Legal Defences

Mental Impairment Defence

The Legal Defence of Mental Impairment in Victoria

The mental impairment defence arises when the accused suffers a mental disease, disorder or disturbance that causes them to not know what they’re doing or that what they’re doing is wrong. Mental impairment may be used as a legal defence to any criminal offence in Victoria.

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What is the Legal Defence of Mental Impairment in Victoria?

In Victoria, mental impairment may be used as a legal defence to all criminal offences committed on or after 18 April 1998. It is a statutory defence, legislated under section 20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (Vic).

The defence arises where the accused was suffering from a mental impairment at the time of committing the alleged offence and either:

Did not understand the nature and quality of what they were doing; or
Understood what they were doing but did not understand that it was wrong.

Mental impairment is considered to mean a mental disease, disorder or disturbance. It may be passing (transitory) or permanent. Similarly, it may be treatable or not.

In Victorian criminal law, it is assumed that all people have the cognitive capacity to make rational decisions and are aware of the outcomes of their actions. The mental impairment defence negates this assumption.

Where the mental impairment defence applies, the Court must find the accused not guilty of the offence. However, the superior court’s retain the power to make supervision orders in such cases.

Mental Impairment in Victorian Criminal Law

Mental impairment is a statutory legal defence in Victoria. It is governed by section 20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (Vic).

The term ‘mental impairment’ is not defined under the Act. However, it is assumed to be similar to ‘disease of the mind’, a term referred to in the defence of automatism. To determine if a mental condition should be considered as a ‘disease of the mind’, Victorian courts apply the sound/unsound mind test. Under the test, a ‘disease of the mind’ is recognised as a mental condition which causes a person’s unsound mind to react to its own delusions or external stimuli (R v Falconer (1990)). Generally, a person is deemed to have a ‘disease of the mind’ if:

  • An underlying pathological infirmity is triggered by external stimuli (this is different to a mental condition that is solely caused by external stimuli, such as a blow to the head) (R v Radford (1985)); and
  • The mental state is prone to recur.

It is unclear how readily the term ‘disease of the mind’ can be applied to the mental impairment defence. However, it is clear that a ‘mental impairment’ must be a mental disease, disorder or disturbance. The person’s ability to understand their actions and the consequences of those actions must be thrown into disorder. Mere excitability, passion, lack of self-control or impulsivity are unlikely to amount to ‘mental impairment’ because they do not amount to a ‘disease of the mind’ (R v Porter (1933)). It can also be assumed that ‘mental impairment’ must involve an intrinsic and recurring pathological infirmity, as opposed to a mental condition solely caused by external forces. Finally, it can be assumed that the impairment may be permanent or temporary, organic or functional, curable or incurable.

Victorian case law (R v Hughes (1989); R v Falconer (1990); and Nolan v R (1997)) holds that diseases of the mind include but are not limited to:

  • Brain Injuries
  • Tumours
  • Schizophrenia
  • Psychomotor epilepsy
  • Hyperglycaemia (excessive blood sugar levels)
  • Cerebral arteriosclerosis

However, some conditions have been found to not constitute a ‘disease of the mind’ and therefore may not constitute a ‘mental impairment’:

  • Hypoglycaemia (excessive insulin intake)
  • Drug-induced psychosis
  • Some forms of epilepsy

If at the time of the offence the accused has self-induced intoxication, this does not constitute a ‘disease of the mind’ and is unlikely to constitute ‘mental impairment’ (R v Meddings [1966]).

Proving Mental Impairment in Court

In Victoria, the defence or the prosecution may raise the issue of mental impairment at any time during the trial (R v Hassan [2004]).

Section 21 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997 (Vic) states that a person is presumed not to have been suffering from a mental impairment until the contrary is proven. As such, whichever party raises the defence carries the onus of rebutting the presumption of sanity. The defence must be proven as a matter of fact and determined by a jury on the balance of probabilities (R v Whelan [2006]).

To rebut the presumption of sanity (and thus prove mental impairment) beyond a reasonable doubt, the party that raises the issue must prove that at the time of offending, a mental impairment caused the accused to:

  • Not know the nature and quality of what they were doing; or
  • Not know that the conduct was wrong.

‘Nature and quality of the act’ refers to the physical nature of the act and the consequences of that conduct. As such, to rely on the defence, it must be proven that at the time of the offence, a mental impairment caused the accused to be:

  • Unable to appreciate the physical nature of what they were doing; and
  • Unable to appreciate the consequences of their behaviour (R v Porter (1933)).

The accused may know the nature and quality of their act without knowing that the act is ‘wrong’. When testing whether the accused knew that their conduct was ‘wrong’, the court applies a test of reasonableness. That is, did a mental impairment mean that the accused “couldn’t reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people, was wrong” (s20(1)(b)). The term ‘wrong’ is defined as meaning contrary to the ordinary principles of a reasonable person. It does not necessarily mean contrary to the law or morality (R v White and Piggin (2003)).

It must also be noted that the defence of mental impairment is not established until it is also proven that the accused ‘engaged in the conduct constituting the offence’ (s20(1)). That is, to rely on the mental impairment defence, it must also be proven beyond a reasonable doubt that the accused committed the act or omission which constitutes the offence charged.

If mental impairment is proven, the court must find the accused not guilty of the charge. It must specify in the verdict that the accused has been found not guilty on the basis of mental impairment.

However, different courts have different powers in regard to mental impairment:

  • The Magistrates’ Court must discharge the accused if they find the person not guilty due to mental impairment for a summary offence or indictable offence triable summarily. In such matters, the Magistrates’ Court does not have the power to make supervision orders.
  • The superior courts must find the accused not guilty if the mental impairment defence is proven beyond a reasonable doubt. However, the superior courts retain the power to make supervision orders, which can carry the same maximum duration as the maximum term of imprisonment prescribed to the offence under legislation.

Obtaining Legal Advice

Mental impairment is a difficult defence to prove and requires professional consideration. If you or a loved one has been charged or is under investigation for allegedly committing a criminal offence, please contact our team at Sher Criminal Lawyers. We will help you achieve the best possible outcome by applying our extensive legal experience and expertise. One of our criminal law specialists will assess whether mental impairment or any other defence is relevant to your case before planning a unique and calculated legal strategy. We are available 24/7 and offer free consultations by way of Zoom, Facetime or in person at our Melbourne and Moorabbin offices.

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