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Codifying Custom

Codifying decentralized forms of law, such as the common law
and customary international law, has been a cornerstone of the
positivist turn in legal theory since at least the nineteenth century.
Commentators laud codification’s purported virtues, including
systematizing, centralizing, and clarifying the law. These attributes
are thought to increase the general welfare of those subject to legal
rules and therefore to justify and explain codification. The literature,
however, overlooks codification’s distributive consequences. In so
doing, it misses a common motive for codification: to define legal
rules in a way that advantages individual codifying institutions,
regardless of how it affects the general welfare.

This Article fills the gap in the literature by examining three
rationales for why states codify customary international law: (1) a
desire to clarify the substantive content of customary law in order to
promote cooperation (the Clarification Thesis); (2) a desire to
enhance compliance through mechanisms such as monitoring,
enforcement, and dispute-resolution provisions (the Compliance
Thesis); and (3) a desire to define the content of customary rules for
a state’s individual benefit (the Capture Thesis). While codification’s
proponents conceive of the enterprise in terms of the Clarification
and Compliance Theses, I argue that states frequently use
codification to capture customary international legal rules to benefit
themselves at the expense of the general welfare. As states with
divergent views on how to interpret a customary rule pursue
conflicting codification efforts, they entrench schisms in the law along
regional or ideological lines, thereby delegitimizing customary rules
and increasing fragmentation. Thus, far from being an unqualified
boon to benevolent legal ordering, codification can replicate, magnify,
or alter the power dynamics present in forming bare customary law.
Indeed, the fragmentation of customary law that can result from
codification actually prevents a unified understanding of customary
law from emerging—the exact opposite of codification’s ostensible
purpose. This Article uses the Capture Thesis to explain important
developments in customary international law, including the outlawing
of the slave trade in the nineteenth century, the rise of bilateral
investment treaties, and the inability to reach an agreement on a
multilateral investment treaty.