Links Between Loneliness and Prosocial Behavior from Adolescence to Young Adulthood: A Ten-Year Longitudinal Study

Background: There is extensive evidence linking loneliness to negative mental health outcomes in adolescents (Goossens, 2018), while prosocial behavior (PB) represents a protective factor (Eisenberg et al., 2005). The literature suggests a bidirectional relationship, wherein loneliness may inhibit social engagement, and reduced PB may, in turn, perpetuate social isolation. However, the nature of this reciprocal association at the intra-individual level remains poorly understood, particularly across developmental transitions from adolescence to adulthood. Purpose: This longitudinal study examined the bidirectional within-person associations between loneliness and PB from adolescence through young adulthood. Methods: The sample comprised 434 participants in Italy (Mage = 15.87, SD = 0.79; 52.8% female) assessed using self-reported measures across five waves over a ten-year period. A Random Intercept Cross-Lagged Panel Model with latent variables was employed to examine the bidirectional associations between loneliness and PB, controlling for sex assigned at birth and age. Results: Longitudinal scalar invariance was achieved. At the between-person level, a significant negative correlation was found between loneliness and PB (β = -.52, p < .001). At the within-person level, significant cross-lagged effects revealed bidirectional temporal dynamics: high levels of loneliness predicted subsequent decreases in PB (β = -.15 to -.31, p < .01), while increases in PB predicted subsequent low levels of loneliness (β = -.13 to -.23, p < .05). Conclusion: Findings provide evidence for reciprocal within-person associations between loneliness and PB from adolescence to young adulthood. This highlights the importance of developing interventions that simultaneously address both constructs to promote healthy developmental trajectories.