Individual versus group strategy-proofness: When do they coincide?

A social choice function is group strategy-proof on a domain if no group of agents can manipulate its final outcome to their own benefit by declaring false preferences on that domain. There are a number of economically significant domains where interesting rules satisfying individual strategy-proofness can be defined, and for some of them, all these rules turn out to also satisfy the stronger requirement of group strategy-proofness. We provide conditions on domains guaranteeing that for all rules defined on them, individual and group strategy-proofness become equivalent. We also provide a partial answer regarding the necessity of our conditions. © 2010 Published by Elsevier Inc. ✩ We are grateful to three anonymous referees and many colleagues for their useful comments in different congresses and workshops where the paper was presented. Salvador Barberà gratefully acknowledges support from the Spanish Ministry of Science and Innovation through grant “Consolidated Group-C” ECO2008-04756, and by the Generalitat de Catalunya, Departament d’Universitats, Recerca i Societat de la Informació through the Distinció per a la Promoció de la Recerca Universitària, grant SGR2009-0419 and the Barcelona GSE Research Network. Dolors Berga acknowledges the support of the Spanish Ministry of Science and Innovation through grant SEJ2007-60671 and of Generalitat de Catalunya, through grant SGR2009-0189. She also acknowledges the Research Recognition Programme of the Barcelona GSE. Bernardo Moreno acknowledges financial support from Junta de Andalucía through Proyecto de Excelencia 522 and the ECO2008-03674 from the Spanish Ministry of Science and Innovation. * Corresponding author at: Universitat Autònoma de Barcelona, Edifici B, 08193 Bellaterra, Spain. Fax: +34 935813767. E-mail addresses: [email protected] (S. Barberà), [email protected] (D. Berga), [email protected] (B. Moreno). Please cite this article in press as: S. Barberà et al., Individual versus group strategy-proofness: When do they coincide?, J. Econ. Theory (2010), doi:10.1016/j.jet.2010.03.006 0022-0531/$ – see front matter © 2010 Published by Elsevier Inc. doi:10.1016/j.jet.2010.03.006 ARTICLE IN PRESS YJETH:3818 JID:YJETH AID:3818 /FLA [m1+; v 1.119; Prn:1/04/2010; 10:05] P.2 (1-27) 2 S. Barberà et al. / Journal of Economic Theory ••• (••••) •••–••• JEL classification: D71