Interindividual Differences in Treatment Effects of a Personality Change Intervention on Life 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 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 global and general life satisfaction. 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 factors. We found a significant average treatment effect on general, but not global, life satisfaction (d = 0.21). Notably, we identified significant heterogeneity in this effect across participants: For approximately 30% of participants, the PEACH intervention showed positive effects on general life satisfaction, while for 65% it showed no effects, and for 5% it showed negative effects. Men, individuals not in a relationship, and those higher in Openness at baseline benefited most from the intervention. Taken together, these findings suggest that personality change interventions can enhance life 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.