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  • Cited by 3
  • Print publication year: 2011
  • Online publication date: December 2011

4 - Investment treaty interpretation and customary investment law: Preliminary remarks

from Part II - Shifts in fundamental character



The historical development of investment protection law has resulted in a somewhat peculiar interplay of different treaty and customary law strata. As has been suggested elsewhere:

the shift has been away from vague and crude substantive rules (set out in customary law, general principles and evocations of equity) discretionarily enforced by the home State, and in the direction of ‘treatified’ law of investment protection, implemented by means of investor–State arbitration . . . The results of the more recent efforts of [treaty] law-making sometimes accept and incorporate the classic [customary] rules; sometimes clarify the classic ambiguities or replace the unsatisfactory solutions; sometimes permit different approaches in parallel; and quite often maintain constructive ambiguity regarding the precise relationship between different rules.

Particularly during the last decade, States, investment treaty tribunals and legal writers have grappled with perhaps not entirely foreseen theoretical and practical implications of the relationship between investment rules set out in treaty and customary law. Different facets of the relationship raise different legal questions, ranging from the more traditional inquiries about lawmaking (and the contribution of treaties to custom) to perspectives of conflict (and the exclusion of custom by treaties) and interpretation (of treaties by reference to custom). This chapter engages in the latter exercise, relying on the practice and case law of the last decade to suggest an approach that an interpreter of an investment treaty should take regarding customary investment protection law.

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