This paper studies whether alternative integration-policy framings affect cooperation in ethnically diverse groups. We conduct a lab-in-the-field experiment with 390 adolescents in mixed classrooms in Italy. Within each class, students were randomly assigned to small groups that received either a neutral condition, a common-identity framing emphasizing shared school belonging, or a multicultural framing highlighting family origins and local cultural diversity, and then played a repeated public goods game with and without punishment. In the neutral condition, students with an immigrant background contributed about 17 percent more than natives. Framing diversity through a multicultural lens increased natives' contributions by about 13 percent relative to their baseline level, nearly eliminating the initial cooperation gap, whereas the common-identity framing had no detectable effect. When punishment was introduced, treatment effects on contributions became small, but multicultural framing increased the sanctioning of free riders, particularly among natives. The results suggest that cooperation in diverse settings depends not only on minority integration but also on how majority-group members respond to diversity. Policies that recognize multicultural identities, rather than emphasizing generic shared belonging alone, can strengthen cooperative norms in heterogeneous environments.