This paper studies whether integration-policy framings affect cooperation in 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 groups receiving either a common-identity framing emphasizing shared school belonging, a multicultural framing highlighting family origins and cultural diversity, or a neutral framing, and then played a public goods game with and without punishment. At baseline, immigrants contributed about 17 percent more than natives. Framing diversity through a multicultural lens increased natives’ contributions by about 13 percent, nearly eliminating the initial cooperation gap, whereas the common-identity framing had no detectable effect. When punishment was introduced, the 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 shared belonging alone, can strengthen cooperative norms in heterogeneous environments.