Group Obvious Strategy-proofness: Definition and Characterization

We introduce the concept of group obvious strategy-proofness, an extension of Li (2017)’s notion of obvious strategy-proofness, by requiring that truth-tellingremains an obviously dominant strategy for any group of agents in the extensive game form implementing the social choice function. We show that this stronger condition isno more restrictive: the set of all group obviously strategy-proof social choice functions coincides with the set of all obviously strategy-proof social choice functions. Building on this equivalence result and existing results on obvious strategy-proofness, we derive further equivalence results concerning the implementability of social choice functions via round-table mechanisms: strategy-proofness, group strategy-proofness, obvious strategy-proofness, and group obvious strategy-proofness are all equivalent.