Chapter 9. Causal inference in environmental conservation: The role of institutions The on going degradation of global public goods such as biodiversity and climate regulation due to the loss of natural tropical ecosystems has generated demand for evidence on the effectiveness of alternative policy instruments for environmental conservation. Economists initially responded with ex post evaluations using quasi experimental methods to identify average causal effects on outcomes such as deforestation. In this chapter we demonstrate how careful attention to institutions enhances both the credibility and the policy relevance of these evaluations. Policy instruments such as protected areas decentralization and payments for ecosystem services are designed to change formal property rights institutions. Their causal effects are shaped by both formal and informal institutions especially when they are applied to ecosystems that are also central to local livelihoods. Program evaluation should consider how these institutions define (1) assignment or selection of people and places (2) specific treatments through variation in institutional details that generate heterogeneous effects (3) moderators that influence potential outcomes both with and without treatment again generating heterogeneous effects and (4) mechanisms or the means by which instruments affect the ultimate outcomes.