Why and When Liars Experience More Cognitive Load than Truth Tellers
Lying is a cognitive task that results in increased cognitive load. The term cognitive load can be defined as a multidimensional construct representing the load that performing a particular task imposes on the actor's cognitive system (Paas, Renkl, & Sweller, 2003; Paas, Tuovinen, Tabbers, & van Gerven, 2003; Paas & Van Merriënboer, 1994). There are many factors that contribute to a liar's cognitive load while being interviewed. First, formulating the lie itself is cognitively taxing. Liars need to make up their stories while monitoring their fabrications so that they are plausible and adhere to everything the observer knows or might find out. In addition, liars must remember their earlier statements and what they told to whom, so that they appear consistent when retelling their story. Liars should also avoid making slips of the tongue and should refrain from providing investigators with new leads (Vrij, 2008).
Second, liars are typically less likely than truth tellers to take their credibility for granted (DePaulo et al., 2003; Gilovich, Savitsky, & Medvec, 1998; Kassin, 2005; Kassin, Appleby, & Torkildson-Perillo, 2010; Kassin & Gudjonsson, 2004; Kassin & Norwick, 2004; Vrij, Mann, & Fisher, 2006b). There are at least two reasons for this. The stakes (i.e. negative consequences of getting caught and positive consequences of getting away with the lie) are sometimes higher for liars than for truth tellers. Smugglers are probably keener to make an honest impression on customs officers than non-smugglers, because the negative consequences for having to open their suitcases are much higher for smugglers than for non-smugglers. In addition, truth tellers typically assume that their innocence shines through (Granhag, Strömwall, & Hartwig, 2007; Kassin, 2005; Kassin & Gudjonsson, 2004; Kassin & Norwick, 2004; Vrij et al., 2006b), which could be explained by the illusion of transparency (Gilovich et al., 1998), the belief that ‘one's inner feelings will manifest themselves on the outside', and the belief in a just world (Lerner, 1980), the belief that people ‘will get what they deserve, and deserve what they get'. As such, liars will be more inclined than truth tellers to monitor and control their demeanour in order that they appear honest to the interviewer (DePaulo & Kirkendol, 1989). Monitoring and controlling behaviour is cognitively demanding (Baumeister, 1998). For example, a guilty suspect may experience powerful emotions (e.g. fear, remorse, anger or even excitement) which must be hidden or faked (Gambos, 2006; Mohamed et al., 2006; Porter & Ten Brinke, 2010; Richards, 2010). Consider a woman publicly pleading for the safe return of her partner who, in reality, she has murdered (see also Vrij & Mann, 2001). She must monitor her body language and emotional expressions while keeping the details of the story straight. A high level of cognitive load accompanies high-stakes deception.
Third, because liars do not take their credibility for granted, they may monitor the interviewer's reactions more carefully in order to assess whether they appear to be getting away with their lie (Buller & Burgoon, 1996; Schweitzer, Brodt, & Croson, 2002). Carefully monitoring the interviewer also requires cognitive resources. Fourth, liars may be preoccupied by the task of reminding themselves to act and role-play (DePaulo et al., 2003), which requires extra cognitive effort. Fifth, deception requires a justification, whereas truth telling does not (Levine, Kim, & Hamel, 2010). People typically lie for psychological or material reasons (DePaulo, Kashy, Kirkendol, Wyer, & Epstein, 1996). For example, they lie because they are too embarrassed to tell the truth (psychological reasons) or they lie to make money (material reason). Considering such justifications is mentally taxing. Sixth, liars have to suppress the truth while they are lying and this is also cognitively demanding (Spence & Kaylor-Hughes, 2008; Spence et al., 2001; Verschuere, Spruyt, Meijer, & Otgaar, 2011). Finally, while the truth often comes to mind automatically, activation of a lie is more intentional and deliberate and thus requires mental effort (Gilbert, 1991; Walczyk, Roper, Seemann, & Humphrey, 2003; Walczyk et al., 2005).
Obviously, lying is not always more cognitively demanding than truth telling (McCornack, 1997). Perhaps the seven reasons given as to why lying is more cognitively demanding could give us insight into when it is more cognitively demanding. That is, lying is more cognitively demanding to the degree that these seven principles are in effect. For at least some of these seven principles to be fulfilled, two elements are required. First, lying is likely to be more demanding than truth telling only when interviewees are motivated to be believed. Only under those circumstances can it be assumed that liars take their credibility less for granted than truth tellers and hence will be more inclined than truth tellers to monitor their own non-verbal and verbal behaviour and/or the interviewer's reactions. Second, for lying to be more cognitively demanding than truth telling, liars must be able to retrieve their truthful activity easily and have a clear image of it. Only when liars' knowledge of the truth is easily and clearly accessed will it be difficult for them to suppress the truth. On the other side of the equation, truth tellers also need to have easy access to the truth for their task of truthfully reporting an event to be relatively undemanding. If truth tellers have to think hard to remember the event in question (e.g. because it was not distinctive or it occurred long ago), their cognitive demands may exceed the cognitive demands that liars require for fabricating a story.
In forensic settings, we can reasonably assume that interviewees will be motivated to be believed, but we cannot assume that they will always be able to easily retrieve the target event from memory, as this will vary from one case to another. However, interviewers can check this. For example, at the beginning of the interview, they can tell an interviewee that they would like to discuss with her/him what she/he did last Tuesday night. They then can give the interviewee some time to think about it. Only when the interviewee says they remember what she/he did on that night can the cognitive lie detection approach be implemented.
