NET Institute* www.NETinst.org Working Paper #11-25 October 2011 Risk and Reciprocity Over the Mobile Phone Network: Evidence from Rwanda Joshua Blumenstock Nathan Eagle Marcel Fafchamps University of California, Berkeley Santa Fe Institute Oxford University * The Networks, Electronic Commerce, and Telecommunications (“NET”) Institute, http://www.NETinst.org , is a non-profit institution devoted to research on network industries, electronic commerce, telecommunications, the Internet, “virtual networks” comprised of computers that share the same technical standard or operating system, and on network issues in general.