Efficient and stable collective choices under gregarious preferences

We consider collective choice problems where a set of agents have to choose an alternative from a …nite set and agents may or may not become users of the chosen alternative. An allocation is a pair given by the chosen alternative and the set of its users. Agents have gregarious preferences over allocations: given an allocation, they prefer that the set of users becomes larger. We require that the …nal allocation be e¢ cient and stable (no agent can be forced to be a user We are grateful to two anonymous referees for their thoughtful comments and suggestions that greatly improved the paper. We have also received very useful comments from Pablo Amorós, Carmen Beviá, Sandro Brusco, Luis Corchón, Humberto Llavador, Bernardo Moreno, David PérezCastrillo, Socorro Puy, Joaquim Silvestre, Marc Vorsatz, and Ashel Wolinsky. The work of Massó is partially supported by the Spanish Ministry of Education and Science through grant SEJ200501481/ECON and FEDER and through project CONSOLIDER-INGENIO 2010 (CDS2006-00016), and by the Generalitat de Catalunya through grant SGR2005-00454 and through the Barcelona Economics Program (XREA). The work of Nicolò is partially supported by the Italian Ministry of University and Research through grant 2005137858. yDepartament d’Economia i d’Història Econòmica and CODE, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Spain. Fax: (34) 935812461. E-mail: [email protected]. zCorresponding author. Dipartimento di Scienze Economiche, Università di Padova, via del Santo 33, 35123 Padova, Italy. Fax: 00390498274211. E-mail: [email protected].