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A new method of detecting lies shows that lie tellers who are asked to multi-task during an interview are easier to spot.

It is well documented that lying during interviews consumes more cognitive energy than telling the truth. A new study from the University of Portsmouth has found that investigators who used this finding to their advantage by asking a suspect to perform an additional, secondary task while being questioned were more likely to expose the lie tellers. The extra brain power needed to focus on a secondary task (other than lying) was particularly difficult for lietellers.

The secondary task used in this experiment was to recall a seven-digit car registration number. The secondary task was only effective if the lietellers were informed of its importance.

Professor Aldert Vrij, from the Department of Psychology at the University of Portsmouth, who designed the experiment, said: “Over the past 15 years we have shown that lies can be detected by outwitting lie tellers. We demonstrated that this can be done by forcing lying tellers to divide their attention between formulating an utterance and a secondary task.

“Our research has shown that truths and lies can seem equally plausible as long as lietellers have a good opportunity to think about what to say. When the opportunity to think becomes less, truths often seem more plausible than lies Lies seemed less plausible than truths in our experiment, especially when the interviewees also had to perform a secondary task and were told that this task was important.

The 164 participants in the experiment were first asked to give their level of support or opposition on various societal issues that were in the news. They were then randomly assigned to a truth or lie condition and asked about the three topics that mattered most to them. Truth tellers were instructed to report their true opinions while lie tellers were instructed to lie about their opinions during interviews.

Those who performed the secondary task received a seven-digit registration number and had to remind the interviewer. Half of them received additional instructions that if they could not remember the car’s registration number during the interview, they could be asked to write down their opinions after the interview.

Participants were given the opportunity to prepare for the interview and were told that it was important to be as persuasive as possible during the interviews, which was prompted to enter a lottery.

The results revealed that the lietellers’ stories seemed less plausible and less clear than the truthtellers’ stories, especially when the lietellers were given the secondary task and told it was important.

Professor Vrij said: “The pattern of results suggests that introducing secondary tasks into an interview might make it easier to detect lies, but such tasks should be introduced carefully. It seems that a secondary task will only be effective if the lietellers do not neglect it. This can be achieved either by telling respondents that the secondary task is important, as demonstrated in this experiment, or by introducing a secondary task that cannot be overlooked (such as grasping an object, holding an object aloft or driving a car simulator). Secondary tasks that do not meet these criteria are unlikely to facilitate lie detection.”

The research was published in the International Journal of Psychology and Behavior Analysis.

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Material provided by University of Portsmouth. Note: Content may be edited for style and length.