Enda Ridge Michael G. Madden Gerard Lyons The Evolution of Probabilistic Reciprocity in a Multi-agent Environment with Neighborhoods. 123-136 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3191&spage=123 2004 conf/cia/2004 CIA db/conf/cia/cia2004.html#RidgeML04