Coalitional Manipulation and Target Set Correspondences

Abstract

We study coalitional manipulation of interval-valued social choice correspondences on the single-peaked domain. In the single-valued model, strategy-proofness and group strategy-proofness coincide within the standard class of peaks-only social choice functions, so generalized median voter rules retain their robustness to coalitional manipulation. This equivalence breaks down once the outcome is allowed to be set-valued: generalized median correspondences may be strategy-proof while remaining vulnerable to coordinated deviations. We show that, among generalized median correspondences allowing genuine set-valued choice, pairwise strategy-proofness already forces anonymity. With voter sovereignty, this restriction becomes sharp and characterizes target set correspondences. Thus, the target-set structure emerges as the form of set-valued flexibility compatible with pairwise coalitional robustness.