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Mr Andreas Desyllas

LL.M. Student - National and Kapodistrian University of Athens, Faculty of Law

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Burden of proof

I. Definition


The notion of burden of proof determines which party is responsible to prove an assertion made in judicial proceedings.1 On the other hand, the standard of proof 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 The onus probandi principle is a risk-defining rule, since the party who finally must prove a fact takes the risk of its non-establishment.5 

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, with rare exceptions provided.6 In some jurisdictions, the burden of proof is considered as part of the substantive law, while in others is considered as part of their procedural laws. In a similar vein, there is no uniformity on the application of burden of proof in common law and civil law systems.

III. Allocation of burden of proof


In general, the burden of affirmatively establishing the elements of an assertion rests upon the party alleging them, whether it is the claimant or respondent.7 Due to this, the burden of proof usually lies with the claimant according to the said principle because it is the party asserting the claim.8 In any event, a variety of rules regarding allocation have been observed in international adjudication. For example, in common law systems, the rule on allocation includes the burden of production, as well as the burden of persuasion,9 which remains largely irrelevant in civil law systems. 


Following the common law model, it is important to distinguish between the legal and the evidential burden of proof. The legal burden never shifts; by contrast, the evidential burden may shift, depending upon the state of evidence.10 The allocation of the burden, however, remains more problematic in terms of jurisdiction rather than the merits.11

A. Jurisdiction


For the purposes of jurisdiction, tribunals tend to assess whether a claimant’s case is reasonably arguable on its face.12 Although the respondent is the party challenging jurisdiction, it is for the claimant to prove the opposite, so the legal burden of proof continues to be borne by the latter. Nevertheless, the evidential burden may be shifted to the respondent in case where the claimant is asked to prove a negative13 or when the respondent itself raises defenses,14 whereas the legal burden still lies with the respondent when it comes to proving an affirmative defense.15

B. Merits


The approach to merits is the same as jurisdiction concerning the onus probandi principle. 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).16 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.17


In any event, the shifting of the burden of proof remains a delicate issue especially since the responding party is a State; for instance, in cases of “inferences of fact” or “circumstantial evidence”, a tribunal would rarely shift the burden of proof.18 Reasons as such have given rise to an opposite view having been supported by some tribunals, meaning that the allocation of burden of proof is an absolute.19 

C. ICSID procedure


Particularly under ICSID procedure, the claimant has to overcome two different thresholds: first, that the jurisdictional requirements of both the ICSID Convention and the treaty at hand are met, and second, that the claimant has a prima facie cause of action under the said treaty.20


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.

Brower, Ch., Evidence Before International Tribunals: The Need for Some Standard Rules, The International Lawyer, Vol. 28, No. 1, 1994, pp. 47-58

Brown, Ch., A Common Law of International Adjudication, Oxford University Press, 2007, pp. 93-98.

Kazazi, M., Burden of Proof and Related Issues: A Study on Evidence Before International Tribunals, Brill, 1996, pp. 116-117, 370-371.

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.

Tsatsos, A., Burden of Proof in Investment Treaty Arbitration: Shifting?, Transnational Dispute Management, Vol. 17, Issue 2, 2020. Previously published in Humboldt Forum Recht, 2009, pp. 91-104.

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