Methodological considerations

The main aim of this thesis was to examine how different mnemonics employed in an immediate interview affect reporting specific types of detail in both immediate and delayed interviews. As a result of this aim Studies I, II, and III did not contain standard control groups in which participants would be provided with the instructions eliciting less complete

accounts. An absence of control conditions made it impossible to conclude whether the use of mnemonic techniques in the immediate interviews as such, have influenced the interviewees’ responses. This approach to the design of the studies can be considered a limitation in that it confines us to conclude that these effects would not be achieved without the use of

mnemonics in the immediate interview. However, it is reasonable to assume that mnemonics used in the initial interviews were helpful to detect deception in the delayed interviews for two reasons. First, research has shown that liars tend to show a stability bias (Vrij et al., 2009; Harvey et al., 2017; Nahari, 2018), whereas truth tellers’ memory of an event becomes weaker over time, resulting in them reporting fewer details over time (Ebbinghaus,

1885/1913; Lawson & London, 2015; Tuckey & Brewer, 2003). The combined effect is that over time the difference between truth tellers and liars in reporting details becomes less pronounced. It can be assumed that in the absence of immediate interviewing, the two-week delay employed in these experiments was a considerable amount of time to show a substantial decrease in recalled information for truth tellers. However, in all three experiments, truth

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tellers’ decline in reporting details was rather small, with the result that truth tellers reported more details than liars even after a delay. The results further showed that the majority of details truth tellers reported in the delayed interviews were repetitions of what they said in the immediate interviews. This suggests that truth tellers had relatively good memories of their initial responses. Second, a comparison between two types of immediate questioning in Study III showed that the report everything mnemonic was more effective (although modestly) than asking spatial questions to discriminate between truth tellers and liars after the delay. Given the magnitude of the effect sizes, it can be argued that in the spatial questions condition the differences between truth tellers and liars in the delayed interviews were also large. Indeed, the way these questions were formulated (e.g., ‘describe the interior of the staff room’) suggests that they were in line with investigative interviewing guidelines, and after asking such questions relatively complete answers could be expected (Oxburgh et al., 2010). Nevertheless, no reason can be identified why the quality of the immediate interview (e.g., eliciting short answers from the interviewees) would not affect deception detection after the delay.

In Studies I to III, all participants were instructed that the apartment they broke in to was a staff room of a community centre. This was to minimise the risk of liars telling an embedded lie. However, this instruction could not eliminate the possibility for some

participants of using embedded lies. For example, some interviewees could report details of staff rooms they have genuinely visited. However, the obtained effect sizes of the differences between truth tellers and liars with respect to the amount of reported details suggest that possible use of embedded lies did not substantially influence the direction of the findings of this research.

Noteworthy, in each experiment interviewers conducted interviews with more than one participant. This could be considered as a limitation because the interviewer could figure

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out whether a person was telling the truth or lying after interviewing a few participants. Specifically, reports of all truth tellers were similar because their accounts were based on a genuine memory of the same event (i.e., a video of the break in), whereas each lying

participant had to create an individual cover story that would be different to the video break in. Thus, it is reasonable to presume that the interviewers became aware of the veracity conditions in the process of data collection, which could have unintentionally impacted the way in which they interviewed. However, the interviewer is believed not to have affected the results of Studies I, II and III for two reasons. First, all interviewers (research assistants who were university students or staff members) were trained by the author of the thesis prior to the beginning of data collection of each study. Specifically, in every interview they were strictly required to use prepared scripts. Moreover, the interviewers were instructed to actively listen to the responses of the interviewees’, make pauses long enough to continue or finish the interview only when being sure that the participant’s response had been fully provided. Second, even if the interviewers started realising who truth tellers and liars were in the process of questioning different participants, they were still blind to the aims and hypotheses of the experiments. Therefore, it is believed the risk of the interviewers’ knowledge to somehow affect the responses of the interviewees (e.g., liars’ reporting fewer details than truth tellers) was low.

A staged filmed event was used as a stimulus material across the three experiments. Participants were instructed to passively watch the staged event and to pretend their active involvement in the break-in. The benefit of such a controlled setting is that all participants witnessed exactly the same event, ruling out differences in exposure between participants or conditions. However, it is unknown whether real participation in the break-in would have resulted in different outcomes in the three experiments. It may have affected the amount of details reported in an undefined manner: It could result in more details being reported due to

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participants paying more attention to the event, or fewer details being reported due to distractions.

Another aspect of ecological validity is that the majority of participants across the studies comprised students or members of the general public rather than legal professionals. Samples with more representative participants (e.g., intelligence officers with a greater insight into covert operations than laypeople) would bring more value into the current findings. It is unknown how the knowledge and/or experience in the subject matter would affect deceptive responses. Again, the amount of details reported may be affected in an undefined direction: liars could provide convincing accounts with many details, or, in contrast, be more careful and report fewer details to avoid the risk of incriminating themselves.