The effects of dual-task interviews on cognitive load and cues to deception

  • Gary Lee Joseph Lancaster

Student thesis: Doctoral Thesis


This thesis used a ‘cognitive load plus’ approach to explore whether deliberate
interventions, (e.g. secondary tasks and unanticipated interview questions) would (i) affect liars and truth tellers cognitive load differently during an interview and (ii) lead to differences in behaviour between liars and truth tellers which could be used to detect deception. Chapter 1 provides a brief introduction to the cognitive load approach to detecting deception and the growing body of research in this area. Next, Experiment 1 is reported in which a computer based target tracking secondary task was performed by (N = 45) lying and (N = 45) truth telling interviewees while they responded to an interviewer’s questions about a mission briefing they were given prior to being interviewed. Truth tellers described their true mission to the interviewer whereas liars gave a fictitious cover story. The main findings, supportive of the cognitive load approach, were that (i) liars made significantly less frequent eye-contact with the interviewer than truth tellers and (ii) liars’ behaviour was rated differently to truth tellers by an independent observer. However, against prediction, there were no differences between liars and truth tellers in terms of their computer task performance or in the numbers of details provided in their verbal statements. These findings informed the design of Experiment 2A which examined the effect of asking unanticipated interview questions on lie detection. In an activity room truth tellers (N = 40) performed five simple tasks (e.g. pairing up playing cards). Liars (N = 40) observed the activity room through a window and were asked to generate a story in which they would claim to have performed five tasks. The interview schedule included four thematically related pairs of questions. The first question of each pairing was easy for interviewees to anticipate as it reflected how events are typically recalled (e.g. in normal chronological order). The second question included a perspective shift (temporal or spatial), which made the question harder for interviewees to anticipate (e.g. recalling the event in reverse order). Eye-gaze was also manipulated by asking half the interviewees to maintain constant eye-contact with the interviewer. Lastly, interviewees’ performed an object sorting secondary task while answering the interview questions. Compared to truth tellers, liars’ verbal responses showed a greater net decline in details, (termed Verbal decline), across three out of four question pairs. Liars also sorted significantly fewer objects per minute on the secondary task. However, there was no relationship between eye-gaze and veracity. In Experiment 2B, 80 observers each saw a single edited video clip of either a liar or truth teller answering one pair of Expected/Unexpected questions from Experiment 2A. Observers were instructed to make a veracity judgement while focussing only on the presence or absence of Verbal decline in the interviewee’s statement. Observers’ accuracy for detecting deception was compared with accuracy rates obtained from a discriminant analysis of the manually coded Verbal decline in Experiment 2A. Observers identified liars with 65% accuracy which compared favourably with the 67.5% accuracy of manual coding. However truth tellers were identified by observers with 60% accuracy which was significantly below the 82.5% rate of manual coding.
Experiment 3 retained the sorting task, eye-gaze manipulation and activity room scenario from Experiment 2A while introducing three new pairings of expected/Unexpected interview questions and two further unpaired questions requiring interviewees to (i) estimate the durations of two tasks they claimed to have completed and (ii) to suggest a future task completion strategy for one task they claimed to have performed in the activity room. Once again liars sorted significantly less correct objects per minute than truth tellers and there was a partial replication of the Experiment 2A findings for Verbal decline. For the new questions: liars showed greater Verbal decline than truth tellers on one of the three new question pairings and liars were significantly less accurate than truth tellers at estimating duration. However, against prediction, liars’ suggested task strategies did not differ in complexity from truth tellers and again there was no relationship between eye-gaze and veracity. Overall, the present thesis’ findings were largely supportive of the theory that lying can be more cognitively demanding than truth telling and that monitoring secondary task performance provides an objective measure of an interviewee’s cognitive load. Also it was demonstrated that strategic use of perspective shifted interview questions elicited a cue (Verbal decline) which could be used to detect deception. The practical and theoretical implications of the present experiments are discussed in a final chapter.
Date of Award30 Sept 2011
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorAldert Vrij (Supervisor), Lorraine Hope (Supervisor) & Bridget Waller (Supervisor)

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