Heterogeneity of Effects in a Prosociality‐Based Intervention to Reduce Loneliness and Increase Social Contact

ABSTRACT Objective This study evaluates the effects of an act of kindness intervention on increasing daily social contact and reducing loneliness among community adults experiencing loneliness. It also explores heterogeneity in effects and potential moderators, including individual differences in baseline social health and intervention implementation. Method In a randomized controlled trial, 208 adults were randomly assigned to perform daily acts of kindness for others (Kindness condition) or take a short break for themselves (Control condition) for 2 weeks. Dairy assessments of loneliness and social contact were collected 3 days before and after the intervention. Results We did not find consistent average effects. Although loneliness decreased in the Kindness condition, the reduction was not significantly greater than in the Control condition. In contrast, a group difference was observed in social contact, which remained stable in the Kindness condition but declined in the Control condition. Notably, significant individual differences emerged: the intervention was more effective for participants with higher baseline social anxiety and loneliness, and when a greater proportion of prosocial acts targeted weak social ties and a smaller proportion targeted strangers. Conclusions These findings highlight the importance of identifying individual differences—for whom and under what conditions prosociality‐based interventions are most effective.