The notion of burden of proof determines which party is responsible to prove an assertion made in judicial proceedings.1 Standard of proof, on the other hand, defines how much evidence is needed to establish either an individual issue or the party’s case as a whole.2
In the sense of Article 38(1)(c) of the Statute of the International Court of Justice, the general principle regarding the burden of proof stems from the maxim onus probandi actori incumbit, namely “he who asserts must prove.”3 The said maxim is considered to be a general principle of international law itself.4 Investment tribunals have held that it does not apply to “obvious or notorious facts”5 and that it only applies to factual questions as opposed to legal questions.6
II. International practice
The aforementioned principle is widely recognized in international adjudication. Yet, most of the procedural rules employed in international arbitral practice do not contain any provision in relation to the burden of proof,7 with rare exceptions provided.8 In some jurisdictions, the burden of proof is considered as part of the substantive law, while in others it is considered as part of their procedural laws but investment tribunals have held that they are not bound by national rules of evidence.9 In a similar vein, there is no uniformity on the application of burden of proof in common law and civil law systems. For example, in common law systems, the rule on allocation includes the burden of production, as well as the burden of persuasion,10 which remains largely irrelevant in civil law systems.
III. Allocation of burden of proof
As a matter of principle, the burden of affirmatively establishing the elements of an assertion rests upon the party alleging them, whether it is the claimant or the respondent.11 This principle does not usually impose a burden beyond a balance of probabilities12 but the failure to provide the required evidence (in a timely manner)13 is generally attributed to the party bearing the burden of proof14 and can result in the tribunal dismissing the claim(s).15
The burden of proof often lies with the claimant because it is the party asserting the claim.19 However according to the principle of onus probandi actori incumbit, it may also lie with the respondent, if it is asserting affirmative defences or claims of its own. Once the party bearing this burden of proof has provided evidence in support of its claim, the burden then shifts to the other, allowing this party to rebut the evidence with its own.20 See further analysis in sub-sections B, C, D and E below.
1. Claimant's burden
With regards to jurisdiction, the claimant must:
The jurisdictional facts that the claimant bears the burden of proving include:
2. Respondent's burden
Nevertheless, the evidential burden may be shifted to the respondent when it objects to the claimant’s allegations33 or when the claimant is asked to prove a negative.34 The respondent also bears the burden of proof when it raises an affirmative defense,35 i.e. claims of abuse of process,36 illegality or fraud. See further Legality of investment, Section VII.
The approach to merits is the same as jurisdiction concerning the onus probandi principle. The claimant that alleges a violation of its substantive rights bears the burden of proving the breach,37 and its allegations must be specific.38 The respondent that advances defences, such as necessity,39 bears the burden of providing evidence in support of its arguments.40
This principle holds true even in cases where obtaining evidence is difficult.41 However, in some cases, including those related to conspiracy42 or corruption, tribunals have considered that circumstantial evidence may suffice in discharging the burden of proof as long as it is clear and non-speculative.43 See further Corruption, Sections IV and V.
2. Shifting the burden of proof
Applying the distinction between the legal and evidential burden, in a case where a party proves prima facie the asserted facts, the burden of proof may shift to the other party (speaking from an evidential perspective).44 Additionally, it may also shift when a party cannot fully prove something but can demonstrate that the other party has access or control over the missing evidence, in the name of fairness and good faith.45 See also Adverse inference. Similarly, it has been suggested that the burden may shift if one party has better access to evidence than the other.46
The shifting of the burden of proof remains a delicate issue especially since the responding party is a State. To this extent, tribunals are cautious when shifting the burden of proof, as to not "bend over backwards" in analysing inferences of fact or circumstantial evidence.47 Tribunals thus tend to require specific elements of proof for there to be a shift.48 See further Standard of proof
3. Application to different standards of protection
According to the onus probandi principle, claimants must bear the burden of proving violations of the different standards of protection accorded to them, including:
The party advancing claims to reparation bears the burden of proving its loss (see also Causation)56 and the quantum of its claimed damages.57 Tribunals have considered that this burden should not be impossible to discharge and thus that scientific certainty is not required. See further Standard of proof, Section III.C.
If the other party challenges these claims or advances affirmative defences, it bears the burden of proving the contrary.58 For example, the respondent State that claims that the investor did not mitigate its damages bears the burden of proof.59 The burden can also be shared60 or shifted61 when a party’s inability to provide evidence proving its claims in damages is attributable to the other’s conduct.
E. Other requests from the parties
Tribunals have analysed the principle of onus probandi actori incumbit with regards to a variety of other requests during and after the arbitral procedure, including:
IV. Consequences of misallocating the burden of proof
Some ICSID tribunals have considered that the misallocation of the burden of proof may justify the annulment of an award in certain circumstances while others have refused. See Serious departure from a fundamental rule of procedure, Section II.A.4. Domestic set-aside tribunals constituted under non-ICSID proceedings have also upheld a stringent standard.72
Böckstiegel, K., Presenting Evidence in International Arbitration, ICSID Review-Foreign Investment Law Journal, Vol. 16, Issue 1, 2001, pp. 1–9.
Born, G., Chapter 4: On Burden and Standard of Proof, in Kinnear, M., Fischer, G.R. et al. (eds), Building International Investment Law: The First 50 Years of ICSID, Kluwer Law International, 2015, pp. 43-54.
Brown, Ch., A Common Law of International Adjudication, Oxford University Press, 2007, pp. 93-98.
Schreuer, Ch. et al., The ICSID Convention: A Commentary, 2nd ed., Cambridge University Press, 2009, pp. 669-670.
Sourgens, F., Evidence in Investor-State Arbitration – The Need for Action, Kluwer Arbitration Blog, 2017.
Sourgens, F.G., Duggal, K. and Laird, I.A. (eds.), Evidence in International Investment Arbitration, Oxford University Press, 2018.
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