Defining Mental Impairment

The Victorian Law Reform Commission (VLRC) is reviewing the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (CMIA). The Attorney-General has asked the VLRC to report on whether changes to the legislation are required to ensure it operates justly, effectively and consistently with the principles that underlie it.  As part of this review, the VLRC has been asked to consider whether the CMIA should define ‘mental impairment’ and if so, how.

The defence of mental impairment
It has long been recognised that a person should not be held criminally responsible if at the time of committing an offence, the person lacked the necessary intent to commit the offence due to a ‘mental impairment’.

The defence of mental impairment is outlined in s 20 of the CMIA, which provides that the defence is established if at the time of engaging in conduct constituting the offence, the person was suffering from a mental impairment that had the effect that:
• the person did not know the nature and quality of the conduct, or
• the person did not know that the conduct was wrong.
The CMIA further outlines in s 20(1)(b) that a person will not know their conduct was wrong where they were ‘unable to reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people, was wrong’.

In the Supreme and County Courts, a person found not guilty by reason of mental impairment may be placed on a supervision order, or may be unconditionally released. Where a person is found not guilty by reason of mental impairment in the Magistrates’ Court, however, the person must be unconditionally discharged.  As part of this review, the VLRC has also been asked to consider whether the Magistrates’ Court should be permitted to make supervision and other orders in relation to a person found not guilty by reason of mental impairment.

‘Disease of the mind’
The statutory defence of mental impairment is based on the common law test known as the M’Naghten Rules. The case of M’Naghten stated that a person was not criminally responsible if:
…at the time of committing the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he [or she] was doing; or, if he [or she] did know it, that he [or she] did not know what he [or she] was doing was wrong [emphasis added].
There has been much debate about which mental conditions constitute a ‘disease of the mind’. The case law establishes that a causal relationship is required between the mental condition and a person being unable to understand that their conduct was wrong.  While mental illness clearly falls within the scope of the defence, it remains uncertain whether other conditions such as cognitive impairment, intellectual disability or severe personality disorder constitute a mental impairment.

Is a definition of mental impairment required?
Prior to the commencement of the CMIA in 1997, a parliamentary committee had recommended that a statutory definition of mental impairment be introduced to clarify the types of conditions that fall within the scope of the defence.  Despite this recommendation, Victoria remains one of the few Australian jurisdictions without a statutory definition of the term.

Those supporting a definition of mental impairment argue that it is unclear which mental conditions fall within the scope of the defence, particularly in relation to intellectual disability and cognitive impairment. A recent Parliamentary Law Reform Committee report acknowledged there is ambiguity in the meaning of the term ‘mental impairment’ and recommended that it be defined to ‘encompass mental illness, intellectual disability, acquired brain injuries and severe personality disorders’.  The report argued that a definition would ensure Victoria is consistent with other Australian jurisdictions that ‘expressly recognise intellectual disability and some other cognitive impairments such as ABIs [acquired brain injuries] and senility as conditions that may qualify a defendant for the defence’.

In 2004, the VLRC also examined this issue in the Defences to Homicide report, but recommended retaining the current defence of mental impairment without defining mental impairment. The report argued that difficulties in adequately defining the term ‘mental impairment’ would mean that any definition would be as problematic and subject to criticism as if there were no definition at all.  Consultations undertaken as part of this review indicated that the defence was working well in practice, and that leaving the term undefined would ensure flexibility in its application.  The review instead recommended that a provision be added to the CMIA specifying that the term mental impairment includes, but is not limited to, the common law notion of ‘disease of the mind’.


The VLRC is currently in the process of writing a consultation paper that will ask a range of questions regarding the operation of the CMIA, including whether a definition of mental impairment is required. It is expected that the consultation paper will be publicly available at the end of June 2013, with formal consultations planned for the months of June to August 2013. For more information, see, or contact team leader Nina Hudson:

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