It is a defence to a criminal act if, at the time which the act was committed, the person was suffering from a mental impairment that had the effect that they either did not know the nature and quality of what they were doing or they did not know that their conduct was wrong. Essentially that the accused could not reasonably conceive that their conduct, as perceived by reasonable people, was wrong.
Mental impairment defences are controlled by section 20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997. It is for people who were suffering from a mental impairment at the time they committed the criminal act. The issue of mental impairment may be brought up at any stage during a trial by defence or prosecution.
The basic principle is that an accused is considered not to be suffering from a mental impairment until the opposite is proven. Whichever side raises the question of mental impairment must, in effect, rebut the presumption of sanity. It is a defence which must be proved on the balance of probabilities. If a jury finds the accused not guilty they must specify in their verdict whether they have done so on the basis of mental impairment.
The Act does not define the term ‘mental impairment’, however it implies an unhealthy mind as opposed to a healthy mind affected by a non-recurring mental malfunction caused by external forces. To fit within the definition of a mental impairment, the accused must have been suffering from some form of mental disease, disorder or disturbance, rather than mere excitability, passion, stupidity, obtuseness, lack of self control or impulsiveness. A mental impairment exists where a person’s ability to understand is thrown into derangement or disorder.
Mental impairment may be transitory or permanent, capable of being treated or not. Conditions which have been held to be diseases of the mind and are included as types of mental impairment are:
- Psychomotor epilepsy
- Cerebral arteriosclerosis
Conditions which have been held not to be diseases of the mind and therefore not included as types of mental impairment are:
- Some forms of epilepsy
- Drug-induced psychosis
- Some instances of sleep walking
- Nature and quality of the act
The nature and quality of the act refers to the physical character, consequences and significance of a person’s. This is not an issue about the morality of the accused’s conduct. To establish the defence the accused must have been unable to appreciate the physical nature of what they were doing, and the consequences of their behaviour.