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How to Catch a Liar: The Cognitive Clues to Deceit

Posted: 04/12/11 09:56 AM ET

One of my guilty pleasures is the long-running TV show "NCIS," a drama focused on the Naval Criminal Investigative Service. The hero is Special Agent Leroy Jethro Gibbs, a former Marine and disciplined detective with an uncanny ability to observe and interrogate criminal suspects. He doesn't say much or display much emotion in the interrogation room -- indeed, his cool demeanor is his trademark -- yet he is a keen lie-spotter.

Psychological scientists are fascinated by real-life versions of the fictional Gibbs. Detecting lies and liars is essential to effective policing and prosecution of criminals, but it's maddeningly difficult. Most of us can spot barely more than half of all lies and truths through listening and observation -- meaning, of course, that we're wrong almost as often as we're right. A half-century of research has done little to polish this unimpressive track record.

But scientists are still working to improve on that, and among them is cognitive psychologist Aldert Vrij of the University of Portsmouth, in the U.K. Vrij has been using a key insight from his field to improve interrogation methods: The human mind, despite its impressive abilities, has limited capacity for how much thinking it can handle at any one time. So demanding additional, simultaneous thought -- adding to cognitive "load" -- compromises normal information processing. What's more, lying is more cognitively demanding than telling the truth, so these compromised abilities should show up in detectable behavioral clues.

Why is lying more demanding? Well, imagine for a few minutes that you're guilty of a murder, and Gibbs is cross-examining you. To start, you need to invent a story, and you also have to monitor that tale constantly so that it's plausible and consistent with the known facts. That takes a lot of mental effort that innocent truth-tellers don't have to spend. You also need to actively remember the details of the story you've fabricated, so that you don't contradict yourself at any point. Remembering a fiction is much more demanding than remembering something that actually occurred.

That's just to start. Because you're naturally worried about your credibility, you're most likely trying to control your demeanor. Surprisingly, "looking honest" saps mental energy. And what's more, you're not just monitoring yourself; you're also scanning Gibbs' face for signs that he's seeing through your lie. Like an actor, you have the mental demands of staying in character. And finally, you have to suppress the truth so that you don't let some damning fact slip out, another drain on your mind's limited supply of fuel. In short, telling the truth is automatic and effortless, and lying is the opposite of that. It's intentional, deliberate and exhausting.

So how can Gibbs exploit the differing mental experiences of liars and truth-tellers? Here are a few strategies that Vrij and his colleagues have been testing in the laboratory, which they describe in the most recent issue of the journal Current Directions in Psychological Science.

One intriguing strategy is to demand that suspects tell their stories in reverse. Narrating backward increases cognitive load because it runs counter to the natural forward sequencing of events. It also disrupts the normal reconstruction of past events using mental schemas, which give coherence to isolated events. Since liars already have depleted cognitive resources, they should find this unfamiliar mental exercise more taxing than truth-tellers do -- which should increase the likelihood that they will somehow betray themselves. And in fact, that's just what happens in the lab: Vrij ran an experiment in which half the liars and truth-tellers were instructed to recall their stories in reverse order. When observers later looked at videotapes of the complete interviews, they detected more clues to deceit in the liars who were burdened by this mental task. Indeed, observers correctly spotted only 42 percent of the lies in the control condition -- way below average, which means they were hard to spot -- but a remarkable 60 percent when the liars were compromised by the reverse storytelling.

Another strategy for increasing liars' cognitive burden is to insist that suspects maintain eye contact. When people have to concentrate on telling their story accurately -- which liars must, more than truth-tellers -- they typically look away to some motionless point, rather than directly at the conversation partner. That's because keeping eye contact is distracting, and makes narration more difficult. Vrij also tested this strategy in the lab, and again observers detected more clues to deceit in those who were required to look the interrogator in the eyes.

NCIS Special Agent Gibbs may be a fictional version of what psychological scientists call "wizards"-- those rare people who have extraordinary lie-detection skills. Researchers have been trying, without a lot of success, to unravel these wizards' strategies, but until do, less sophisticated lie-catchers may be able to exploit the mind's cognitive weaknesses to catch the bad guys in their web of lies.