Past research has relied on normative analyses for establishing the positive relationship between psychological flexibility (PF) process and well-being. These analyses, however, assume that aggregate-level effects (i.e., this positive relationship) also apply to most individuals, limiting our ability to explore the dynamic and complex nature of PF processes that interact with individual factors. Idionomic analyses, which use idiographic effects as a foundation from which to draw normative conclusions, provide a possible solution. This study used idionomic analysis to assess the degree of non-random individual heterogeneity in the relationship between each PF process and mood, how this relationship changes over time, and the unique effects of each process. From a pre-existing dataset, we analysed data from 103 psychiatric patients ( M age = 37 years, SD = 11.7 years; 47% male) who completed an ACT intervention. Experience sampling methods were used to repeatedly measure PF processes and mood at pre- and post-intervention. Traditional normative analyses showed an average positive relationship between each PF process and mood. However, idionomic analyses uncovered significant person-specific heterogeneity for several PF processes. The strength and direction of each relationship varied depending on the person, the specific PF process, and the temporal effects that were masked in the aggregate findings. We highlight the value of idionomic analyses in capturing, with greater depth and precision, within-person heterogeneity in the PF process-mood relationship. Idionomics has the potential to advance our development of empirically-based and personalised interventions, thus going beyond the general statement of “it depends” to a specification of how it does so. • Explores the link between psychological flexibility (PF) processes and mood using ESM • We compare traditional normative and novel idionomic statistical approaches • Idionomics explored PF processes with more depth and sensitivity than normative approaches • The link varies depending on the individual, each PF process, and over time • We encourage researchers and clinicians to review their use of normative approaches

Psychological flexibility reliably predicts well-being, but the strength, nature and sequence of relationships between components of these constructs are likely to considerably vary between individuals. This study used emerging idionomic methods to examine within-person links between psychological flexibility/inflexibility sub-processes and hedonic well-being using ecological momentary assessment data ( n = 167; 76% female college students; M age = 23.8 years; sampling design: 3 prompts daily for one week; total measurements = 2,252). We employed advanced statistical modelling techniques to understand complex time-series data, including modelling of individual behaviour (i-ARIMAX), meta-analyses to pool individual data to see overall heterogeneity (RE-MA), and multilevel modelling (Multilevel-VAR) to compare the groups which emerged from the data. Results aligned with past literature, demonstrating that psychological flexibility and inflexibility uniquely predicted hedonic well-being, though with substantial heterogeneity. Replicating past findings (Sahdra et al., 2024; see also Catts et al., 2025), we found that for Stoics, values operated independently from affect in within-person networks, while Non-Stoics showed strong value-affect connections. Novel analyses revealed that among Stoics, stress positively connected to acceptance, which then linked to committed action. While loneliness increased sadness for Stoics, they uniquely intensified committed action when sad—an effect only visible through within-person analysis. This contrasted with Non-Stoics, where sadness negatively impacted committed action. This study contributes to growing evidence supporting the advantages of an idionomic approach over purely nomothetic group analyses, particularly in revealing individual, idiosyncratic patterns. • Examined psychological flexibility's impact on hedonic well-being. • Used idionomic methods to examine within-person patterns • Identified Stoic and Non-Stoic groups with psychological flexibility sub-processes • Key differences present in psychological flexibility networks between groups • Research supports an idionomic approach over purely nomothetic methods

Gambling harm and post-traumatic stress disorder (PTSD) appear to be prevalent among veterans. Globally, help-seeking rates for gambling are low, and veterans may experience obstacles in accessing mental health support due to stigma. Digital health interventions may increase treatment uptake and improve outcomes for veterans. Here, we report findings from a pilot feasibility study of a novel smartphone application-based intervention, “ACT Vet”, based on Acceptance and Commitment Therapy (ACT) for veterans experiencing PTSD symptoms, gambling harm, or both. A 10-week, within-subjects design was employed with 24 veterans (21 men, 2 women, 1 undisclosed; M age = 45.29 years; SD = 10.70). Outcome measures assessed PTSD symptoms, gambling severity, psychological flexibility, anxiety, alcohol use, suicidality, and loneliness. We also examined participants’ quality of life and app usability and acceptability ratings. Findings demonstrated significant reductions in both PTSD and gambling symptoms across the intervention, with a corresponding increase in psychological flexibility. Alcohol use also decreased post-intervention. High usability scores suggest the app was well-received by participants. Overall, the sustained improvements post-intervention indicates the successful deployment of ACT-based methods in an app format. ACT Vet has potential scalability as a first-line digital intervention for PTSD and/or gambling harm. • ACT Vet is the first smartphone-based Acceptance and Commitment Therapy (ACT) intervention specifically designed to support UK Armed Forces veterans experiencing PTSD symptoms and/or gambling harm. • ACT Vet provides a tailored, evidence-based intervention aligned with the six core ACT principles. • Symptom improvements were maintained up to six weeks post-intervention, demonstrating the potential for brief ACT-based digital interventions to create lasting change. • ACT Vet has the potential to serve as a scalable, low-cost digital intervention that enhances self-management of PTSD and/or gambling harm.