@techreport{NBERw13135,
 title = "Directed Altruism and Enforced Reciprocity in Social Networks: How Much is A Friend Worth?",
 author = "Stephen Leider and Markus M. Möbius and Tanya Rosenblat and Quoc-Anh Do",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Working Paper Series",
 number = "13135",
 year = "2007",
 month = "May",
 URL = "http://www.nber.org/papers/w13135",
}