Repetitive negative thinking (RNT) is a problematic thinking style that encompasses rumination and worry. RNT is characterized by continuously dwelling on negative content, such as problems or unpleasant experiences. RNT is associated with negative affect and various psychopathological symptoms, and it even predicts the onset of these symptoms, making it a transdiagnostic risk factor for psychopathology. This qualifies RNT as an ideal candidate for interventions because its reduction can help avoid worsened affect and symptoms, and may even prevent psychopathology. Mindfulness interventions are promising to reduce RNT because mindfulness teaches us to become aware of experiences (e.g., thoughts and feelings) and approach these experiences in an observing and non-judgmental way instead of getting stuck in negativity. Previous findings suggest that mindfulness interventions may indeed be helpful to reduce RNT and symptoms, and to improve affect. However, it remains largely unanswered whether brief mindfulness interventions in daily life lead to immediate benefits for RNT and affect. Two randomized controlled trials were conducted as part of this dissertation to answer this question. In both studies, participants completed brief audio-guided mindfulness interventions multiple times per day over several days. Immediately after each intervention, participants reported their RNT and affect via experience sampling method (ESM). STUDY 1 investigated the effects of a mindfulness intervention in N = 91 non-clinical participants. Over 10 days, participants were randomized at each assessment to complete either a mindfulness intervention or an active control task consisting of listening to neutral background sounds. Results of STUDY 1 showed that participants reported less RNT and less negative affect after completing the mindfulness intervention as compared to the control condition. However, the associations between RNT and negative affect were not impacted by the mindfulness intervention. STUDY 2 investigated the effects of a detached mindfulness intervention in N = 100 participants with elevated trait RNT. The study consisted of a 5-day baseline phase with only ESM assessments and a 5-day intervention phase, where participants additionally engaged in either a detached mindfulness or an active control task, depending on which group they were randomized to. The control task was matched to the mindfulness intervention except for the mindfulness instructions and corresponded to a guided imagery task. Results of STUDY 2 showed that participants of both groups reported stronger reductions in RNT and in negative affect, and stronger improvements in positive affect during the intervention phase compared to the non-intervention baseline phase. However, there were no differences between the groups. The integration of the studies’ findings allows to draw conclusions about the effectiveness of mindfulness interventions and about the mechanisms causing benefits. Effectiveness. Findings suggest that detecting an effect of the mindfulness interventions depended on the control condition used as a comparison. Our guided mindfulness interventions led to less RNT and negative affect compared to both (i) not engaging in any task (STUDY 2) and (ii) listening to neutral background sounds (STUDY 1). However, the mindfulness intervention was equally effective as a matched control task that excluded mindfulness instructions (i.e., guided imagery; STUDY 2). Mechanisms. The studies of this dissertation do not allow us to conclusively determine which mechanisms caused the benefits across both mindfulness interventions and the guided imagery task. It could be that experiencing momentary mindfulness led to lower RNT and better affect. However, other mindfulness-unspecific mechanisms, such as expectations about the helpfulness of the tasks or distraction from current RNT and affect, may also have been (partly) responsible for the observed benefits. Overall, findings of this dissertation indicate that brief guided mindfulness interventions and guided imagery tasks are helpful to immediately reduce RNT and improve affect in daily life. Whether these tasks caused benefits by increasing momentary mindfulness or via other mechanisms remains to be elucidated.
