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Criminal defenses
Part of the common law series
Defenses to crime
Actual innocence
Excuse and exculpation
Defenses that deny the act:
Alibi  · Mistaken identity
Frameup  · Falsified evidence
False confession  · Automatism
Defenses that negate intent:
Infancy  · Entrapment
Insanity  · Mental disorder
M'Naghten Rules
Diminished responsibility
Mistake of law  · Mistake of fact
Intoxication
Defenses that justify the act:
Self defense  · Consent
Duress  · Necessity
Provocation
See also Criminal Law
Criminal Procedure
Other areas of the common law
Contract law  · Tort law  · Property law
Wills and trusts  · Evidence
Portals: Law  · Criminal justice

In the criminal laws of Australia and Canada, the defence of mental disorder (sometimes called the defence of mental illness) is a legal defence by excuse, by which a defendant may argue that he or she should not be held criminally liable for breaking the law, as he or she was mentally ill at the time of their allegedly "criminal" actions.

These are a statutory version of the M'Naghten Rules which define insanity in most common law countries, but they were renamed due to the stigma that the label of "insanity" inflicts on the accused.

In Australia there are nine law units. All may have varying rules (see [1]). In South Australia, the Criminal Law Consolidation Act 1935 (SA) provides that:

269C—Mental competence

A person is mentally incompetent to commit an offence if, at the time of the conduct alleged to give rise to the offence, the person is suffering from a mental impairment and, in consequence of the mental impairment—

(a) does not know the nature and quality of the conduct; or

(b) does not know that the conduct is wrong; or

(c) is unable to control the conduct.

269H—Mental unfitness to stand trial

A person is mentally unfit to stand trial on a charge of an offence if the person's mental processes are so disordered or impaired that the person is—

(a) unable to understand, or to respond rationally to, the charge or the allegations on which the charge is based; or

(b) unable to exercise (or to give rational instructions about the exercise of) procedural rights (such as, for example, the right to challenge jurors); or

(c) unable to understand the nature of the proceedings, or to follow the evidence or the course of the proceedings.

In Victoria the current defence of mental impairment was introduced in the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 which replaced the common law defence of insanity and indefinite detention at the governor's pleasure with the following:

the accused was suffering from a mental impairment; and

the mental impairment affected the accused so he or she either did not understand the nature and quality of the conduct, or did not know that it was wrong.

These requirements are almost identical to the M'Naghten Rules, substituting "mental impairment" for "disease of the mind".

The defence of mental disorder is codified in section 16 of the Canadian Criminal Code which states, in part:

16. (1) 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.

To establish a claim of mental disorder the party raising the issue must show on a balance of probabilities first that the person who committed the act was suffering from a "disease of the mind", and second, that at the time of the offence they were either 1) unable to appreciate the "nature and quality" of the act, or 2) did not know it was "wrong".

The meaning of the word "wrong" was determined in the Supreme Court case of R. v. Chaulk [1990] 3 S.C.R. which held that "wrong" was NOT restricted to "legally wrong" but to "morally wrong" as well.

This defence was very little used until 1992, as before that date a finding of insanity meant that the accused could be confined indefinitely to a mental institution. The punishment for being found insane was thus often considerably greater than being found guilty. Allegations of insanity were thus often brought by the crown, rather than the defence. In 1992 this system was changed as courts ruled that permanent institutionalization was wrong. A system of regular review and assessment was introduced and time confined to an institution was capped at the equivalent of the sentence if they had been found guilty. The crown was also forbidden from making accusations of mental disorder. Since these changes the number of mental disorder pleas has greatly increased.

• Victorian Law Reform Commission. Defences to Homicide: Final Report, [2]
• Justice department review of section 16