Inequality of opportunity, particularly when overlaid with socioeconomic, ethnic, or cultural differences, may limit the scope of cooperation between individuals. A central question, then, is how to overcome such obstacles to cooperation. We study this question in the context of Germany, by asking whether the propensity of immigrant youth to cooperate with native peers was affected by a major integration reform: the introduction of birthright citizenship. Our unique setup exploits data from a large-scale lab-in-the-field experiment in a quasi-experimental evaluation framework. We find that the policy caused male, but not female, immigrants to significantly increase their cooperativeness toward natives. We show that the increase in out-group cooperation among immigrant boys is an outcome of more trust rather than a reflection of stronger other-regarding preferences towards natives. In exploring factors that may explain these behavioral effects, we present evidence that the policy also led to a near-closure of the educational achievement gap between young immigrant men and their native peers. Our results highlight that, through integration interventions, governments can modify prosocial behavior in a way that generates higher levels of efficiency in the interaction between social groups.