Prior work in ethnic politics and political economy has shown that in-group ethnic favoritism can lead to adverse political and social outcomes. However, in many of these studies, ethnicity could not easily be disentangled from other dimensions of identity, such as religion or nationality. As a result, the independent effects of ethnic differences on social cohesion remain unclear. In this paper, we leverage Singapore's unique demographic composition and housing policies to examine how ethnicity and national identity shape altruism toward members of ethnic outgroups. We conducted a field experiment in which we sent 3,150 misdirected letters to public housing residents. Each letter contained vouchers and cues indicating the intended recipient’s identity. We tracked their return as a measure of altruism. Our findings offer mixed evidence of ethnic favoritism: on average, individuals identifying as ethnically Chinese or Indian display higher levels of altruism toward their ethnic outgroups, whereas individuals identifying as Malay tend to favor their coethnics. In addition, we find that nationality (though not shared nationality) and social class (though not shared social class) are strong predictors of altruistic behavior. In particular, individuals with higher income and Singaporean nationality are more likely to return misdirected letters (i) when they perceive that the intended recipients belong to a lower socioeconomic class; (ii) when the voucher value is larger; and (iii) as their own education and household income increase. We discuss the implications of these findings for ethnic politics, social cohesion, and nation-building.