Both the International Criminal Tribunal for the former Yugoslavia (ICTY) and the International Criminal Court (ICC) have struggled to combine vertical and horizontal modes of liability. At the ICTY, the question has primarily arisen within the context of ‘leadership-level’ joint criminal enterprises (JCEs) and how to express their relationship with the relevant physical perpetrators (RPPs) of the crimes. The ICC addressed the issue by combining indirect perpetration with co-perpetration to form a new mode of liability known as indirect co-perpetration. The following article argues that these novel combinations – vertical and horizontal modes of liability – cannot be simply asserted; they must be defended at the level of criminal-law theory. Unfortunately, courts that have applied indirect co-perpetration have generally failed to offer this defence and have simply assumed that modes of liability can be combined at will. In an attempt to offer the needed justification, this article starts with the premise that modes of liability are ‘linking principles’ that link defendants with particular actions, and that combining these underlying linking principles requires a second-order linking principle. The most plausible candidate is the personality principle – a basic principle that recognizes the inherently collective nature of leadership-level groups dedicated to committing international crimes. Like Roxin's theories describing the collective organizations that can be used as a form of indirect perpetration, the personality principle treats the horizontal leadership group as an organization or group agent whose collective nature potentially justifies the attribution of vertical modes of liability to all members of the horizontal group. Although this article does not defend the doctrine of indirect co-perpetration, it does conclude that combined vertical and horizontal modes of liability, whether at the ICTY or ICC, implicitly or covertly rely on something like the personality principle in order to justify collective attribution to the horizontal collective.
