Substantial impairment of the mind is a partial defence to murder, resulting in murder being downgraded to manslaughter. A murder charge can be reduced to manslaughter in New South Wales if the court is satisfied, more likely than not, that the defendant's capacity to understand events, judge whether the act was wright or wrong, or control him or herself was “substantially impaired” by a “mental health or cognitive impairment”, where the impairment was so substantial that it warrants lesser criminally responsible than murder, under section 23A Crimes Act. Murder carries a penalty of up to life imprisonment. Manslaughter carries up to 25 years in jail.
After the prosecution successfully proves the offence of murder beyond reasonable doubt, the murder charge will be reduced to manslaughter if the accused person can prove on the balance of probabilities (more likely than not) that:
(1) at the time of committing the act causing death, the accused person's capacity to understand events, judge whether the acts were wrong, or control him/herself was substantially impaired by an abnormality of mind (mental health impairment or cognitive impairment) from an underlying condition, and
(2) that impairment was so substantial that it warrants murder to be reduced to manslaughter. This is a value judgement applying community standards decided by the jury.
This is known as the partial defence to murder under section 23A Crimes Act 1900 (NSW).
The defence can only be raised where all other defences/partial defences to a murder charge, including self-defence or extreme provocation, have first been raised and resolved unfavourably to the accused person.
Furthermore, the defence of substantial impairment to a charge of murder can only be used through giving evidence to prove it if the defence gives notice to the prosecution of an intention to do so and files a copy of that notice with the court. If no notice is served, then the accused person will not be allowed to adduce evidence to support this defence unless the court grants leave as outlined in section 151 Criminal Procedure Act 1986 (NSW).
MEANING OF “SUBSTANTIAL IMPAIRMENT”
When considering “substantial impairment”, “impaired” is to have its ordinary meaning, namely, proof of a capacity less or lower than the normal range. “Substantial” also has its ordinary meaning of being of substance and not slight or insignificant.
The mental impairment or cognitive impairment must also come from a pre-existing condition, which, on the one hand, is not of a passing or brief kind, but on the other is not necessarily permanent.
WARRANTING REDUCTION TO MANSLAUGHTER
As to the second element noted earlier, the main question the jury is to ask is “has the accused satisfied you in the circumstances of this case that any impairment to his or her capacity (if you find that it's likely to have existed) was such that he or she should not be condemned or blamed as a murderer, and that rather, he or she should be treated as having been guilty of manslaughter?”. This question is to be approached in a broad and common sense way in the sense that manslaughter is regarded as a less serious crime than murder, and that the community places less blame and condemnation upon a person guilty of manslaughter than of murder.
WHAT IS THE REASONING FOR THIS TYPE OF PARTIAL DEFENCE?
One of the main factors to consider what the appropriate punishment should be for an offender is the extent of blame attaching to the crime.
The crime of murder is more serious than the crime of manslaughter. Which is why, Manslaughter is punished less severely than murder. This is largely due to the criminality or culpability of the offender. Therefore, a person who commits an act causing death caused by a mental health or cognitive impairment impairing his or her capacity to understand the events, judge whether or not the acts were right or wrong, or to control his or her action, is less responsible, according to the standards prevailing in our community, than a person who has full capacity in those respects.
CAN INTOXICATION BE USED AS EVIDENCE?
According to section 23A(3) Crimes Act 1900 (NSW), if a person was intoxicated at the time of the act(s) causing death, and the intoxication was self-induced intoxication, the effects of that intoxication are not to be taken into consideration when the court determined whether or not the person is liable for murder under the same section.
Here is more on the law and meaning of “cognitive impairment” or “mental health impairment”.