A divergence of views among microeconomists in general and game theorists in particular regarding the explanatory objectives of microeconomic theory has become apparent in recent years. This divergence concerns, most fundamentally, the question whether institutions, legal or customary rules, or social norms are to be classified among the endogenous as opposed to the exogenous variables in the framework of microeconomic analysis. 1 The majority of economists are probably agnostic or ambivalent on this question, not having confronted, or not having had to confront, the issue in their own work. Many have sidestepped it by treating institutions as immutable or by restricting their analyses to a given rule regime. But rules do vary and change, and among those who are concerned with studying variation in institutions, two diverging views are increasingly identifiable, sometimes coexisting even within the writings of the same author.