Abstract: Multilingualism is the global norm, but the implications of this for cooperation and public goods provision have not been studied before. We test whether the language in which a public goods game is played affects subjects’ contributions amongst a bilingual population in eastern Uganda, finding that subjects contribute 30% more on average in the national language. This treatment effect is solely driven by those most associated with the local Gisu identity, for whom contributions are 43-74% higher in the national language. This difference fits with Gisu culture’s high value on self-reliance and low value on reciprocity and cooperation, due to a violent history of intense competition over land. Language is thus shown to affect cooperation, but only for individuals who both have different latent norms and for whom language activates these norms.