This presentation investigates doctrinal and legal justifications for the exclusion of a diminished accountability defense for core crimes under the ICC jurisdiction. The ICC Statute states that individuals will not be criminally responsible for their actions when suffering from a mental disease or defect which completely destroys the capacity to appreciate the lawfulness of the conduct, or capacity to control this conduct so as to conform to the requirements of the law. This study concludes that the defense is indistinct from the common law defense of insanity. As a result this omission excludes an entire category of sentencing mitigating factors and it is therefore inconsistent with most common and civil law traditions and represents a clear departure from the ICTY/ICTR jurisprudence. Importantly, it is significantly detrimental to the rights of the accused and it is also prejudicial to the presumption of sanity. Moreover, considering the in dubio pro reo principle, whereby the most favorable construction of the law must be made in the interest of the accused, Court’s unqualified discretion in assessing and admitting health sector evidence is also questioned vis-à-vis national norms under complementarity, concluding that the parameters of international criminal law, although sui generis, are insufficiently precise and require a realignment with contemporary criminal and human right norms.