How to Spot a Liar

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The best poker players say tics and flutters in an opponent's face--the so-called poker tells--can telegraph when a player is bluffing. Scientists agree that the face tells tales we may wish it didn't. San Francisco psychologist Paul Ekman has codified 46 facial movements into more than 10,000 microexpressions in what he calls the Facial Action Coding System (FACS). He and Frank, who helped devise the catalog, say they can detect deception with 76% accuracy. According to Ekman, thousands of people have been trained in FACS, including Transportation Security Administration personnel. While similar behavioral screening has been used in British airports for several years, FACS is only now being rolled out as a terrorist-screening tool in a dozen U.S. airports.

Each of these systems comes with uncertainties and limitations. Researchers working with EEGs, for example, concede that not all truths read the same way in the brain. A truthful answer about where you were born may produce a quicker--seemingly more honest--signal than an equally truthful one about how you spent your last birthday. Moreover, your brain and someone else's may not answer the same question at the same speed. Each test must thus be painstakingly calibrated for each subject. Not only is that impractical, but it also introduces a whole new level of variability--like trying to diagnose a fever if all of us had a different basal body temperature.

The shortcomings of fMRIs may be more serious. Physical anomalies such as evidence of a stroke or tumor can interfere with the scan's accuracy. And the test is administered in a decidedly unnatural way--with the subject lying down inside a giant magnet. Since speaking aloud activates regions of the brain that could swamp lie-detection results, subjects are asked yes-or- no questions and then instructed to push a button to answer. Maybe the brain operates the same way with a push-button fib as with a verbal one--but maybe it doesn't. And because we all do a certain amount of self-censorship--telling white lies to avoid hurt feelings, for example--signs of activity in the relevant brain regions do not necessarily make you a criminal. "All fMRI lie-detection studies report findings in parts of the anterior cingulate," says University of South Carolina psychologist Jennifer Vendemia. "Well, that's good because if you don't have activation there, you're probably dead."

And don't even get critics started on the shortcomings of reading faces or heat around the eyes. The same honest anxiety that can produce false positives on a polygraph can also increase blood flow in the periorbital region. Facial analysis is problematic, since there's no way to standardize the skills of human analysts, and nobody can say for certain if cooler liars give up fewer clues than nervous ones. "It's not as simple as a Pinocchio phenomenon," says Frank.