RESUMEN
We investigated behavioral and neural mechanisms in the relation between social media use (SMU) and self-concept, as well as longitudinal developmental outcomes. Adolescents and young adults (N = 150, 11-21 years old at T1) rated themselves on 60 traits in the academic, physical and prosocial domain, and also indicated how they thought peers would judge them (reflected-peer-judgements). Longitudinal questionnaires (1- and 2-year follow-up) were collected to assess positive (prosocial behavior, self-concept clarity) and negative (clinical symptoms) long-term outcomes. Results indicated that heavier self-reported SMU was linked with lower difference scores between self-judgements and reflected-peer-judgements. Lower SMU was related to more positive ratings from self-judgements vs. reflected-peer-judgements. SMU was also associated with less positive self-concept, particularly in the academic domain (boys and girls) and physical domain (girls). Neurally, increased SMU was linked to heightened mPFC-activity during self-judgements compared to reflected-peer-judgements, and increased activity during physical compared to academic and prosocial self-judgements. Longitudinal analyses indicated no evidence for long-term effects of social media use, self/reflected-peer-difference scores and mPFC-activity on clinical symptoms, prosocial behavior or self-concept clarity. This study highlights the complex relationship between social media use and wellbeing and future research is needed to confirm the lack of long-term effects.