mathematically tractable, the top-down modeling of economics focuses solely on the prediction of behavior.

Theory building in other social sciences and particularly in psychology, on the other hand, is a more data-driven and less elegant enterprise, which has as its goal not only prediction but also explanation of observed behavior. Theory and research thus focus on the processes that give rise to observed behavior. It is this focus on explanation and process that leads psychologists to study a broader range of dependent measures (e.g., not just the magnitude of withdrawals from common-pool resources, but also users’ justifications of such withdrawals as well as judgments of fairness). Kopelman, Weber, and Messick (Chapter 4) provide a very useful classification and road map to the effects of a large number of user and user group characteristics and situational variables that have been shown to affect behavior in commons dilemmas. Agrawal’s critical analysis and synthesis of political science field studies of commons management regimes (Chapter 2) includes user group characteristics as well as resource system characteristics, institutional arrangements, and characteristics of the external environment. In all instances, he finds that the effect of these variables on sustainability of the commons is configural. The effect of even a basic characteristic such as user group size or heterogeneity depends on a range of other contextual and mediating factors. Closed-form solutions for models that would attempt to incorporate all of these variables into a prediction equation would be hard to come by. Nevertheless, awareness of these effects and causal explanations of their origin are of both theoretical and practical importance to researchers, resource users, and policy makers.

In their review of the effects of economic heterogeneity among water resource users on commons dilemma outcomes, Bardhan and Dayton-Johnson (Chapter 3) illustrate the utility of focusing on process-level explanations of behavior and the importance of checking the predictions of economic theory against observed regularities (in their case, regularities observed in large-n field studies). The authors draw some careful distinctions between different processes by which economic heterogeneity might affect the resolution of common-pool resource management dilemmas (e.g., effects on incentives for cooperation versus effects on social norms and sanctions; effects on choice of institutions versus effects on their implementation and enforcement). Such distinctions allow them to reconcile apparently contradictory predictions about the effect of economic heterogeneity made by different theories. In their evaluation of the explanatory potential of these alternative process explanations for behavior observed in several international field studies, Bardhan and Dayton-Johnson show that some mechanisms (e.g., “Olson effects”) may have greater theoretical plausibility than practical reality.

Agrawal (Chapter 2) explicitly addresses the relationship between theory-driven and data-driven research approaches. He argues that the two are not just synergistic in the insights they provide, but that they require each other at an



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