7.3 Mental impairment

Mental impairment ¹³⁹

A person is not criminally responsible for the commission of an offence if, at the time the offence was committed, the person suffered from a mental impairment which precluded criminal responsibility.

Mental impairment and criminal responsibility: The relationship between criminal responsibility and mental impairment is not left at large. Mental impairment has no effect on culpability unless, as a consequence of the impairment, the person:

1. did not know the nature and quality of the conduct constituting the offence; or
2. did not know that the conduct was wrong; or
3. the person was unable to control their conduct.

Section 7.3(1) is based on the principles derived from the rules propounded by the House of Lords in McNaghten, ¹⁴⁰ in 1843. The McNaghten rules have been extended, however, by the provision of an excuse for a person whose conduct was beyond their control, as a consequence of mental impairment.   The first of the tests for determining criminal responsibility   in s7.3)(1) paraphrases the first of the McNaghten rules. Unlike some earlier Australian codifications of the first McNaghten rule, which require proof that insanity deprived the defendant of the capacity for knowledge,¹⁴¹ the Code simply requires proof that the defendant did not know the nature and quality of their conduct. The second of the tests amplifies the original McNaghten requirement that the mental impairment deprive the defendant of knowledge that the conduct was wrong. A defendant who knows that reasonable people consider the conduct to be wrong is taken to know that it is wrong, no matter what the defendant’s personal convictions or personal morality on that score.¹⁴²   However,  a person who is unable to reason with   a moderate degree of sense and composure cannot be said to know how others might judge the defendant’s conduct. The Code adopts the formulation of the test used by Sir Owen Dixon in his jury direction in Porter, in 1933.¹⁴³ The extension of the defence to include those who are unable to reason with a moderate degree of sense and composure about   right and wrong does not go so far as to excuse those who know their conduct to be wrong but lack any feeling, understanding or moral appreciation of its wrongfulness.¹⁴⁴ The third of the tests, which has no common law counterpart, allows mental impairment to excuse when the effect of the impairment is that the defendant cannot control their conduct. Here Chapter 2 follows the majority of Australian jurisdictions which have enacted similar extensions of the McNaghten Rules.¹⁴⁵

The McNaghten Rules require proof of a “disease of the mind” as a foundation for the defence. Common law and statute have widened the applications of the concept of “disease of the mind” and the Code provision incorporates these developments in its definition of “mental impairment”, which includes senility, intellectual disability, mental illness, brain damage and severe personality disorder: s7.3(8). The Code goes beyond Australian law in other jurisdictions in recognising the possibility that a mental impairment defence can be based on evidence of severe personality disorder.¹⁴⁶ The concluding provision, which defines “mental illness”, adopts a common law formula derived from the judgement of King CJ in Radford.¹⁴⁷

The prosecution may allege mental impairment as an alternative to guilt: In modern caselaw on mental state defences, mental impairment is frequently alleged by the prosecution in answer to a defendant’s denial of fault or a defence of automatism or other claim of involuntariness.¹⁴⁸ In this respect, mental impairment stands apart from the other defences. A defendant who escapes conviction because of mental impairment gains no more than a qualified acquittal and remains liable to the imposition of custodial or other controls. The Code adopts and extends the common law, allowing the prosecution to seek a special verdict of acquittal on the ground of mental impairment with the leave of the court: s7.3(4). Since the special verdict  may be sought by the prosecution and resisted by the accused, it is necessary for the court to reach an affirmative conclusion that mental impairment is the only ground for acquittal before the special verdict can be returned: s7.3(5). The question whether the defendant’s  mental state at the time of  the offence is one of mental impairment is a question of fact for the court: s7.3(2).

A mental impairment defence must be proved on the balance of probabilities: The burden of persuasion rests on the proponent of the defence, whether it be the defendant or prosecution. The standard of proof is the same, whether the defendant or prosecution seeks the special verdict: s7.3(3). The rule  takes the traditional form of a “presumption” that the defendant was not mentally impaired at the time of the offence. So far as the prosecution is concerned, the proof provisions of 7.3 Mental impairment are an exception to the proof provisions in Part 2.6 – Proof of Criminal Responsibility.¹⁴⁹Priority of defences: The remaining provisions on mental impairment are meant to ensure that defendants who were mentally impaired at the time of the offence cannot avoid the special verdict by relying on evidence of their mental impairment to deny a fault element.   A defendant who seeks to   deny that their conduct was voluntary cannot do so if the only ground for denial is evidence of mental impairment: 4.2 Voluntariness; s7.3(6). Once again, the Code formulation follows Australian common law.¹⁵⁰ The Code departs from the common law, however, by providing that evidence of mental impairment cannot provide a basis for a denial of intention or other fault elements.¹⁵¹ So, for example, a mentally ill defendant who killed in consequence of a delusion that their victim was a ghost or zombie would   not be able to escape liability on the ground that there was no intention to kill a person: s7.3(7).¹⁵² Application of these rules, which deprive the defendant of the chance of an unqualified acquittal, requires the trial judge  to determine whether the evidence excludes any possibility that the defendant was mentally sound at the time of the offence. If it is possible that a reasonable jury might not be satisfied, on the balance of probabilities, that the defendant’s mental state was one of impairment, as defined in the Code, evidence of that mental state is admissible to support a denial of voluntariness or fault: s7.3(3).1) 213-218.