The effects of a secondary task on true and false opinion statements

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.