Group Obvious Strategy-proofness: Definition and Characterization

• Authors: R. Pablo Arribillaga, Jordi Massó and Alejandro Neme
• Journal of Mathematical Economics, Vol. 123, 103216, April 2026

We introduce the concept of group obvious strategy-proofness, an extension of Li (2017)’s notion of obvious strategy-proofness, by requiring that truth-telling remains 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 is no 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 on existing results on obvious strategy-proofness via extensive game forms with perfect information, we derive additional equivalences concerning the implementability of social choice functions: in this class of games, strategy-proofness, group strategy-proofness, obvious strategy-proofness, and group obvious strategy-proofness are all equivalent.