How to expose a liar: Have them multitask while being questioned

PORTSMOUTH, United Kingdom — Looking to catch a suspected liar? New research suggests asking the suspect to perform multiple tasks at once may help uncover the truth. Scientists at the University of Portsmouth report liars who had to multitask during an interview were easier to spot.

Previous scientific studies reveal that lying requires more mental energy than telling the truth. Study authors decided to take advantage of this by asking people to perform an additional, secondary, task during an interview. Sure enough, those who were genuinely lying were easier to pick out using this strategy.

Study authors theorize the extra brain power needed to perform the secondary task (which was totally unrelated to the lie) made maintaining the lie during the interview that much harder.

More specifically, the secondary task participants had to perform during the interview was remembering a seven-digit car registration number. Interestingly, the secondary task only effectively tripped up the liars if researchers told them this was an important matter.

“In the last 15 years we have shown that lies can be detected by outsmarting lie tellers. We demonstrated that this can be done by forcing lie tellers to divide their attention between formulating a statement and a secondary task,” says the designer of the experiment, Professor Aldert Vrij, in a university release.

“Our research has shown that truths and lies can sound equally plausible as long as lie tellers are given a good opportunity to think what to say. When the opportunity to think becomes less, truths often sound more plausible than lies. Lies sounded less plausible than truths in our experiment, particularly when the interviewees also had to carry out a secondary task and were told that this task was important,” the study author continues.

‘Important’ tasks make it hard to keep lies straight

A total of 164 people took part in this project. To start, each person had to give their opinions on various societal topics recently in the news. Then, researchers randomly assigned them to either a “truth” or “lie” condition before interviewing the participants about the three news stories they seemed to be most passionate about. Those assigned to the truth condition discussed their legitimate opinions, but the lying group had to lie about their opinions during the interviews.

Participants performing the secondary task, involving the seven-digit car registration number, had to recite it from memory back to the researcher. Study authors told half the group that if they could not remember the car registration number during the interview, they may have to write down their opinions afterward.

Everyone had a chance to get ready for the interview, and study authors told each participant that it was very important to be as convincing as possible.

Ultimately, the liars’ stories sounded much less plausible and clear than those telling the truth, especially liars who were also engaging in an “important” secondary task.

“The pattern of results suggests that the introduction of secondary tasks in an interview could facilitate lie detection but such tasks need to be introduced carefully. It seems that a secondary task will only be effective if lie tellers do not neglect it,” Prof. Vrij concludes.

“This can be achieved by either telling interviewees that the secondary task is important, as demonstrated in this experiment, or by introducing a secondary task that cannot be neglected (such as gripping an object, holding an object into the air, or driving a car simulator). Secondary tasks that do not fulfill these criteria are unlikely to facilitate lie detection.”

The study is published in the International Journal of Psychology & Behavior Analysis.

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