Ethnic balancing in the security sector increasingly accompanies power sharing agreements after civil war, but new challenges arise as these institutions must sustain cooperation amidst increasing ethnic heterogeneity. Inclusive involvement in security sector institutions may reduce discrimination against minority groups. But pressure to assimilate may also foment "loyalty conflict" among minority group members, exacerbating discrimination. We test these competing logics using surveys and lab-in-the-field experiments with teams of Liberian National Police officers. Consistent with a logic of loyalty conflict, we find that teams with minority police officers are more rather than less discriminatory against minority civilians. This effect is not driven by heterogeneity, but rather by the presence of minority police officers per se. We also find that teams that include minority police officers are no more or less cooperative than those that do not, and that heterogeneous teams are no more or less cooperative than homogeneous ones. We argue that these effects are likely a result of professionalization processes that encourage conformity and loyalty to an existing police subculture.