Section 28: Legal Defence of Mental Health Impairment or Cognitive Impairment

Prior to 27 March 2021, the ‘M’Naghten Defence’ of mental illness was legislated through section 38 of the Mental Health (Forensic Provisions) Act 1990 (NSW), which read:

1. If in an indictment or information, an act or omission is charged against a person as an offence and it is given in evidence on the trial of the person for the offence that the person was mentally ill, so as not to be responsible, according to law, for his or her action at the time when the act was done or omission made, then, if it appears to the jury before which the person is tried that the person did the act or made the omission charged, but was mentally ill at the time when the person did or made the same, the jury must return a special verdict that the accused person is not guilty by reason of mental illness (emphasis added).

Interestingly, the old law did not define ‘mental illness’. Lawyers who sought this defence, therefore, had to go back and rely on the very old and outdated definition established in M’Naghten of ‘defect of reason, from disease of the mind’- a definition which modern-day psychiatrists found difficult to support.

Lawyers who sought the defence under the old law also often faced difficulty when Magistrates and Judges refused to accept their client’s diagnosis of anxiety and clinical depression as ‘mental illnesses’, instead remarked that such diagnosis were ‘struggles of everyday life’ and ‘commonplace in society’.

The new legislation makes it all very clear.

Since 27 March 2021, the defence of mental illness is now contained in section 28 of the Mental Health and Cognitive Impairment Forensic Provisions Act 2020 (NSW) (‘the Act’).

What does the law say?

Section 28 states as follows:

(1) A person is not criminally responsible for an offence if, at the time of carrying out the act constituting the offence, the person had a mental health impairment or a cognitive impairment, or both, that had the effect that the person –

(a) did not know the nature and quality of the act, or

(b) 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).

(2) The question of whether a defendant 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.

(3) Until the contrary is proved, it is presumed that a defendant did not have a mental health impairment or cognitive impairment, or both, that had that effect.

(4) In this Part,

“act” includes–

(a) an omission, and

(b) a series of acts or omissions.

Section 4 of the Act defines mental health impairment as follows:

(1) For the purposes of this Act, a “person has a mental health impairment” if–

(a) the person has a temporary or ongoing disturbance of thought, mood, volition, perception or memory, and

(b) the disturbance would be regarded as significant for clinical diagnostic purposes, and

(c) the disturbance impairs the emotional wellbeing, judgment, or behaviour of the person.

(2) A mental health impairment may arise from any of the following disorders but may also arise for other reasons–

(a) an anxiety disorder,

(b) an affective disorder, including clinical depression and bipolar disorder,

(c) a psychotic disorder,

(d) a substance-induced mental disorder that is not temporary.

(3) A person does not have a mental health impairment for the purposes of this Act if the person’s impairment is caused solely by–

(a) the temporary effect of ingesting a substance, or

(b) a substance use disorder.

Section 5 of the Act defines cognitive impairment as follows:

(1) For the purposes of this Act, a “person has a cognitive impairment” if–

(a) the person has an ongoing impairment in adaptive functioning, and

(b) the person has an ongoing impairment in comprehension, reason, judgment, learning or memory, and

(c) 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 in subsection (2) or for other reasons.

(2) A cognitive impairment may arise from any of the following conditions but may also arise for other reasons–

(a) intellectual disability,

(b) borderline intellectual functioning,

(c) dementia,

(d) an acquired brain injury,

(e) drug or alcohol-related brain damage, including foetal alcohol spectrum disorder,

(f) autism spectrum disorder.

The new law takes out most of the guesswork by specifying that disorders including anxiety and clinical depression, do meet the definition of ‘mental health impairment’ and therefore do satisfy the preliminary limb of the defence of mental illness.

The new law also specifies that temporary effects of ingesting a substance or substance use disorders do not meet the definition of ‘mental health impairment’.

If the preliminary limb is established, a lawyer will of course still need to satisfy the judge or jury that their client’s mental health impairment and/or cognitive impairment caused them to either:

1. Not know the nature and quality of their act, or
2. Not know that the act was wrong.

If the judge or jury is satisfied with the above, on the balance of probabilities, then the defence of mental illness will be made out and the defendant be found not guilty of the offence.

It is important to be aware that an acquittal by reason of mental illness does not mean a person will be released into the community.

Rather, the person will be dealt with under the provisions of the Mental Health Act 2007 (NSW), which may involve involuntary detention in a mental health facility.

The Mental Health Act details the procedures for assessment and review of persons who have been found not guilty by reason of mental health.