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Mind

Human Lie Detectors

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People who think they know a lie when they hear one are fooling themselves. According to psychologist Nancy Etcoff, the average joe is terrible at telling when another person is being deceitful. The superior truth detectors are certain kinds of aphasics— those who, because of damage to the brain's left hemisphere, have lost the ability to understand spoken sentences.

Etcoff, of Massachusetts General Hospital East in Charlestown, recently showed a test video to both a group of aphasics and a group of people with normal language skills. The video showed volunteers describing their emotional states as they watched a movie. Half the time the volunteers were interviewed while watching gruesome scenes of limb amputation and lying about their feelings; the other half of the time they were viewing peaceful nature films and responding honestly. Aphasics correctly identified lies in up to three quarters of all cases, whereas normal subjects were correct just half of the time— no better than chance.

Loss of language may free aphasics to pick up on unspoken cues to deceit— a fleeting expression of sadness, a raised pitch of voice— that most other people ignore. "When we're talking to someone else, we're getting information overload— what they say, their facial expressions, their gestures," Etcoff explains. "But we really focus in on language, and language may camouflage some of the other cues." Also, local brain damage might trigger changes in the nerve cell wiring that enhance lie detection. Etcoff hopes future studies with brain imaging tools will settle the question.

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