Rationalizing sharing rules

A partnership can yield a return—a loss or a profit relative to the partners' investments. How should the partners share the return? We identify the sharing rules satisfying classical properties (symmetry, consistency, and continuity) and avoiding arbitrary bounds on a partner's share. We show that any such rule can be rationalized in the sense that its recommendations are aligned with those maximizing a separable welfare function. Among these rules, we characterize those formalizing different notions of proportionality and, in particular, a convenient subclass specified by a single inequality aversion parameter. We also explore when a rule can be rationalized by a more general welfare function. Our central results extend to a wider class of resource allocation problems.