Pair-efficient reallocation of indivisible objects

We revisit the classical object reallocation problem under strict preferences. When attention is constrained to the set of Pareto-efficient rules, it is known that top trading cycles (TTC) is the only rule that is strategy-proof and individually rational. We relax this constraint and consider pair efficiency. A rule is pair-efficient if it never induces an allocation at which a pair of agents gain from trading their assigned objects. Remarkably, even in the larger set of pair-efficient rules, we find that TTC is still the only rule that is strategy-proof and individually rational. Our characterization result gives strong support to the use of TTC in object reallocation problems.