In the Trial Judgement, the Trial Chamber expressed reservation about the doctrine of joint criminal enterprise (“JCE”)[1] and applied, instead of JCE, a mode of liability it defined as “co-perpetratorship”.[2] The Appeals Chamber held that this mode of liability was “new to the jurisprudence of the Tribunal”,[3] that “[t]he introduction of new modes of liability into the jurisprudence of the Tribunal may generate uncertainty, if not confusion, in the determination of the law by parties to cases before the Tribunal as well as in the application of the law by Trial Chambers”,[4] and therefore decided proprio motu to assess whether this mode of liability was consistent with the case-law of the Tribunal[5] and whether, as defined by the Trial Chamber, it had support in customary international law. It held the following:

62. Upon a careful and thorough review of the relevant sections of the Trial Judgement, the Appeals Chamber finds that the Trial Chamber erred in conducting its analysis of the responsibility of the Appellant within the framework of “co-perpetratorship”. This mode of liability, as defined and applied by the Trial Chamber, does not have support in customary international law or in the settled jurisprudence of this Tribunal, which is binding on the Trial Chambers. By way of contrast, joint criminal enterprise is a mode of liability which is “firmly established in customary international law”[6] and is routinely applied in the Tribunal’s jurisprudence.[7] Furthermore, joint criminal enterprise is the mode of liability under which the Appellant was charged in the Indictment, and to which he responded at trial.[8] In view of these reasons, it appears that the Trial Chamber erred in employing a mode of liability which is not valid law within the jurisdiction of this Tribunal. This invalidates the decision of the Trial Chamber as to the mode of liability it employed in the Trial Judgement.