Interindividual Differences in Treatment Effects of a Personality Change Intervention on Life and Domain Satisfaction

Personality change interventions have shown promising average effects not only on personality traits, but also on well-being. However, little is known about the extent to which these effects differ between individuals and what baseline characteristics are associated with such differences. To answer these questions, we analyzed data from the PEACH intervention, a 10-week smartphone-based personality change intervention (N = 873). Using the EffectLiteR framework for causal inference, we estimated average and individual treatment effects of the intervention (vs. waitlist) on life satisfaction, average life domain satisfaction, and satisfaction with individual domains. Moreover, we tested whether interindividual differences in treatment effects could be predicted by demographic characteristics, baseline personality traits, baseline well-being, motivational factors, and engagement preferences. We found a significant average treatment effect on average domain satisfaction (d = 0.20), concentrated in work- and emotion-related domains, but not life satisfaction. Notably, we identified significant heterogeneity in this effect across participants: For approximately 27% of participants, the PEACH intervention showed positive effects on average domain satisfaction, while for 66% it showed no effects, and for 7% it showed negative effects. Men, individuals rated higher in Openness by observers, and individuals with lower self-reported change goal attainability at baseline benefited most from the intervention. Taken together, these findings suggest that personality change interventions can enhance domain satisfaction for some, but not all, individuals, underscoring the need for personalized approaches and the identification of subgroups for whom interventions may be ineffective or even counterproductive.