How does inequality affect our sense of moral obligation?

Tomasello's novel and insightful theory of obligation explains why we sometimes sense an obligation to treat each other equally, but he has not yet explained why human morality also allows and enables much inequality in wealth and power. Ullman-Margalit's (1977) account of norms of partiality suggested a different source and kind of norms that might help to fill out Tomasello's picture.