Between Fact and Opinion: The Sui Generis Approach to Expert Witness Testimony in International Criminal Trials

It is a fundamental tenet of the law of evidence, spanning all jurisdictions, that witness testimony should ideally be delivered in open court by the individual who observed the event in question, or by the expert whose technical knowledge is relied upon. A notable exception to this principle has emerged in the field of international criminal justice, where courts and tribunals may allow ‘summarising witnesses’ to present a summation of witness testimony. In the case of Ayyash et al., the Special Tribunal for Lebanon extended the principle, allowing voluminous expert opinion evidence to be presented in factual summation. This article analyses such approaches, utilising doctrinal methods alongside empirical Wigmorean analysis, to assess the probity of these sui generis practices. The results are placed in legal and theoretical perspective, demonstrating that international courts and tribunals are departing from an overarching obligation to integrate international and domestic standards in respect of expert testimony.