It is well documented that lying during interviews takes up more cognitive energy than telling the truth. Investigators can use this finding to their advantage by asking a suspect to carry out an additional, secondary, task while being questioned became less plausible in their responses, found a UK study.
The extra brain-power needed to concentrate on a secondary task (other than lying) was particularly challenging for lie tellers. The secondary task used in this experiment was to recall a seven-digit car registration number. The secondary task was found to be effective only if liars were told it was important.
Professor Aldert Vrij, from the Department of Psychology at the University of Portsmouth, who designed the experiment said: “In the past 15 years we have shown that lies can be detected by outsmarting the liars. We demonstrated that this can be done by forcing them 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 then interviewed on the three topics about which they felt most strongly. Truth tellers were instructed to report their true opinions whereas liars were instructed to lie about their opinions during the interviews.
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 registration number during the interview, they might 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 convincing as possible during the process – the interview was incentivised by being entered into a prize draw.
The results revealed that liars’ 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.
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.
Study details
The Effects of a Secondary Task on True and False Opinion Statement
Aldert Vrij, Haneen Deeb, Sharon Leal and Ronald P. Fisher.
Published in the International Journal of Psychology & Behavior Analysis on 28 March 2022
Abstract
Background
In this experiment, we examined the effect of carrying out a secondary task on the arguments truth tellers and lie tellers put forward when discussing their opinions about societal issues. There is evidence to suggest that lying is more cognitively demanding than truth telling. Investigators can take advantage of the additional cognitive load imposed on lie tellers by imposing additional cognitive load, which should be particularly debilitating for lie tellers.
Method
In the experiment, participants told the truth or lied about some societal issues. Two-thirds of participants were asked to also remember and recall a car registration number during the interview. For one third of participants this secondary task was made important (secondary task and incentive). The pre-registered hypothesis we tested was that the most pronounced differences between truth tellers and lie tellers would occur in this secondary task and incentive condition, followed by the control condition (no secondary task) followed by the secondary task without an incentive condition. The dependent variables were the number of words uttered and number of arguments reported and the plausibility, immediacy, directness and clarity of the statement.
Results
The differences between conditions were small but followed the predicted pattern of results. The effects were most pronounced for the variables plausibility, immediacy, directness and clarity.
Conclusion
The pattern of results suggests that the introduction of secondary tasks in 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 we did in the present experiment) or by introducing a secondary task that interviewees cannot neglect (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 distinguishing between truth tellers and lie tellers.
The Effects of a Secondary Task on True and False Opinion Statement (Open access)
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