Population Monotonicity and Egalitarianism for Binary Social Choice Problems

We consider the class of binary social choice problems. A society must choose one of two public projects, money being available to perform side payments and each agent having quasi-linear preferences. Moulin [1987a] formulates the problem and characterizes the egalitarian solution on the basis of agreement. This axiom requires that changes in the preferences of some members of the society should affect the agents whose preferences have not changed in the same direction; all gain or all lose. In this paper, we present population monotonicity. This axiom requires that upon the arrival of new agents, all of the original agents should be affected in the same direction; all gain or all lose.