Group insurance and lending with endogenous social collateral

マイクロクレジットの個人貸付とグループ貸付に関する最近の論文：

Paal and Wiseman "Group insurance and lending with endogenous social collateral" JDE 2011
http://www.sciencedirect.com/science/article/pii/S0304387809001230

Abstract
We construct a dynamic model of self-enforcing insurance provision and lending to a community of borrowers who are connected by risk-sharing arrangements that are themselves subject to enforcement problems, as in Kocherlakota (1996). We show that an outside lender offering constant-consumption contracts can earn a higher profit if he conditions his repeated interactions with each borrower on the history of his interactions with all the group members (a joint liability contract), rather than on his history with that borrower only (individual liability contracts). This result holds even in the absence of informational asymmetries. The observation driving it is that with individual liability contracts, a joint welfare-maximizing group may prefer to have one or more group members default on their contracts, so that the group can consume a mix of outside funds and the defaulters' stochastic income. One contribution of our work is to give precise economic content to the concept of “social collateral” as the per-agent surplus from group risk-sharing over autarky. The group can deter its members from defaulting on their contracts with the principal by threatening to reduce that surplus.

Madajewicz "Joint liability versus individual liability in credit contracts" JEBO 2011
http://www.sciencedirect.com/science/article/pii/S0167268110002088

Abstract
I offer an explanation for the coexistence of joint-liability and individual-liability microcredit contracts. I show that both contracts maximize welfare when credit is rationed due to limited liability, but for different borrowers. Borrowers monitor each other when liability is joint, while the lender monitors individual loans. Joint liability offers poorer borrowers larger loans with less monitoring effort than would have to be exerted by the lender. Individual liability offers the wealthier among credit-constrained borrowers larger loans even without monitoring. The theory explains why individual loans serve the wealthier among poor borrowers and are larger, and why businesses funded with individual loans grow more.

最近の開発経済学ではフィールド実験が主流となってきたけれど、
モデルを作ってみれば、ある政策の効果はいろんなパラメータに依存することが分かってくるので、
フィールド実験の論文があふれかえっている今こそ、
改めて理論論文の価値が見直されるべきだと思う。