How to tell if someone is lying? See if they copy your body language, says study

Researchers think it’s a way of coping with ‘cognitive overload’

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Ever wondered how you can tell someone is lying to you? Fibbers have physical giveaways, a new study has suggested.

Researchers at Erasmus University Rotterdam recorded the head, chest and wrist movements of a group of around 50 university students using a wireless accelerometer, during one-to-one conversations with other participants.

Prior to these conversations, the students had been asked to solve a wooden puzzle within five minutes. However, the students weren't told that the task was actually far too complex to complete in the time available.    

Assistant professor Sophie van der Zee placed puzzle solutions in the room so students could find them, which encouraged them to cheat on the task. Afterwards, she asked them not to tell her supervisor that she'd "accidentally" left the answers lying around, claiming she didn't want to get into trouble.

Later, researchers asked each student to tell another student how they solved the puzzle, while they were recorded. If they did as van der Zee asked them, this involved lying to the other student.

Van der Zee's team found that, when a student was lying, their movements tended to mirror those of the person they were speaking to. If they were telling the truth, however, their movements tended to be different.

"Liars often deliberately change their behaviour into a way they think truth-tellers behave, but this particular copycat behaviour is something they wouldn’t even try to manipulate because they don’t realise they’re doing it," van der Zee told New Scientist.

"And that could make it an interesting cue for detecting deceit."

The researcher suggested that participants may have been mimicking body movements because lying requires so much concentration and copying requires less thought when you're experiencing "cognitive overload". 

The team concluded that the students who were lying had been unaware that they were mirroring body language.

"When asked directly at debrief, none of our participants, including interviewers, mentioned deliberately using mimicry, stressing the unconscious, automatic aspects of nonverbal coordination in this setting," the study authors wrote.