This article revisits the relationship between income per capita and civil con ‡ict. We establish that the empirical literature identi…es two di¤erent patterns. First, poor countries have a higher propensity to su¤er from civil war. Second, civil war occurs when countries su¤er negative income shocks. In a formal model we examine an explanation often suggested in the informal literature: civil wars occur in poor countries because the opportunity cost of …ghting is small. We show that while this explanation fails to make sense of the …rst empirical pattern, it provides a coherent theoretical basis for the second. We then enrich the model to allow for private imperfect information about the state of the economy and show that mutual fears exacerbate the problem caused by negative income shocks.

We study the design of randomized controlled experiments in environments where outcomes are significantly affected by unobserved effort decisions taken by the subjects(agents). While standard randomized controlled trials (RCTs) are internally consistent, the unobservability of effort provision compromises external validity. We approach trial design as a principal-agent problem and show that natural extensions of RCTs -which we call selective trials-can help improve the external validity of experiments. In particular, selective trials can disentangle the effects of treatment, effort, and the interaction of treatment and effort. Moreover, they can help experimenters identify when measured treatment effects are affected by erroneous beliefs and inappropriate effort provision.

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This paper studies how agents with conflicting interests learn to cooperate when the details of cooperation are not common knowledge. It considers a repeated game in which one player has incomplete information about when and how her partner can provide benefits. Initially, monitoring is imperfect and cooperation requires inefficient punishment. As the players' common history grows, the uninformed player can learn to monitor her partner's actions, which allows players to establish more efficient cooperative routines. Because revealing information is costly, it may be optimal not to reveal all the existing information, and efficient equilibria can be path-dependent. (JEL C73, D82, D83, D86)

We thank Esther Duflo for her leadership on the handbook, and for extensive comments on earlier drafts. Chassang and Snowberg gratefully acknowledge the support of NSF grant SES-1156154. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.