Psychological scientists are fascinated, as am I, by the tactics used by investigators in trying to determine if a suspect is lying. Detecting lies and liars is essential to effective policing and prosecution of criminals, but it is maddeningly difficult. Most of us can correctly spot barely more than half of all lies and truths through listening and observation—meaning we are wrong almost as often as we are right. And half a 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 social psychologist Aldert Vrij of the University of Portsmouth in England. 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 piling on demands for additional, simultaneous thought—or cognitive “load”—compromises normal information processing. Because lying is more cognitively demanding than telling the truth, these compromised abilities should be revealed in detectable behavioral clues.
Why is lying more demanding? Imagine for a few minutes that you’re guilty of a murder, and being cross-examinined. To begin, you need to invent a story, and you also have to monitor that tale constantly so it is plausible and consistent with the known facts. That task takes a lot of mental effort that innocent truth tellers do not 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. Because you’re worried about your credibility, you’re most likely trying to control your demeanor, and “looking honest” also saps mental energy. And you’re not just monitoring yourself; you’re also scanning the faces of investigators for signs that he/she might be seeing through your lie.
That’s not all. 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, the truth is automatic and effortless, and lying is the opposite of that. It is intentional, deliberate and exhausting.
So how can investigators 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 a recent issue of the journal Current Directions in Psychological Science.
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