A problem-based learning (PBL) assumption is that silence is incompatible with collaborative learning. Although sociocultural studies have reinterpreted silence as collaborative, we must understand how silence occurs in PBL groups. This essay presents students' explanations of dominance, leadership, and silence as PBL group roles. An ethnographic investigation of PBL groups, informed by social constructionism, was conducted at two dental schools (in Australia and Ireland). The methods used were observation, interviews, and focus groups. The participants were volunteer first-year undergraduates. Students attributed dominance, silence, and members' group roles to personal attributes. Consequently, they assumed that groups divided naturally into dominant leaders and silent followers. Sometimes silence had a collaborative learning function, but it was also due to social exclusion. This assumption enabled social practices that privileged some group members and marginalized others. Power and participation in decision making in PBL groups was restricted to dominant group members.