Two statements of the Duggan-Schwartz theorem

Abstract: The Duggan-Schwartz theorem (Duggan and Schwartz, 1992) is a famous result concerning strategy-proof social choice correspondences, often stated as "A social choice correspondence that can be manipulated by neither an optimist nor a pessimist has a weak dictator". However, this formulation is actually due to Taylor (2002), and the original theorem, at face value, looks rather different. In this note we show that the two are in fact equivalent.