matter, but its report is not expected until at least the end of 2001.

So what is the precise problem with polygraph testing? According to critics, it lies not in the hardware, but in the interpretation of the test results.

Using a polygraph as a lie detector involves studying a subject’s physical response to specific questions. The series of questions asked usually starts with queries unrelated to whatever matter is being investigated in order to establish a baseline of physical reading, including blood pressure, sweat gland activity and breathing. Then other questions more directly address the areas of suspicion.

But interpreting the results of the exam is a subjective task and confounding factors are numerous. A fair number of fearful but innocent subjects could produce false positives and psychologically-savvy or unstable guilty subjects could yield false negatives. And there is a burgeoning industry of books and webpages promising to teach how to fool polygraphs.

With an error rate that defies calculation, polygraph tests are much more useful as tools of intimidation than as instruments of truth. Experts agree that, faults aside, the tests can convince guilty parties to confess who would otherwise have remained stoical. But few polygraph examiners speculate on what effect this intimidation has on the innocent.

Previous big stories, like the case of Wen Ho Lee, elicited a number of reports on the doubts scientists and judges held about the effectiveness of polygraph testing. Since those doubts have not abated, it is strange that so little skeptical attention was paid to polygraphs this time around.

Howard Fienberg is a research analyst with the non-profit non-partisan think-tank Statistical Assessment Service (STATS), in Washington, DC.

Related links:

American Polygraph Association

Stop Polygraphs

(1) Denver Post, 2 May 2000

(2) Boston Globe, 15 August 2000

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Topics Politics


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