A handgrip task was used to measure self-control performance following depletion (see Hagger et al., 2010). To allow a more precise determination of when the handgrip was released, we replaced the digital handgrip in Study 2 with a manual hand exerciser (consisting of two handles and a metal spring) and inserted a small wad of paper between the far end of the handles. Self-control performance was measured by the duration of time the participant was able to continuously squeeze the handgrip using the dominant hand until the wad fell out.
Each participant was asked to perform the handgrip task three times (i) at the beginning of the study (baseline handgrip stamina), (ii) immediately following the mindfulness induction (as a measure of performance in the second self-control task), and (iii) at the completion of the study (to test whether the effect of mindfulness induction would persist). Self-control performance was calculated twice as the changes in handgrip duration compared with baseline duration.
4.3.5 Power calculation
A similar previous study (Friese et al., 2012) of the effect of mindfulness induction on self-control performance following ego-depletion assigned 66 participants to one of three conditions (i.e., no depletion, depletion, and depletion plus mindfulness). Thus an additional 22 participants (88 in total) would be necessary for four conditions in the current study. When calculated with G*Power 3 (2013), a total 128 participants is required to detect a medium effect (d = .25) for four group and 1 covariate (sex), and given the power of .80 and an alpha level of .05. Based on these considerations, the current study would require between 22 to 32 participants (11 to 16 males or females) for each group.
4.3.6 Procedure
Potential participants from University of Nottingham (UoN) were contacted via posters/leaflets on the university campus and departmental mailing lists (Appendix 4.2). Those from Nottingham Trent University [NTU]) were recruited by asking the researcher’s colleagues (NTU students) to help distribute the leaflets around their campus. Participants were told the study investigated the effects of personality traits and experimental treatments on the way people perform in a competitive reaction- time task. To be able to participate, they have to speak British English as their first language. They were given a £5 inconvenience allowance and a chance to win £50
incentive for the fastest participants’ reaction-time across the entire study. Alternatively, experimental credit points were given for UoN Psychology students signing up via the Research Participation Scheme.
Participants came into the lab in the School of Pychology. At first, information about the study was presented (Appendix 3.8). After giving informed consent (Appendix 3.9), they were asked to provide demographic information (i.e., sex, age, current mindfulness practice) and completed an online questionnaire of the trait measures (Appendix 2.1). Immediately after the trait measures, participants were asked to furnish a baseline handgrip measure. The experimenter stopped the stopwatch as soon as the wad inserted between the grip handles fell out.
Next, all participants were asked to watch the 6 min attention control videotape. Participants were left alone to do this task and were asked to open the cubicle when they had finished watching the video clip. The experimenter then returned to the cubicle and asked participants to complete two questions measuring the effectiveness of the depletion procedure, three dummy questions about the person perception judgments of the interviewee on the videotape, and the first PANAS (Watson et al., 1988),
Half of the participants in the ego-depletion group and half of those in the no- depletion condition were then given a mindfulness induction task, while the rest of the participants were given the neutral education information. Then the experimenter left the room again, while participants listened to and practised the appropriate audio guided instruction for 15 minutes. When the participant opened the cubicle door, the experimenter entered the cubicle and asked participants to complete the TMS (Lau et al., 2006), followed by a handgrip task.
Next, the participants were told that the experimenter needed to prepare their TCRT opponent. After leaving the room for a while (ostensibly to prepare the opponent), the experimenter returned. Participants were then left alone to perform the TCRT task. When the participant opened the cubicle door to indicate the end of the TCRT
task, the experimenter asked the participants to complete the second PANAS and perform a final measure of handgrip stamina. Finally participants were debriefed and given help and support information (Appendix 3.10).
4.3.7 Data analysis
Statistical analyses were performed on the data using IBM SPS statistics 20.0 (2011). First, associations between personality variables, and between personality variables and the experimental outcomes (i.e., aggressive behaviour and self-control performance) were examined using zero-order correlations or independent t-tests (for sex and self-harm). The efficacy of the experimental conditions was assessed using independent t-tests or one-way ANOVAs.
4.3.7.1 Moderation of mindfulness induction on the link between depletion and
aggressive behaviour
For the primary hypotheses, we used self-control ego-depletion as the predictor, mindfulness induction as the moderator, and aggressive behaviour as the outcome. Aggressive behaviour was measured in terms of (i) blast intensity on no provocation, low provocation, high provocation (Hypothesis 2a), (ii) maximum blast latency (Hypothesis 2b), and (iii) indirect aggression ratings/damage to opponent’s reputation (Hypothesis 2c). For Hypothesis 2a and 2c, the moderating effect of mindfulness induction on the link between self-control ego depletion and aggressive behaviour is tested with a bootstrapping method, using model “number 1” in the A. F. Hayes’s (2012a) PROCESS macro for simple moderation (explanation for this model can be found in section 2.3.6, Chapter 2). Hypotheses 2a and 2c would be supported if the interaction coefficient between depletion and mindfulness induction is statistically different from zero. Specifically for Hypothesis 2c (five items
of indirect aggression), we tested if the effect would survive Bonferroni corrections set at p <.05 /5 items, or p < .01.
Hypothesis 2b would be supported if the interaction between mindfulness and depletion significantly predicts the maximum blast latency (i.e., the number of trials participants waited before delivering the maximum blast). To test this prediction, we conducted a censored survival analysis using Cox regressions to assess the extent to which participants would be more likely to administer the maximum blast to the opponent as a function of experimental conditions.
Hypothesis 1a, 1b, and 1c would be supported if there is a significant main effect of depletion on blast intensity, maximum blast latency, and ratings of the opponent. To provide support for Hypothesis 3, the moderation models were repeated while accounting for the influence of the hypothesised covariates based on the prior significant zero-order correlations between covariates and aggressive behaviour.
4.3.7.2 Moderation of mindfulness induction on the link between depletion and
self-control performance
The moderating role of mindfulness induction on the link between ego-depletion and self-control performance were also examined using bootstrapping method. Self- control ego-depletion was used again as the predictor, mindfulness induction as the moderator, and self-control performance as the outcome. Specifically, self-control performance was measured in terms of (i) Change 1 (changes in handgrip duration prior to the TCRT task relative to baseline), and (ii) Change 2 (post-TCRT task handgrip duration relative to baseline). We expected that the interaction coefficient between depletion and mindfulness induction on Change 1 (Hypothesis 4a) and Change 2 (Hypothesis 4b) would be statistically different from zero after accounting for the potential influence of the hypothesised covariates, based on the prior significant zero-order correlations between covariates and self-control performance.
4.4 RESULTS