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Before you take a polygraph, the examiner interviews you extensively. The examiner says that first he'll explain how the machine works. Then he'll tell you the questions he's going to ask so there will be no surprises, and the two of you will go over the questions to make sure you understand them before the actual test. The entire pretest phase generally runs about an hour, according to Webb.

This seems reassuring, since you may have been concerned that the examiner will suddenly ask who broke that slide projector in seventh grade, or whether you have sex fantasies about carp, or whether it was you who said that when they were passing out brains, your boss was standing in line to get seconds on anal-retentive.

This is also when you hear about "control" questions. These are questions about wrongdoing so sweeping that almost no one can honestly answer "No." "Have you ever told a lie to get out of trouble?" "Have you ever broken a traffic law?" They are intended to evoke an emotional reaction that the polygraph can detect when you answer "No," which can then be compared with your reaction when the tester asks you the questions he is really interested in.

At this time you might find yourself confiding to the examiner that, OK, maybe you have run a stop sign or two. (One American Polygraph Association official has testified that in pre-employment screening, three out of four applicants admitted to stealing something from a previous employer.) The examiner will appear very concerned, and ask for all the details. Your dossier has been opened, and you haven't even gone on the box yet.

The examiner also runs an "acquaintance test," usually a demonstration with a card or a number. He hooks you up to the polygraph and has you pick a card from a deck. The examiner then asks a series of questions to determine the card: "Is it a red card?" "Is it a black card?" Is it a face card?" Your job is to answer "No" every time. Then the examiner names the card, ostensibly because the charts spike when you're lying.

This is intended to convince you of the polygraph's infallibility, which makes the testing more effective, because the better you think it works, the more apt you are to be nervous about being caught in a lie. Of course, if the examiner already knew that the card was the queen of diamonds because you picked it from a stacked deck, your faith might be misplaced. In laboratory studies, without a stacked deck examiners can tell from the polygraph charts which card you picked between 30 percent and 73 percent of the time.

The examiner determines the exact form of the control questions, the non-control questions and all the follow-up questions. He may ask some questions repeatedly. As a result, every polygraph testing session is different.

Examiners don't get much feedback other than the confessions they extract at the time of testing. As Webb says, "If I were to test someone who was in fact guilty of a crime, and I mistakenly test them as not guilty, they're not likely to tell me about it. By the same token, if I test someone as guilty and they're in fact innocent, like many people who are guilty, they'd say they're innocent."

On the other hand, Webb says, in law enforcement polygraphs, the examiner may get confirmation in the form of later confessions, new evidence or convictions. Since innocent people can be convicted, most scientists don't like this last form of confirmation. In criminal cases the examiner also has information about the crime and the suspect; he needs this information to formulate the questions, but it means that he has a lot to rely on besides the machine's readings in declaring that the person is "deceptive" or "not deceptive" or in extracting a confession -- anything but a double-blind experiment.

Webb also points to studies of polygraph testing as proof of their validity. But there are a number of problems with the studies: They tend to look at either criminal investigations or staged crimes. In staged crimes volunteer subjects may be told to go into a room where half of them "steal" an object and then are tested to find out which are the "thieves." It seems unlikely that people would feel the same kind of anxiety about staged misdeeds that they feel about real misdeeds.

The biggest problem with the studies, however, is the results. In studies, polygraph diagnoses are often wrong, with rates hovering around 80 percent correct -- and sometimes much lower.

False positives -- people whom the examiner says are "deceptive" but who are in fact telling the truth -- are more common than false negatives -- people whom the examiner says test "not deceptive" but who are in fact lying. Of one study situation Lykken says, "An accused person who is innocent who takes a polygraph test has almost a 50-50 chance of failing it." Says Lykken, "Those odds are worse than Russian roulette!"

Moreover, two examiners looking at the same polygraph charts will not always agree on what they mean. In fact, one study found that 10 to 20 percent of the time the same examiner may read the same charts differently if he reads them after a six-month interval.

Thus polygraph testing gets poor marks for both reliability and validity. While the polygraph has always been wrapped in the trappings of science, scientists reject it. A 1997 poll of psychologists and psychophysiologists showed that most of them view it as junk science that should not be admissible in court.

Lykken compares it to astrology: You may think there's some validity to astrology, but would you use it to find spies? You would? Well, Mrs. Reagan, you're entitled to your opinion.

Manuel Garcia, a physicist at the Lawrence Livermore Nuclear Laboratory (LLNL) compares it to phrenology. Edward Teller, director emeritus of the LLNL, put it more tactfully in a letter to DOE Secretary Bill Richardson, calling it "a clumsy and imperfect tool" that produces "rather dubious evidence."

Nuclear lab workers who fail polygraphs, but for whom there is no evidence of guilt, can't be fired but may be transferred to less sensitive work -- a transfer that would destroy the careers of most scientists.

The courts tend to agree with the scientists; polygraph results are not admissible in court in most situations. Sometimes they are allowed as evidence, if both sides agree beforehand, in cases in which people volunteer to take a test, hoping they will be vindicated. But since even innocent people can test as "deceptive," most lawyers strongly advise their clients to shun the box.

Next page | Polygraphs don't have to work, people have to believe they do