Subsequent Practice and Evolutive Interpretation: Techniques of Treaty Interpretation over Time and Their Diverse Consequences

Abstract

This paper compares two different means of treaty interpretation by which a treaty or treaty provision may change over time: the interpretation and reinterpretation of a treaty on the basis of its evolutive character, and the (re)interpretation of a treaty on the basis of the subsequent practice of the parties. I contend that evolutive interpretation and interpretation based on subsequent practice do not simply refer to two different and distinct phenomena – as a practical matter they constitute two different “techniques of interpretation” which may or may not both be applicable in a particular case, and may sometimes both be applicable but be mutually exclusive. The basic problem of the paper revolves around the following question: where the evidence is uncertain, or ambivalent, which technique – if any – should be applied? My goal is to show that although both techniques may be applicable to a treaty in a given case, the application of one or the other doctrine will have different consequences in the short and long term. In so doing, I will first expound the immediate effects of the techniques by examining them individually with an eye to their evidentiary criteria and their relative expansive potentialities. I shall then illustrate and compare their respective long-term consequences, which I categorize as “vertical” (with respect to successive interpretations of the particular treaty in question) and “horizontal” (referring to effects on the interpretation of other treaties). Ultimately, I want to argue that neither means of interpretation is more appropriate across the board, and therefore the choice between the two techniques should be informed by the consideration of their consequences in light of the object and purpose of the particular treaty to be interpreted.

Keywords: Treaty Interpretation, Evolutive Interpretation, Dynamic Interpretation, Subsequent Practice, Vienna Convention, Costa Rica v. Nicaragua