“…1 In experimental research, it has also been shown that people often reduce others' payoffs at personal cost when the cost imposed on others is larger than that incurred by the decision maker (e.g., Zizzo & Oswald 2001, Abbink et al 2010, Abbink & Herrmann 2011, Brañas-Garza et al 2014, which is consistent with individuals having a preference for increasing their relative standing. However, the opposite is often observed as well: people are willing to sacrifice their own payoffs in order to increase the welfare of others (e.g., Piliavin & Charng 1990, Forsythe et al 1994, Ledyard 1995, Camerer 2003, Staffiero et al 2013, Brethel-Haurwitz & Marsh 2014, which could be modeled as if others' payoffs enter positively into the individuals' utility functions. 2 Based on all this evidence, we shall assume that individuals aim at increasing their relative standing within the group they belong to (i.e., they are "competitive") 3 but also at enhancing the performance of the group, or social efficiency (i.e., they are "cooperative"; see McClintock 1972, Van Lange et al 1997, Charness & Rabin 2002.…”