Substantial Impairment
Substantial impairment is a partial defence that’s available for murder only. If successful, the outcome reduces a murder conviction to manslaughter.
The partial defence of “substantial impairment by abnormality of mind” will succeed if the accused establishes on the balance of probabilities the following two-limb test:
1. The mental health or cognitive impairment substantially (not totally) impaired the accused at the time of the offending.
This includes their capacity to:
(i) understand events, or
(ii) judge whether their actions were right or wrong, or
(iii) control themselves
Which was substantially impaired by an abnormality of mind arising from an underlying condition, and
2. This was so substantial as to warrant liability being reduced.
The difference between arguing this defence as opposed to mental health or cognitive impairment is that the outcome of this defence does not guarantee medical or psychiatric treatment. The accused will be released once their sentence has been served. However, with the mental health or cognitive impairment defence, the accused could face indefinite detention receiving treatment or be detained in a psychiatric facility.
If the defence of substantial impairment by abnormality of mind is raised successfully, you will be convicted of manslaughter rather than murder, which carries lesser penalties.