RCT Protocol for 'OverThinking': A Mobile EMI for Reducing Experiential Avoidance

The goal of this trial is to examine whether a mobile app intervention for rumination can modify a maladaptive feature of repetitive negative thinking (RNT) - this is understood by the link between daily RNT and well-being, depressive and anxiety symptoms. The main questions it aims to answer are: 1. If the intervention can modify the maladaptive feature of repetitive negative thinking - the investigators anticipate the link between daily rumination and maladaptive outcomes (poorer wellbeing, higher depressive/anxious symptoms) will be weaker in the intervention groups. 2. By examining (1) above, to confirm the role played by avoidance in repetitive negative thinking. 3. By splitting the intervention condition into two groups (one receiving concurrent support from a therapist, one not), to evaluate if interventions such as these might be enhanced by therapist support. 4. To examine if the impact of the intervention on depressive and anxious symptoms might be mediated by changes in beliefs regarding emotions (e.g. the valuation of negative emotions). Researchers will compare across four groups to examine the above effects: (1) intervention condition (therapist support); (2) intervention condition (no therapist support); (3) Partial intervention (daily sampling and emotion valuation questions); (4) Control group (only daily sampling questions). Participants will: * Be requested to participate in a four-week intervention, providing daily assessments of depressive and anxiety symptoms, affect, repetitive negative thinking, and rumination outcomes. * Use a mobile application during the intervention period, which has been designed by the research team (intervention groups), or provide daily assessment scores only (control groups). * Be assessed at pre-intervention (all groups), post-intervention (all groups), 1-month follow-up (all groups), and 3-month follow-up (intervention groups only).