with the observation that public good problems are essentially two-trait in nature [47]. First, ... ( more at http://rspb.royalsocietypublishing.org/content/280/1765/20131151.full.pdf+html )Received May 7, 2013.Accepted May 28, 2013. 2013 The Author(s) Published by the Royal Society. All rights reserved.ReferencesDunbar RIM, Shultz S. 2010 Bondedness and sociality. Behaviour 147, 775803. doi:10.1163/000579510X501151 (doi:10.1163/000579510X501151)CrossRefWeb of ScienceShultz S, Dunbar R. 2010 Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality. Proc. Natl Acad. Sci. USA 107, 21 58221 586. doi:10.1073/pnas.1005246107 (doi:10.1073/pnas.1005246107)Abstract/FREE Full TextDunbar RIM. 1992 Neocortex size as a constraint on group-size in primates. J. Hum. Evol. 22, 469493. doi:10.1016/0047-2484(92)90081-J (doi:10.1016/0047-2484(92)90081-J)CrossRefWeb of ScienceDunbar RIM. 1998 The social brain hypothesis. Evol. Anthropol. 6, 178190. doi:10.1002/(SICI)1520-6505(1998)6:5<178::AID-EVAN5>3.0.CO;2-8 (doi:10.1002/(SICI)1520-6505(1998)6:5<178::AID-EVAN5>3.0.CO;2-8)CrossRefWeb of ScienceBarton RA. 1996 Neocortex size and behavioural ecology in primates. Proc. R. Soc. Lond. B 263, 173177. doi:10.1098/rspb.1996.0028 (doi:10.1098/rspb.1996.0028)Abstract/FREE Full TextByrne RB, Whiten A (eds) 1988 A Machiavellian intelligence: social complexity and the evolution of intellect in monkeys, apes and humans. Oxford, UK: Oxford University Press.Barton RA, Dunbar RIM. 1997 Evolution of the social brain. In Machiavellian intelligence. II. Extensions and evaluations (eds Whiten A, Byrne RB), pp. 240263. Cambridge, UK: Cambridge University Press.Sallet J, et al. 2011 Social network size affects neural circuits in macaques. Science 334, 697700. doi:10.1126/science.1210027 (doi:10.1126/science.1210027)Abstract/FREE Full TextLewis PA, Rezaie R, Brown R, Roberts N, Dunbar RI. 2011 Ventromedial prefrontal volume predicts understanding of others and social network size. Neuroimage 57, 16241629. doi:10.1016/j.neuroimage.2011.05.030 (doi:10.1016/j.neuroimage.2011.05.030)CrossRefMedlineWeb of SciencePowell J, Lewis PA, Roberts N, Garcia-Finana M, Dunbar RIM. 2012 Orbital prefrontal cortex volume predicts social network size: an imaging study of individual differences in humans. Proc. R. Soc. B 279, 21572162. doi:10.1098/rspb.2011.2574 (doi:10.1098/rspb.2011.2574)Abstract/FREE Full TextBickart KC, Wright CI, Dautoff RJ, Dickerson BC, Barrett LF. 2011 Amygdala volume and social network size in humans. Nat. Neurosci. 14, 163164. doi:10.1038/nn.2724 (doi:10.1038/nn.2724)CrossRefMedlineKanai R, Bahrami B, Roylance R, Rees G. 2012 Online social network size is reflected in human brain structure. Proc. R. Soc. B 279, 13271334. doi:10.1098/rspb.2011.1959 (doi:10.1098/rspb.2011.1959)Abstract/FREE Full TextFreeberg TM, Dunbar RI, Ord TJ. 2012 Social complexity as a proximate and ultimate factor in communicative complexity. Phil. Trans. R. Soc. B 367, 17851801. doi:10.1098/rstb.2011.0213 (doi:10.1098/rstb.2011.0213)Abstract/FREE Full TextDunbar RIM. 2012 Bridging the bonding gap: the transition from primates to humans. Phil. Trans. R. Soc. B 367, 18371846. doi:10.1098/rstb.2011.0217 (doi:10.1098/rstb.2011.0217)Abstract/FREE Full TextDunbar RIM. 2008 Mind the gap: or why humans aren't just great apes. Proc. Br. Acad. 154, 403423.Ihara Y. 2011 Evolution of culture-dependent discriminate sociality: a gene-culture coevolutionary model. Phil. Trans. R. Soc. B 366, 889900. doi:10.1098/rstb.2010.0247 (doi:10.1098/rstb.2010.0247)Abstract/FREE Full TextSigg H, Stolba A. 1981 Home range and daily march in a hamadryas baboon troop. Folia Primatol. (Basel) 36, 4075. doi:10.1159/000156008 (doi:10.1159/000156008)CrossRefMedlineWeb of ScienceTajfel H, Billig MG, Bundy RP, Flament C. 1971 Social categorization and intergroup behavior. Eur. J. Soc. Psychol. 1, 149177. doi:10.1002/ejsp.2420010202 (doi:10.1002/ejsp.2420010202)CrossRefWeb of ScienceRiolo RL, Cohen MD, Axelrod R. 2001 Evolution of cooperation without reciprocity. Nature 414, 441443. doi:10.1038/35106555 (doi:10.1038/35106555)CrossRefMedlineWeb of ScienceAntal T, Ohtsuki H, Wakeley J, Taylor PD, Nowak MA. 2009 Evolution of cooperation by phenotypic similarity. Proc. Natl Acad. Sci. USA 106, 85978600. doi:10.1073/pnas.0902528106 (doi:10.1073/pnas.0902528106)Abstract/FREE Full TextNettle D, Dunbar RIM. 1997 Social markers and