FACULTY FEATURE ARTICLE 5 Arrow’s Impossibility Theorem: Two Simple Single-Profile Versions

In this paper we provide two simple new versions of Arrow’s impossibility theorem, in a model with only one preference profile. Both versions are transparent, requiring minimal mathematical sophistication. The first version assumes there are only two people in society, whose preferences are being aggregated; the second version assumes two or more people. Both theorems rely on assumptions about diversity of preferences, and we explore alternative notions of diversity at some length. Our first theorem also uses a neutrality assumption, commonly used in the literature; our second theorem uses a neutrality/monotonicity assumption, which is stronger and less commonly used. We provide examples to illustrate our points. In 1950 Kenneth Arrow ([Ar1],[Ar2]) provided a striking answer to a basic abstract problem of democracy: how can the preferences of many individuals be aggregated into social preferences? The starkly negative answer, known as Arrow’s impossibility theorem, was that every conceivable aggregation method has some flaw. That is, a handful of reasonable-looking axioms, which on