Defence of Mental Impairment

Accused persons will have a defence to what would otherwise be 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 did not know that their conduct was wrong. This means that the accused could not reasonably conceive that their conduct, as perceived by reasonable people, was wrong.

The defence of mental impairment is governed by section 20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997. The defence applies to people who were suffering from a mental impairment at the time they committed the criminal act. 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 party raising the question of mental impairment must rebut the presumption of sanity. The defence of mental impairment must be proved on the balance of probabilities. Where 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 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.

A mental impairment may be permanent or temporary, curable or incurable. Conditions which have been held to be diseases of the mind and are included as types of mental impairment are:

  • Schizophrenia
  • Psychomotor epilepsy
  • Cerebral arteriosclerosis
  • Hyperglycaemia

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
  • Hypoglycaemia
  • Drug-induced psychosis
  • Some instances of sleep walking
  • Nature and quality of the act

Nature and quality refers to the physical character and significance of a person’s actions and the consequences of those actions. It does not refer to the moral quality of their conduct. The accused must have been unable to appreciate the physical nature of what they were doing, and the consequences of their behaviour.

For more information, read our blog article on the mental impairment defence.