Research has shown an easy method to help investigators weed out liars: ask a suspect to carry out an additional task while being questioned, and you’re more likely to accurately tell whether they are telling the truth or not.
Previous studies have shown that being dishonest and deceptive can strain the brain, requiring considerably more cognitive energy than simply telling the truth. By giving a person a second task to carry out while lying, the researchers believe they struggle to muster the cognitive effort needed to fabricate stories, making their lies less polished and easier to spot.
“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,” Professor Aldert Vrij, study author from the Department of Psychology at the University of Portsmouth, said in a statement.
“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 insight comes from a study reported in the International Journal of Psychology & Behavior Analysis, by psychologists from the University of Portsmouth in the UK.
The team gathered 164 people to take part in an experience that first saw them asked about their levels of support or opposition on various controversial topics in the news, from COVID passports and immigration to Brexit and Boris Johnson.
The participants were then randomly allocated to play the role of truth-tellers or liars. When questioned about three of the topics, truth-tellers simply reported their genuine feelings on an issue, while liars gave false information about their opinions.
The researchers then had to guess who was being sincere and who was lying. To give them an incentive, the participants were told they were in for a chance of winning a cash prize if they successfully convinced the researchers of their position. Two-thirds of participants were asked to also remember and recall a car registration number during the interview. Half of this group was also told that this task was particularly important.
Overall, the results showed that liars' stories were perceived as less plausible and less clear than truth tellers' stories, especially when lie tellers were given the secondary task and told that it was important.
“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," explained Professor Vrij.
"It seems that a secondary task will only be effective if lie tellers do not neglect it. 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 fulfil these criteria are unlikely to facilitate lie detection."
An earlier version of this article was published in May 2022.
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