This paper examined British children's (8- to 10-year-olds) and adolescents' (13- to 15-year-olds, N = 340
Female N = 171, 50.3%) expectations, evaluations and reasoning about a bystander peer who challenges the social exclusion of an immigrant or non-immigrant peer by a peer group of non-immigrant students. Participants read a hypothetical scenario in which a peer was excluded from an afterschool club by the peer group. The scenarios were either intergroup or intragroup contexts. Participants' expectations of a peer bystander challenging the social exclusion by the peer group, their perception of how the peer group would evaluate the challenger, and their reasoning around their expectations were measured. Adolescents were less likely to expect a peer bystander to challenge exclusion compared to children. Participants' perceptions of how the group would evaluate the challenger were significantly lower in intergroup compared to intragroup contexts. In intergroup contexts, adolescents with low expectations of challenging favoured group dynamics and group repercussions reasoning over moral reasoning, while children did not use group repercussions reasoning.