Ulric B. and Evelyn L. Bray Social Sciences Seminar
Abstract: We argue that Arrow's (1951) independence of irrelevant alternatives condition (IIA) is unjustifiably stringent. Although, in elections, it has the desirable effect of ruling out spoilers (Candidate A spoils the election for B if B beats C when all voters rank A low, but C beats B when some voters rank A high - - A "siphons" off support from B), it is stronger than necessary for this purpose. Worse, it makes a voting rule insensitive to voters' preference intensities. Accordingly, we propose a modified version of IIA to address these problems. Rather than obtaining an impossibility result, we show that a voting rule satisfies modified IIA, Arrow's other conditions, and May's (1952) axioms for majority rule if and only if it is the Borda count (Borda 1781), i.e., rank-order voting.