Independence of irrelevant expansions

Nash characterized the only bargaining solution to satisfy a well-known list of axioms. Independence of Irrelevant Alternatives states invariance of the solution outcome under certain contractions of the bargaining problem. A dual of this axiom is proposed here, stating invariance under certainexpansions of the bargaining problem andNash's solution is characerized by substituting this axiom for IIA in Nash's original list. After a transposition from the domain of bargaining solutions to the domain of choice rules, and a weakening of Invariance with respect to Positive Affine Transformations toTranslation Invariance, this new list of axioms is shown to characterizeUtilitarian rules.