Defenses and the Burden of Proof in Public International Law Litigation

Modern treaties include an increasingly complex set of rules, carve-outs and exceptions, ensuring, for example, the policy space of states. Their impact may be tempered also by outside instruments or custom. International law disputes, in turn, are increasingly fact-intensive, addressing, for example, complex scientific, environmental or economic questions. This double evolution enhances the role, in international litigation, of defenses and the importance of who carries the burden of proof for them.

Some international tribunals may spend more time than necessary on questions of burden of proof and different types of claims in defense (dispute settlement at the World Trade Organization (WTO) comes to mind); most, however, tend to under-estimate the issue and do not devote it enough attention (for example, the International Court of Justice). 

The burden of proving a defense is said to be on the party invoking it. As trite as this proposition may sound, in international law, it hides a far more complex litigation reality. Distinctions must be made both in terms of (i) types of claims in defense, and (ii) types of burdens this may impose (or not) on the respondent. 

This paper distinguishes six different “claims in defense”: (i) objections to jurisdiction, (ii) objections to admissibility, (iii) exemptions/alternative rules, (iv) absence of breach, (v) exceptions, and (vi) defenses under secondary rules.  

For each of these six “claims in defense”, five types of “burdens” on the parties or tribunal are identified: (i) burden of raising a claim in defense, (ii) burden of production of evidence, (iii) burden of persuasion (or real burden of proof), (iv) standard or quantum of proof and (v) standard of review.  

Although for some claims in defense some types of burden are on the defendant, this is certainly not the case for all. The Table below links the types of claims in defense to the respective burdens they conventionally impose on the parties or tribunal (exceptions to the general rule are not included in the Table but discussed in the text of the paper).

Defendants generally have the burden to raise objections to admissibility but not objections to jurisdiction (which the tribunal must examine at its own initiative). In addition, even if the burden of persuasion for admissibility objections generally rests on defendants, the burden of persuasion in respect of jurisdictional requirements generally rests on claimants.

The burden of persuasion for exceptions and defenses under secondary rules does fall on respondents (except before the international criminal court). However, it is for claimants to prove that a rule applies and not an exemption (reservation, carve-out or alternative rule) or that the constituent elements for breach are met. The difference between exceptions and exemptions is well established in WTO jurisprudence. It needs more attention before other international tribunals.

Whereas the burden of production of evidence may shift within the examination of a single claim in defense (e.g. once a prima facie case is established), the burden of persuasion (or real burden of proof) does not shift. Where a party has the burden of persuasion under a claim in defense, it has the burden of proving the facts (not the law) required for that claim in defense to succeed. Whereas a prima facie case may shift the burden of production (and in some cases, when not refuted, even discharge the burden of persuasion), it should not be generally equated with the quantum or standard of proof required to discharge the burden of persuasion. The fallback standard of proof before international tribunals is “preponderance of the evidence” or “balance of probabilities”. Yet, special cases may justify a lower or higher standard of proof (such as proof beyond reasonable doubt in international criminal proceedings). Increasing deference is provided to respondents by way of less intrusive standards of review (focused on reasonableness; margin of appreciation) and a rejection of the alleged rule that exceptions must, by definition, be interpreted restrictively.

Given the increasing fact-intensive nature of disputes and thus enhanced importance of burden of proof issues (including for claims in defense), tribunals should clarify upfront (i) who has the burden of persuasion and (ii) what precise standard of proof applies (matters on which tribunals have been surprisingly silent), rather than describe (as too often happens in WTO, and more recently also in investor-state, dispute settlement) formalistic Ping-Pong games of prima facie cases and shifting burdens of production, disclosed to the parties only ex post.

The full text of this paper, forthcoming in a book to be published by Oxford University Press (Exceptions in International Law (L. Bartels & F. Paddeu, forthcoming 2017) can be found here.