<jats:title>Abstract</jats:title><jats:p>This paper contributes to the literature on externalities and their classification by reconciling insights from transaction costs theory with James Buchanan’s and Elinor Ostrom’s analyses of property rights and institutional diversity. We critique the dominant Pigouvian analysis, which assumes only two forms of institutions—namely, governments and private markets—that can internalize externalities. We develop a new taxonomy of externalities that provides relevant conceptual space for a wide array of institutions that the market-versus-state dichotomy obscures. The proposed taxonomy considers two key classes of often-conflated attributes: (1) the scale of externalities, and (2) the assignability, enforceability, and tradability of property rights. This approach enriches the Coasean (transaction cost) perspective by allowing us to unbundle transaction costs in a manner that extends its applicability to nonmarket situations in which market-based transactions are either not permitted or technically infeasible. Thus, by integrating insights from two distinct Public Choice schools, we broaden the theory of externalities to not only encompass market exchanges but also to incorporate cases in which property rights are, and will remain, unclear. We conclude that institutional diversity can offer adaptable solutions to tackle medium- and large-scale externalities.</jats:p>
