Recent Catholic social teaching’s treatments of social sin and its proposed remedy, neighbor-love conceived as solidarity, represent genuine advances in this modern Christian tradition. This essay asks what Niebuhr’s ethical analysis might add to, or question about, these Catholic interpretations. After briefly describing how these themes are enunciated in post-Vatican II documents, and Niebuhr’s approach to like issues, I identify several challenges, cautions and additions that Niebuhr might offer to Catholic leaders seeking to understand social sin and to promote solidary (...) action. Suggesting the merits of a more ‘Niebuhrian’ Catholic social ethic, and a more ‘Catholic’ Niebuhrian realism, I argue that articulating a Catholic social agenda with a Niebuhrian perspective can address weaknesses that undercut each tradition’s ability to motivate and sustain effective work for justice. Both traditions also need to better connect social ethics and spirituality, and more systematically account for the social-theoretical assumptions their moral discourse depends upon or implies. (shrink)