Networks promote cooperation in experiments with humans. Proc Natl Acad Sci USA 108:19193?9198. 10. Pacheco JM, Traulsen A, Nowak MA (2006) Active linking in evolutionary games. J Theor Biol 243:437?43. 11. Kossinets G, Watts D (2009) Origins of homophily in an evolving social network1. Am J Sociol 115:405?50. 12. Newcomb T (1961) The Acquaintance Process (Holt, Rinehart Winston, New York, NY). 13. Feld S (1981) The focused organization of social ties. Am J Sociol 86:1015?035. 14. Granovetter M (1985) Economic action and social structure: The problem of embeddedness. Am J Sociol 91:481?10. 15. Felmlee D, Tulathromycin web Sprecher S, Bassin E (1990) The dissolution of intimate relationships: A hazard model. Soc Psychol Q 53:13?0. 16. Hauk E (2003) Multiple prisoner’s dilemma games with (out) an outside option: An experimental study. Theory Decis 54:207?29. 17. Hauk E, Nagel R (2001) Choice of partners in multiple two-person prisoner’s dilemma games. J Conflict Resolut 45:770?93. 18. Coricelli G, Fehr D, Fellner G (2004) Partner selection in public goods experiments. J Conflict Resolut 48:356?78. 19. List J (2006) Friend or foe? A natural experiment of the prisoner’s dilemma. Rev Econ Stat 88:463?71. 20. Slonim R, Garbarino E (2008) Increases in trust and altruism from partner selection: Experimental evidence. Exp Econ 11(2):134?53. 21. Page T, Putterman L, Unel B (2005) Voluntary association in public goods experiments: Reciprocity, mimicry and efficiency*. Econ J 115:1032?053. 22. Cinyabuguma M, Page T, Putterman L (2005) Cooperation under the threat of expulsion in a public goods experiment. J Public Econ 89:1421?435.23. Suri S, Watts DJ (2011) Cooperation and contagion in web-based, networked public goods experiments. PLoS ONE 6:e16836. 24. Traulsen A, Semmann D, Sommerfeld RD, Krambeck HJ, Milinski M (2010) Human strategy updating in evolutionary games. Proc Natl Acad Sci USA 107:2962?966. 25. Watts DJ, Strogatz SH (1998) Collective dynamics of `small-world’ networks. Nature 393:440?42. 26. Newman M (2003) The structure and function of complex networks. SIAM Rev 45: 167?56. 27. Gelman A, Hill J (2007) Data Analysis Using Regression and Multilevel/Hierarchical Models (Cambridge Univ Press, New York), Vol 1. 28. McPherson M, Smith-Lovin L, Cook J (2001) Birds of a feather: Homophily in social networks. Annu Rev Sociol 27:415?44. 29. Skyrms B, Pemantle R (2000) A dynamic model of social network formation. Proc Natl Acad Sci USA 97:9340?346. 30. Immorlica N, Lucier B, Rogers B (2010) Cooperation in anonymous dynamic social networks. HM61713, BI 1482694 cancer Proceedings of the 11th ACM Conference on Electronic Commerce (Association for Computing Machinery, New York), pp 241?42. 31. Santos FC, Pacheco JM, Lenaerts T (2006) Cooperation prevails when individuals adjust their social ties. PLOS Comput Biol 2:e140. 32. Hanaki N, Peterhansl A, Dodds P, Watts D (2007) Cooperation in evolving social networks. Manage Sci 53:1036?050. 33. Bilancini E, Boncinelli L (2009) The co-evolution of cooperation and defection under local interaction and endogenous network formation. J Econ Behav Organ 70(1-2): 186?95. 34. Eshel I, Cavalli-Sforza LL (1982) Assortment of encounters and evolution of cooperativeness. Proc Natl Acad Sci USA 79:1331?335. 35. Cavalli-Sforza LL, Feldman MW (1983) Paradox of the evolution of communication and of social interactivity. Proc Natl Acad Sci USA 80:2017?021. 