School Bullying Involvement and Adolescent Depressive Mood: A Daily Diary Study

School bullying has attracted considerable attention due to its high prevalence and profound psychological consequences among adolescents. Although previous studies have documented associations between different bullying roles and depressive mood, the daily dynamics of these relationships and their underlying mechanisms remain unclear. The present study examined the associations between four types of daily school bullying involvement (perpetration, victimization, bystanding, and defending) and adolescent depressive mood, with a particular focus on the mediating role of school belonging. A total of 453 adolescents (M_age = 14.10; SD = 1.35; 50.77% male) completed 20 consecutive daily diaries (N = 8,276), reporting their daily bullying involvement, perceived school belonging, and depressive mood each day. At the between-person level, bullying perpetration, victimization, and bystanding behavior were significantly associated with higher depressive mood, whereas defending behavior showed no significant association. At the within-person level, one-day lagged analyses showed that daily bullying perpetration, victimization, and bystanding behavior predicted higher depressive mood on the following day, whereas two-day lagged analyses indicated that only bullying perpetration and victimization predicted depressive mood two days later. Multilevel analyses revealed that school belonging showed significant indirect associations linking all four forms of bullying involvement to depressive mood at the between-person level, whereas only daily bullying perpetration predicted subsequent depressive mood through school belonging at the within-person level. These findings highlight the importance of distinguishing between-person and within-person processes, as well as different temporal windows, in understanding how daily bullying involvement is associated with adolescent emotional adjustment.