Social Choice Under Incomplete, Cyclic Preferences

Actual individual preferences are neither complete (=total) nor antisymmetric in general, so that at least every quasi-order must be an admissible input to a satisfactory choice rule. It is argued that the traditional notion of ``indifference'' in individual preferences is misleading and should be replaced by `equivalence' and `undecidedness'. In this context, ten types of majority and minority arguments of different strength are studied which lead to social choice rules that accept profiles of arbitrary reflexive relations. These rules are discussed by means of many familiar, and some new conditions, including `immunity from binary arguments'. Moreover, it is proved that every choice function satisfying two weak Condorcet-type conditions can be made both composition-consistent and idempotent, and that all the proposed rules have polynomial time complexity.