LEGAL PRINCIPLE: CRIMINAL LAW – Defence of Insanity – Burden and Standard of Proof on the Accused
PRINCIPLE STATEMENT
The burden of establishing a defense on the ground of insanity rests squarely on the accused; it will be satisfied if the facts proved by the defense make it "most probable" that the accused was insane at the relevant time. The burden on the accused is not higher than that which rests on a plaintiff or defendant in civil proceedings.
RATIO DECIDENDI (SOURCE)
"I must quickly say that the burden of establishing a defence on the ground of insanity rests squarely on the accused. And it will be satisfied if the facts proved by the defence are such as to make it 'most probable' that the accused was at the relevant time, insane within the meaning of Section 28 above ... The burden on the accused is not higher than that which rests on a plaintiff or defendant in civil proceedings."
EXPLANATION / SCOPE
Insanity is an affirmative defense shifting the burden to the accused to prove. The standard is “most probable” or balance of probabilities—the civil standard, not the criminal “beyond reasonable doubt” standard. The accused must show it’s more likely than not they were insane when committing the act. This lower standard (compared to prosecution’s burden) recognizes the difficulty of proving mental states and that insanity is a defense, not an element of the offense. Evidence might include: psychiatric testimony, witnesses describing abnormal behavior, medical records, or history of mental illness. The prosecution need not disprove insanity unless the defense raises sufficient evidence making it a live issue. Once raised, the accused bears the persuasive burden.