This meta-analysis examines the effectiveness of interventions aimed at reducing loneliness among adults, adolescents, and children. The study reviews six previous qualitative reviews that identified four primary intervention strategies: improving social skills, enhancing social support, increasing opportunities for social contact, and addressing maladaptive social cognition. The meta-analysis aims to quantify the effects of these strategies and explore potential moderator variables. The results show that single-group pre-post and non-randomized comparison studies yield larger mean effect sizes compared to randomized comparison studies. Among randomized studies, interventions addressing maladaptive social cognition were most successful. The analysis also discusses theoretical and methodological issues in designing new loneliness reduction interventions. The findings highlight the importance of social cognition in loneliness and suggest that interventions targeting maladaptive social cognition may be more effective in reducing loneliness.This meta-analysis examines the effectiveness of interventions aimed at reducing loneliness among adults, adolescents, and children. The study reviews six previous qualitative reviews that identified four primary intervention strategies: improving social skills, enhancing social support, increasing opportunities for social contact, and addressing maladaptive social cognition. The meta-analysis aims to quantify the effects of these strategies and explore potential moderator variables. The results show that single-group pre-post and non-randomized comparison studies yield larger mean effect sizes compared to randomized comparison studies. Among randomized studies, interventions addressing maladaptive social cognition were most successful. The analysis also discusses theoretical and methodological issues in designing new loneliness reduction interventions. The findings highlight the importance of social cognition in loneliness and suggest that interventions targeting maladaptive social cognition may be more effective in reducing loneliness.