Heterogeneity in the links of psychological (in)flexibility subprocesses and well-being: Idionomic insights from an experience sampling study

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 = 2252). 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.