Disorder which impairs the human mind and prevents a person from awareness that his act was wrong.
Sanity is presumed at common law.
In the United States, in 1984 and more than a century of following the antiquated M'Naghten Rules, the defence of insanity is now codified, Chapter 313 of Title 18 of the US Code, and which starts with, at §4241:
“At any time after the commencement of a prosecution for an offense and prior to the sentencing of the defendant, the defendant or the attorney for the Government may file a motion for a hearing to determine the mental competency of the defendant. The court shall grant the motion, or shall order such a hearing on its own motion, if there is reasonable cause to believe that the defendant may presently be suffering from a mental disease or defect rendering him mentally incompetent to the extent that he is unable to understand the nature and consequences of the proceedings against him or to assist properly in his defense.”
The concept of insanity and the convoluted cases which went with it (see, for example, R v Chaulk 1990 3 SCR 1303 published at canlii.org/en/ca/scc/doc/1990/1990canlii34/1990canlii34.html), has now been mercifully ousted from Canadian criminal law by ¶16 of the Criminal Code:
“No person is criminally responsible for an act committed or an omission made while suffering from a mental disorder that rendered the person incapable of appreciating the nature and quality of the act or omission or of knowing that it was wrong.
“Every person is presumed not to suffer from a mental disorder so as to be exempt from criminal responsibility (re above)..., until the contrary is proved on the balance of probabilities.
“The burden of proof that an accused was suffering from a mental disorder so as to be exempt from criminal responsibility is on the party that raises the issue.”
In Canadian tort law, the law on the impact of insanity as to liability, appears to be unsettled.
One decision, Buckley v Smith Transport Ltd., Ontario Court of Appeal, cited at 1946 OR 798, offered this broad defence:
“It is always a question of fact to be determined on the evidence, and the burden of proving that a person was without that appreciation and understanding and/or ability is always on those who allege it. Therefore, the question here, to my mind, is not limited to the bare inquiry whether or not Taylor at the time of the collision was labouring under this particular delusion, but whether or not he understood and appreciated the duty upon him to take care, and whether he was disabled, as a result of any delusion, from discharging that duty.”
Others, like Wenden v Trikha, 1991 8 CCLT 2d 138 (ABQB), do not go that far:
“I see no reason why a person whose mental state is such that he does not appreciate that he owes a duty of care to others while operating his motor vehicle, by reason of which he caused loss or damage to others, should not be subjected to the same criteria for establishing civil liability as anyone else, namely the objective standard of the reasonable driver. In my opinion, ...the law of negligence does not and should not take account of the defendant's mental disability in determining the applicable standard of care.”