Background: School bullying is a group phenomenon in which being defended by peer bystanders may buffer against the development of psychological problems in victims.AimsThis registered report examines whether being defended diminished victims' daily mood problems and self-blame, both from a within- and between-person perspective.

Materials and Methods: Daily diary data were collected from n = 1669 Finnish 7th-9th grade students (M age = 14.45; 55.5% girl) across 3 weeks. In n = 1329 out of 12,366 assessments (10.7%), students indicated that schoolmates victimized them on the day of bullying.

Results: Multi-level regression analyses indicated that students reported lower depressed mood, greater positive mood and lower self-blame on days that they were victimized and defended as compared to days when they were victimized but non-defended. Effect sizes were medium for depressed mood and small for positive mood and self-blame. Repeated victims (n = 144) were less likely to blame themselves for victimization on days they were defended, which, in turn, diminished feelings of humiliation (mediation).

Discussion: Our findings indicate that being defended benefits victims of bullying by mitigating mood problems, both directly and indirectly via diminished self-blame.

Conclusion: Anti-bullying programmes that encourage peer defending have the potential to improve victims' psychological adjustment, even on a daily basis.