What is the Defence of Mental Impairment?
People will have a defence to what would otherwise be a criminal act if, at the time they committed the act, they were 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
- did not know that their conduct was wrong.
The defence of mental impairment is governed by the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997.
The question of mental impairment may be raised at any time during the trial by the defence or the prosecution.
A person is presumed not to be suffering from a mental impairment until the contrary is proved. The onus of rebutting the presumption of sanity rests on the party raising the question of mental impairment. The defence of mental impairment must be proved on the balance of probabilities. When mental impairment is in issue, 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 defence applies to people who were suffering from a mental impairment at the time they committed the criminal act.
The Act does not define the term “mental impairment”. It has been held to have the same meaning as “disease of the mind”, which formed the basis of the common law insanity defence. “Disease of the mind” has been held to be synonymous with “mental illness”. It connotes an unhealthy or “infirm” mind, as opposed to a healthy mind affected by a transient, non-recurrent mental malfunction caused by external forces.
To fit within the definition of a “mental impairment”, the accused must have been suffering from some kind of mental disease, disorder or disturbance, rather than “mere excitability, passion…stupidity, obtuseness, lack of self-control and impulsiveness”. A mental impairment exists where a person’s ability to understand is thrown into “derangement or disorder”.
A mental impairment may be permanent or temporary, curable or incurable. Conditions which have been held to be diseases of the mind (and will therefore also be types of “mental impairment”) include:
- Psychomotor epilepsy
- Cerebral arteriosclerosis
- Hyperglycaemia (caused by excessive blood sugar levels)
Conditions which have been held not to be diseases of the mind (and will therefore not be types of “mental impairment”) include:
- Some forms of epilepsy
- Hypoglycaemia (caused by excessive insulin intake)
- Drug-induced psychosis
- Some cases of sleepwalking
- Nature and Quality of the Act
“Nature and quality” has been held to refer to the physical character and significance of a person’s actions, and the consequences of those acts. It does not refer to the moral quality of his or her conduct.
To satisfy this limb of the defence, the accused must have been unable to appreciate the physical nature of what he or she was doing, and the consequences of his or her behaviour. In the case of murder, for example, the accused must have had so little capacity for understanding the nature of life and the destruction of life, that to him or her it was like breaking a twig or destroying an inanimate object.
Knowledge of wrongfulness
People will have a defence to what would otherwise be a criminal act if, at the time they committed the act, they were suffering from a mental impairment that had the effect that they did not know that their conduct was wrong. This means that the person “could not reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people, was wrong”.