582. [I]f the defendant raises the issue of lack of mental capacity, he is challenging the presumption of sanity by a plea of insanity.  That is a defence in the true sense, in that the defendant bears the onus of establishing it – that, more probably than not, at the time of the offence he was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of his act or, if he did know it, that he did not know that what he was doing was wrong.[1]  Such a plea, if successful, is a complete defence to a charge and it leads to an acquittal.  It is submitted by Landžo that Rule 67(A)(ii) has also made diminished mental responsibility a complete defence to any charge (or has perhaps recognised it as such),[2] an argument which the Trial Chamber had accepted.[3]

The Appeals Chamber examined in paragraphs 583, 585-588 the ICTY Statute and Rules of Procedure and Evidence, the English Homicide Act of 1957, the partial defence of diminished responsibility that originated in Scotland in the 19th century, Article 31(1)(a) of the ICC Statute, and the relevant penal laws in many other countries.

590. […] The Appeals Chamber accepts that the relevant general principle of law upon which, in effect, both the common law and the civil law systems have acted is that the defendant’s diminished mental responsibility is relevant to the sentence to be imposed and is not a defence leading to an acquittal in the true sense.  This is the appropriate general legal principle representing the international law to be applied in the Tribunal.  Rule 67(A)(ii)(b) must therefore be interpreted as referring to diminished mental responsibility where it is to be raised by the defendant as a matter in mitigation of sentence.  As a defendant bears the onus of establishing matters in mitigation of sentence, where he relies upon diminished mental responsibility in mitigation, he must establish that condition on the balance of probabilities – that more probably than not such a condition existed at the relevant time.