Some papers on "Generalised Exchange"...

...which seems to cover a lot of what we call "Informal Exchange".

• The Emergence of Generalized Exchange by Indirect Reciprocity, Anders Biel, Daniel Eek, Tommy Gärling and Mathias Gustafsson -- http://www.springerlink.com/content/qp585m5tu4404761/
• Multiplexity, generalized exchange and cooperation in organizations: a case study, Emmanuel Lazegaa and Philippa E. Pattison -- http://dx.doi.org/10.1016/S0378-8733(99)00002-7
• Generalized Exchange and Social Dilemmas, Toshio Yamagishi and Karen S. Cook -- http://www.jstor.org/pss/2786661
• Towards a Theory of Large-Scale Generalized Exchange, Levine, Sheen. and Shah, Sonali -- http://www.allacademic.com//meta/p_mla_apa_research_citation/1/0/7/6/7/pages107678/p107678-1.php
• Generalized Exchange, Social Identity, and Non-Anonymity, Triplett, Jennifer. and Thye, Shane -- http://www.allacademic.com//meta/p_mla_apa_research_citation/1/8/3/0/5/pages183055/p183055-1.php
• Mechanisms of Generalized Exchange, Baker, Wayne E. and Levine, Sheen S -- http://ssrn.com/abstract=1352101
• Generalized Exchange, Peter Bearman -- http://www.jstor.org/stable/2782312
• The emergence of generalized exchange, Nobuyuki Takahashi -- http://www.jstor.org/stable/3003889
• Expectations of a generalized exchange system and ingroup favoritism: an experimental study of bounded reciprocity, Kiyonari T -- http://www.ncbi.nlm.nih.gov/pubmed/12101584
• Spontaneous formation of generalized exchange system: An experimental study of discriminating altruism, Nobuyuki Takahashi, http://sciencelinks.jp/j-east/article/199917/000019991799A0512436.php
• Building Solidarity through Generalized Exchange: A Theory of Reciprocity, Linda D. Molm, Jessica L. Collett, David R. Schaefer -- http://www.journals.uchicago.edu/doi/abs/10.1086/517900