The Impartial Observer Under Uncertainty

Abstract. This paper extends Harsanyi’s Impartial Observer Theorem by introducing Knightian Uncertainty in the form of individual belief systems. It features an axiomatic framework of societal decision-making in the presence of individual uncertainty. The model allows the analysis of scenarios where individuals agree on the ranking but not on the likelihood of social outcomes. The preferences of the impartial observer are expressed by a weighted sum of utilities - each representing individual preferences with different belief systems. To incorporate common criticism of the framework of Harsanyi (1953), our approach is based on the generalized version by Grant et al. (2010). The belief systems are introduced as second-order beliefs following Seo (2009)