Polygraphs and other lie-detection technologies may never really work in the real world.

Why Lie Detection Technology May Never Work

Why Lie Detection Technology May Never Work

The citizen’s guide to the future.
June 4 2013 7:53 AM

Good News for Liars

New technologies for detecting untruths are as problematic as polygraphs.

A computerized polygraph machine being used in a simulated situation on February 26, 2007 in Moscow, Russia.
A computerized polygraph machine being used in a simulated situation on Feb. 26, 2007 in Moscow, Russia. As far as detecting actual lies, polygraphs produce too many false positives.

Photo by Dima Korotayev/Epsilon/Getty Images

In the Watergate tapes, President Richard M. Nixon expressed concern over leaks regarding the Strategic Arms Limitation Treaty talks. He told counselor John Erlichman that he was thinking of having hundreds of government employees undergo polygraph tests to pinpoint the source. “I don’t know anything about polygraphs,” said Nixon, “but I know they’ll scare the hell out of people.”

Nixon was right. Ample research demonstrates that when people are hooked up to a fake but realistic looking apparatus (wonderfully dubbed the “bogus pipeline to truth”), they are likely to tell the truth.

As far as detecting actual lies, however, polygraphs produce too many false positives—that is, they mistake too many truthful people for liars. In the eyes of a lie-detector examiner, innocent people can seem guilty. Under interrogation, they may become frightened, indignant, or agitated. Their hearts pound, their breath labors, and their palms sweat. They may even feel guilty. Conversely, liars are not necessarily anxious; this is especially true of psychopaths and other practiced liars, whose peripheral nervous systems are less responsive to threat than are most individuals’. At bottom, the polygraph is an arousal detector, not a lie detector.


If the body can’t be trusted to reliably betray its secrets, would going straight to the brain, the organ of deceit, be a better way to reveal deception?

One approach is to discover whether suspects are keeping information to themselves. The so-called guilty knowledge test (GKT) simply requires that suspects have a memory for details of the crime—precisely where it happened, what the victim was wearing, the weapon used, and so forth. If the suspect recognized these facts of the crime, a spike of activity registers on an electro-encephalograph. Thus, without saying a word, the suspect’s brain will supposedly implicate him.

In some respects, the brain-wave GKT is an improvement over the polygraph, but its major problem is that it relies on memory. But memory is not like a video-recorder, nor is it a repository for static recollections: It is often a spectacularly fallible instrument. At each stage of memory—encoding the event, storing it, creating a permanent record, or retrieving it—something can go awry. As a consequence, the GKT is subject to the opposite problem that bedevils the polygraph—false negatives, or liars whom the test deems innocent, People who commit crimes might “pass” a brain-wave interrogation simply because, in the heat of passion or rage, they did not note crucial details of the crime. And if something goes unnoticed, the brain cannot encode a memory. Even when details are encoded, they are not always stored permanently. They can undergo normal decay or become contaminated by both earlier and later memories.

Are there reliably different patterns of brain activation when people deceive than when they tell the truth? The short answer is: only in so-called mock-theft experiments in which researchers “plan” a burglary in the lab and enlist subjects as the thieves. For example, they instruct a subject to “steal” an item (e.g., a ring or a watch) from a desk drawer in the lab and then hide it in a locker. A member of the research team observes the mock-theft, so that the ground truth is known to someone. Next, the subject is told to lie to an examiner who asks about the item taken; meanwhile, his brain is being scanned to determine the veracity of his replies.

But it is not easy to generalize from what works in the lab to the real-world elements of lying. Among the reasons: An actual suspect accused of a crime faces an intensely emotional situation with high stakes. Thus it also incorporates the neural correlates of emotion and imagery that would not be found in a less fraught lab lie. Indeed, is it even possible for an instructed lie to be construed as deception in the ordinary sense? In addition, most lab subjects are happy to go along with the testing, whereas real suspects might try to beat the machine by moving their heads, humming, or silently performing multiplication in the hopes of distorting the imaging signal. In one study, investigators found that simply wiggling a single finger or toe could reduce the accuracy of lie detection from near perfect to one-third.