Burden of proof

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 but it is not absolute.⁵ Investment tribunals have held that it does not apply to “obvious or notorious facts"⁶ and that it only applies to factual questions as opposed to legal questions.⁷

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,⁸ with rare exceptions provided.⁹ 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.¹⁰ 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,¹¹ which remains largely irrelevant in civil law systems.

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.¹² This principle does not usually impose a burden beyond a balance of probabilities¹³ but the failure to provide the required evidence (in a timely manner)¹⁴ is generally attributed to the party bearing the burden of proof¹⁵ and can result in the tribunal dismissing the claim(s).¹⁶

Although not always mentioned by tribunals, some authorities have found it conceptually important to distinguish between the legal and the evidential burden of proof.¹⁷ The legal burden never shifts.¹⁸ By contrast, the evidential burden may shift, “depending upon the state of evidence.”¹⁹

The burden of proof often lies with the claimant because it is the party asserting the claim.²⁰ 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.²¹ See further analysis in sub-sections B, C, D and E below.

The burden of proof during the jurisdictional phase must be distinguished from that required for the merits.²²

With regards to jurisdiction, the claimant must:

• raise facts related to the merits as to allow the tribunal to decide whether the claims fall within the boundaries of its jurisdiction²³ (see further Prima facie test); and
• prove any facts related to the tribunal’s jurisdiction.²⁴

The jurisdictional facts that the claimant bears the burden of proving include:

• that it respected or did not have to respect the cooling off period (if required by the investment agreement);²⁵
• the consent of the parties (See Consent to arbitration, Section I);
• that local remedies were exhausted or that it was unable to do so (if required by the investment agreement);²⁶
• its status as a protected investor²⁷ (for instance with regards to control of the investment²⁸ or its nationality);²⁹
• that an investment was made³⁰ ratione temporis;³¹
• that it complied with time bars.³²

Particularly under the ICSID procedure, the claimant must prove that the jurisdictional requirements of both Article 25 of the ICSID Convention and the applicable investment agreement are met.³³ See further Jurisdiction of the arbitral tribunal, Salini test and Double barrelled test.

Nevertheless, the evidential burden may be shifted to the respondent when it objects to the claimant’s allegations³⁴ or when the claimant is asked to prove a negative.³⁵ The respondent also bears the burden of proof when it raises an affirmative defense,³⁶ i.e. claims of abuse of process,³⁷ 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,³⁸ and its allegations must be specific.³⁹ The respondent that advances defences, such as necessity,⁴⁰ bears the burden of providing evidence in support of its arguments.⁴¹

This principle holds true even in cases where obtaining evidence is difficult.⁴² However, in some cases, including those related to conspiracy⁴³ 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.⁴⁴ See further Corruption, Sections IV and V.

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).⁴⁵ 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.⁴⁶ 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.⁴⁷

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.⁴⁸ Tribunals thus tend to require specific elements of proof for there to be a shift.⁴⁹ See further Standard of proof

According to the onus probandi principle, claimants must bear the burden of proving violations of the different standards of protection accorded to them, including:

1. Denial of justice,⁵⁰ where State regulatory acts enjoy a presumption of legality.⁵¹ However the respondent will have to prove the availability of local remedies;⁵²
2. Discriminatory treatment;⁵³
3. Expropriation;⁵⁴
4. Legitimate expectations, by proving that it carried out reasonable due diligence;⁵⁵
5. Minimum standard of treatment (See Minimum standard of treatment, Section III);
6. National treatment (See National treatment, Section VII.C); and
7. Bad faith taxation measures.⁵⁶

The onus probandi principle applies in the damages phase of the proceedings as well.

The party advancing claims to reparation bears the burden of proving its loss (see also Causation)⁵⁷ and the quantum of its claimed damages.⁵⁸ 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.⁵⁹ For example, the respondent State that claims that the investor did not mitigate its damages bears the burden of proof.⁶⁰ The burden can also be shared⁶¹ or shifted⁶² when a party’s inability to provide evidence proving its claims in damages is attributable to the other’s conduct.

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:

• Change of the place of arbitration;⁶³
• Provisional measures (See Provisional measures, Section V);
• Arbitrator challenges (See Disqualification of Arbitrator, Sections VII.A.1 and VII.B.1);
• Request to assume a State’s representation;⁶⁴
• Consolidation;⁶⁵
• Document production;⁶⁶
• Summary procedures;⁶⁷
• Estoppel;
• Counterclaims;⁶⁸
• Recognition and enforcement of non-ICSID awards;⁶⁹
• Stay of enforcement of ICSID awards (See Stay of enforcement of ICSID awards, Section VI.A);
• Stay of enforcement of non-ICSID awards;⁷⁰
• Sovereign immunity from execution;⁷¹
• Annulment of ICSID awards (See Grounds of annulment in ICSID awards, Section II.C); and
• Set aside of non-ICSID awards and ICSID Facility awards.⁷²

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.⁷³

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, 2^nd 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.

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.