Introduction: Peer victimization often ends quickly when peers support the victims (Hawkins et al., 2001) and is less frequent in school contexts where bystanders tend to side with victims (Salmivalli et al., 2011). Although some programs aimed at increasing bystander intervention have been proven effective, a meta-analysis has revealed small effect sizes for K–8 children(Polanin et al., 2012). Hence, understanding factors that are associated with bystander behaviors among students in these age groups is of paramount importance. Drawing on social-cognitive theory (Bandura, 1997), we investigated whether defender self-efficacy was longitudinally associated with defending and passive bystanding.

Method: We collected self-report questionnaire data at one-year intervals at five time points, from fourth to eighth grade (M_age1 = 10.56, M_age2 = 11.55, M_age3 = 12.57,  M_age4 = 13.62, M_age5 = 14.56). Data from 709 Swedish students were analyzed via a longitudinal structural equation modeling approach.

Results: The results revealed several significant associations, both in the direction from defender self-efficacy to the bystander behaviors and vice versa. Greater defender self-efficacy in grades 4 and 6 predicted more defending ingrades 5 and 7, respectively, while less defender self-efficacy predicted more passive bystanding at adjacent time points across all grades. Furthermore, more defending in grades 4 and 5 predicted greater defender self-efficacy in grades 5 and 6, respectively, and more passive bystanding in grade 7 predicted less defender self-efficacy in grade 8.

Conclusion: This study adds important insights to the field, going beyond cross-sectional studies and suggesting reciprocal associations between defender self-efficacy and bystanders behaviors. In general, greater defender self-efficacy seems to motivate students to help victims and to inhibit them from passive bystanding. At the same time, defending the victim is likely to strenghten students’ beliefs of being capable of intervening effectively, while passive bystanding seems, in some occasions, to decrease students’ perceived power to change bullying situations.