The legal defence of mental illness provides that a person is not criminally responsible for an offence if, at the time of their conduct, the person had a mental health impairment or a cognitive impairment, or both, that had the effect that the person:
- Did not know the nature and quality of the act, or
- Did not know that the act was wrong (that is, the person could not reason with a moderate degree of sense and composure about whether the act, as perceived by reasonable people, was wrong).
The question of whether a person had a mental health impairment or a cognitive impairment, or both, that had that effect is a question of fact and is to be determined by the jury on the balance of probabilities.
Until the contrary is proved, it is presumed the person did not have a mental health impairment or cognitive impairment, or both, that had that effect.
What is a ‘mental health impairment’?
A person has a mental health impairment if:
- The person has a temporary or ongoing disturbance of thought, mood, volition, perception or memory,
- The disturbance would be regarded as significant for clinical diagnostic purposes, and
- The disturbance impairs the emotional wellbeing, judgment, or behaviour of the person.
What is a ‘cognitive impairment’?
A person has a cognitive impairment if:
- The person has an ongoing impairment in adaptive functioning,
- he person has an ongoing impairment in comprehension, reason, judgment, learning or memory, and
- The impairments result from damage to or dysfunction, developmental delay, or deterioration of the person’s brain or mind that may arise from a condition set out below or for other reasons.
The law makes clear that a cognitive impairment may arise from any of the following conditions but may also arise for other reasons:
- Intellectual disability,
- Borderline intellectual functioning,
- Dementia,
- Acquired brain injury,
- Drug or alcohol-related brain damage, including foetal alcohol spectrum disorder,
- Autism spectrum disorder.
What happens if the defence is successfully raised?
The defence of mental illness is not one to be raised lightly.
If successfully raised, a person will formally be found ‘not guilty by reason of mental illness.
However, the person will then come under the provisions of the Mental Health Act 2007 and required to undergo assessment, and may be detained for an indefinite period in a mental health facility, subject to regular reviews to assess progress and whether and when the person is safe for release into the community.