To Tell The Truth

The parents of missing intern Chandra Levy do not believe Congressman Gary Condit has revealed all he knows of her whereabouts. Until this week, police investigators were satisfied with Condit’s answers, though the Levys have been demanding that he take a polygraph test. Now, Chief Charles Ramsey says that D.C. police will ask Condit to submit to the test, popularly known as a lie-detector test. Polygraphs are such familiar instruments, both in criminal investigations and job background checks, that few question the reliability or validity of the tests. But how well do these devices really detect lies?

The U.S. Department of Energy administered a polygraph test 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 passing CIA secrets to Russia.

The American Polygraph Association last year claimed that 12 studies demonstrated an average accuracy of 98 percent. On the other hand, University of Minnesota psychologist William Iacono accuses polygraph examiners of “wishful thinking.” He said last year that “almost no published peer-reviewed scientific papers exist that bear out the accuracy of current polygraph techniques.”

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 1 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 this year.

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. Test administrators usually begin by asking questions unrelated to the matter being investigated in order to establish a baseline of physical reading, including blood pressure, sweat gland activity, and breathing. Additional questions more directly address the areas of suspicion. However, interpreting the results of the exam is a subjective task, and confounding factors are numerous. 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 web pages promising to teach how to fool polygraphs.

Because error rates defy 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 silent. Few polygraph examiners, however, speculate on what effect such intimidation has on the innocent.

I can sympathize with the Levys in their demands. Their daughter is missing and there is little concrete evidence of what happened. But the police should not allow the emotional pleas of parents to outweigh the inherent weaknesses of lie detection devices. The old “string ’em up” style lynch mob has given way to a modern “hook ’em up” mob of polygraph proponents. Unfortunately, neither one is very good at uncovering the truth.