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.
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 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—
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:
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 [[Case citation#Canada| 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 indefinite institutionalization without a system of regular review was wrong. A system of regular review and assessment was introduced in 1992. The law now provides that all forensic psychiatric patients will have their case reviewed by a Review Board at least every 12 months (or in some cases up to 24 months). So long as the accused remains a significant threat to public safety (section 672.54 of the Criminal Code), he or she will remain either in custody or under forensic supervision in the community.
In the original 1992 mental disorder amendments, Parliament added a provision to cap the time confined to an institution or under forensic supervision at the equivalent of the maximum sentence allowable if the accused had been found guilty. These capping provisions, however, have never been proclaimed into law. The provinces have resisted capping, likely because the provincial civil mental health structures are not equipped to provide the same level of monitoring and supervision that are available in the forensic psychiatric system.
Either the Crown or the defence may raise the issue of "not criminally responsible by reason of mental disorder", but unless the defence puts mental capacity in issue, the Crown can only raise the issue of insanity after the trier of fact (judge or jury) has made a finding of guilt. Since these changes the number of mental disorder pleas has greatly increased.