Exposing Liars by Distraction – Science Reveals a New Method of Lie Detection

According to an experiment, investigators who asked a suspect to carry out an additional, secondary, task while being questioned were more likely to expose liars.

A new method of lie detection shows that lie-tellers who are made to multitask while being interviewed are easier to detect.

It has been clearly established that lying during interviews consumes more cognitive energy than telling the truth. Now, a new study by the University of Portsmouth has found that investigators who used this knowledge to their advantage by asking a suspect to carry out an additional, secondary, task while being questioned were more likely to expose liars. The extra brain power required to concentrate on a secondary task (other than lying) was particularly challenging for lie-tellers.

The Effectiveness of Secondary Tasks in Lie Detection

In this experiment, the secondary task used was to recall a seven-digit car registration number. The secondary task was only found to be effective if lie tellers were led to believe that it was important.

“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.”

Professor Aldert Vrij, Professor in Psychology

Professor Aldert Vrij, from the Department of Psychology at the University of Portsmouth, who designed the experiment said: “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.

“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 164 participants in the experiment were first asked to give their levels of support or opposition about various societal topics that were in the news. They were then randomly allocated to a truth or lie condition and interviewed about the three topics that they felt most strongly about. Truth tellers were instructed to report their true opinions whereas lie tellers were instructed to lie about their opinions during the interviews.

“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.”

Professor Aldert Vrij, Professor in Psychology

Those doing the secondary task were given a seven-digit car registration number and instructed to recall it back to the interviewer. Half of them received additional instructions that if they could not remember the car registration number during the interview, they may be asked to write down their opinions after the interview.

Participants were given the opportunity to prepare themselves for the interview and were told it was important to come across as convincingly as possible during the interviews – which was incentivized by being entered into a prize draw.

The Impact of Secondary Tasks on Plausibility

The results revealed that lie tellers’ stories sounded less plausible and less clear than truth tellers’ stories, particularly when lie tellers were given the secondary task and told that it was important.

Professor Vrij said: “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. 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.”

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

Reference: “The Effects of a Secondary Task on True and False Opinion Statements” by Aldert Vrij, Haneen Deeb, Sharon Leal and Ronald P. Fisher, 28 March 2022, International Journal of Psychology and Behaviour Analysis.
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