36. Ostrom E, Walker J, Gardner R (1992) Covenants with and without a sword: Selfgovernance is possible.Networks promote cooperation in experiments with humans. Proc Natl Acad Sci USA 108:19193?9198. 10. Pacheco JM, Traulsen A, Nowak MA (2006) Active linking in evolutionary games. J Theor Biol 243:437?43. 11. Kossinets G, Watts D (2009) Origins of homophily in an evolving social network1. Am J Sociol 115:405?50. 12. Newcomb T (1961) The Acquaintance Process (Holt, Rinehart Winston, New York, NY). 13. Feld S (1981) The focused organization of social ties. Am J Sociol 86:1015?035. 14. Granovetter M (1985) Economic action and social structure: The problem of embeddedness. Am J Sociol 91:481?10. 15. Felmlee D, Sprecher S, Bassin E (1990) The dissolution of intimate relationships: A hazard model. Soc Psychol Q 53:13?0. 16. Hauk E (2003) Multiple prisoner’s dilemma games with (out) an outside option: An experimental study. Theory Decis 54:207?29. 17. Hauk E, Nagel R (2001) Choice of partners in multiple two-person prisoner’s dilemma games. J Conflict Resolut 45:770?93. 18. Coricelli G, Fehr D, Fellner G (2004) Partner selection in public goods experiments. J Conflict Resolut 48:356?78. 19. List J (2006) Friend or foe? A natural experiment of the prisoner’s dilemma. Rev Econ Stat 88:463?71. 20. Slonim R, Garbarino E (2008) Increases in trust and altruism from partner selection: Experimental evidence. Exp Econ 11(2):134?53. 21. Page T, Putterman L, Unel B (2005) Voluntary association in public goods experiments: Reciprocity, mimicry and efficiency*. Econ J 115:1032?053. 22. Cinyabuguma M, Page T, Putterman L (2005) Cooperation under the threat of expulsion in a public goods experiment. J Public Econ 89:1421?435.23. Suri S, Watts DJ (2011) Cooperation and contagion in web-based, networked public goods experiments. PLoS ONE 6:e16836. 24. Traulsen A, Semmann D, Sommerfeld RD, Krambeck HJ, Milinski M (2010) Human strategy updating in evolutionary games. Proc Natl Acad Sci USA 107:2962?966. 25. Watts DJ, Strogatz SH (1998) Collective dynamics of `small-world’ networks. Nature 393:440?42. 26. Newman M (2003) The structure and function of complex networks. SIAM Rev 45: 167?56. 27. Gelman A, Hill J (2007) Data Analysis Using Regression and Multilevel/Hierarchical Models (Cambridge Univ Press, New York), Vol 1. 28. McPherson M, Smith-Lovin L, Cook J (2001) Birds of a feather: Homophily in social networks. Annu Rev Sociol 27:415?44. 29. Skyrms B, Pemantle R (2000) A dynamic model of social network formation. Proc Natl Acad Sci USA 97:9340?346. 30. Immorlica N, Lucier B, Rogers B (2010) Cooperation in anonymous dynamic social networks. Proceedings of the 11th ACM Conference on Electronic Commerce (Association for Computing Machinery, New York), pp 241?42. 31. Santos FC, Pacheco JM, Lenaerts T (2006) Cooperation prevails when individuals adjust their social ties. PLOS Comput Biol 2:e140. 32. Hanaki N, Peterhansl A, Dodds P, Watts D (2007) Cooperation in evolving social networks. Manage Sci 53:1036?050. 33. Bilancini E, Boncinelli L (2009) The co-evolution of cooperation and defection under local interaction and endogenous network formation. J Econ Behav Organ 70(1-2): 186?95. 34. Eshel I, Cavalli-Sforza LL (1982) Assortment of encounters and evolution of cooperativeness. Proc Natl Acad Sci USA 79:1331?335. 35. Cavalli-Sforza LL, Feldman MW (1983) Paradox of the evolution of communication and of social interactivity. Proc Natl Acad Sci USA 80:2017?021. 36. Ostrom E, Walker J, Gardner R (1992) Covenants with and without a sword: Selfgovernance is possible.