1. Informal groups cannot rely on external enforcement to insure that members .......... by their obligations.

2. It is generally assumed that these problems are solved by social sanctions' and .......... effects.

3. Roscas are one of the most commonly found informal financial institutions in the developing world; in the absence of an external (social) sanctioning mechanism, roscas are never sustainable, even if the .......... member is excluded from all future roscas.

4. It is .......... assumed that informal groups, such as roscas, rely on social sanctions to solve their enforcement problems.

5. We examine this notion more carefully and distinguish between expulsion from future roscas cycles and .......... social sanctions.

6. We first demonstrate that theoretically expulsion in itself is never a sufficient ..........

7. We then ask whether institutional features of these groups are chosen in some part to prevent members from .......... on their responsibilities.

8. We focus on the allocation of ranks, and we show that a random allocation of ranks, though preferred by a majority of members, tends to .......... the incentives to welch.

9. They are therefore sustainable only if the .......... of social sanctions on their members are sufficiently high.

10. We provide some evidence in favour of our hypothesis, since individuals who are more .......... to those sanctions tend to participate in roscas where the order of ranks is randomly drawn at each cycle.