Defence of Mental Impairment (Victoria) – Comprehensive Notes

1. Criminal Responsibility & the Rationale for a Defence of Mental Impairment

  • Criminal law begins with the presumption of rational capacity:

    • Every individual is assumed able to choose rationally before acting.

    • Blameworthiness requires three classic ingredients:

    • Voluntariness – the act must be willed.

    • Intentionality – the act must be purposeful.

    • Cognitive awareness – the accused must understand what they are doing (nature/quality of the act).

  • Certain mental states can compromise any of these three ingredients, prompting the law to excuse or partially excuse the actor.

    • Examples: psychotic delusions, severe mood disorders, paranoid schizophrenia (as in McNaughton), traumatic brain injury, major neuro-cognitive disorders.

  • Policy & ethical backdrop:

    • Fairness – It offends moral intuition to punish someone incapable of rational choice.

    • Utilitarian concern – Deterrence and retribution lose meaning if the offender cannot appreciate the threat of sanction.

    • Stigma reduction – Modern statutes deliberately avoid the archaic label “insanity” to reduce prejudice.

2. Historical Label Shift: From “Insanity” to “Mental Impairment”

  • Older common-law sources, Hollywood films, and early casebooks use the term “insanity defence.”

  • All Australian jurisdictions now prefer “defence of mental impairment.”

    • Removes derogatory undertones tied to “insanity.”

    • Signals a broader clinical focus that can include intellectual disability, dementia, etc.

3. The McNaughton Case (House of Lords, 1843)

  • Facts

    • Daniel McNaughton mistakenly shot Edward Drummond, believing he was Prime Minister Sir Robert Peel.

    • Diagnosed (retrospectively) as a paranoid schizophrenic with delusions of persecution.

    • Interviewed by 15 psychiatrists, all indicating significant mental disorder.

  • Outcome

    • Acquitted of murder “on grounds of insanity.”

  • The McNaughton Rules – five guiding propositions distilled by the House of Lords:

    1. Presumption of sanity: Every person is sane until the contrary is proved.

    2. Defect of reason from disease of the mind must be shown.

    3. The defect must render the accused incapable of knowing the nature & quality of the act, or incapable of knowing it was wrong.

    4. Jury = fact-finder; expert medical evidence is permitted but cannot usurp the jury’s role.

    5. Experts should, where practicable, examine the accused directly before testifying.

  • Significance

    • Became the foundation for modern tests in the UK, Australia, Canada & many U.S. jurisdictions.

    • Introduced the dual cognitive limbs: “nature/quality” and “right/wrong.”

    • Explicitly set the burden of proof on the accused (later jurisdictions modify this; Victoria uses balance of probabilities).

4. Elements of the Defence of Mental Impairment (General Statement)

To succeed, the accused must establish, on the balance of probabilities (i.e. P > 0.5), that at the time of the offence they:

  1. Were labouring under a defect of reason caused by a qualifying mental impairment; and

  2. As a result either: a. Did not know the nature or quality of the act (e.g.

    • Believed they were squeezing a lemon when actually strangling a person),
      or
      b. Did not know the act was wrong according to ordinary standards of reasonableness.

5. Victoria’s Statutory Framework: Crimes (Mental Impairment and Unfitness to Be Tried) Act 1997 (CMIA)

  • Section 20(1) – Codifies the defence:

    • A person is not criminally responsible if, at the relevant time, they were suffering from a mental impairment that made them:
      a. Unaware of the nature or quality of their conduct; or
      b. Unable to see the conduct was wrong.

  • Section 25 – Abrogates the common-law insanity defence in Victoria; CMIA now exhaustive.

  • Terminology

    • “Mental impairment” can include mental illness, intellectual disability, neuro-cognitive disorders, etc.

    • Focus is on functional impact rather than diagnostic label.

  • Standard of Proof & Onus

    • Accused bears the onus on a civil standard (balance of probabilities).

    • Practical note: Expert psychiatric/psychological evidence is almost always called.

6. Consequences of a Successful Defence in Victoria

  • A verdict is recorded as “not guilty because of mental impairment.”

  • Sentencing court choses between:

    • Custodial Supervision Order (CSO) – Detention in a secure forensic hospital or prison environment (rarely indefinite; must be reviewed).

    • Non-Custodial Supervision Order (NCSO) – Community-based with conditions (treatment, reporting, no-contact orders, etc.).

  • No ordinary conviction is entered; the primary aim shifts to treatment & public safety, not punishment.

7. Comparing McNaughton & CMIA Tests

Criterion

McNaughton (1843)

CMIA s20 (1997)

Source of impairment

“Disease of the mind”

“Mental impairment” (broader)

Cognitive tests

1. Nature & quality; 2. Right/wrong

Identical twin limbs

Fact-finder

Jury

Jury (still), but codified instructions

Expert role

Advisory; must examine accused

Same in substance; governed by Evidence Act + CMIA

Burden of proof

On accused

On accused (balance of probabilities)

Post-verdict outcome

Royal pardon, hospital detention, etc. (case-by-case)

CSO or NCSO under CMIA

8. Practical & Ethical Implications

  • Public protection vs. therapeutic justice: Courts balance community safety with humane treatment.

  • Stigma management: Removing “insanity” reduces pejorative connotations and potential jury bias.

  • Evidentiary complexity: Competing psychiatric testimony can confuse juries; strict directions are essential.

  • Right/wrong limb controversy:

    • Critics argue it is overly cognitive, ignoring volitional impairments where the person knows wrongfulness but cannot control impulses.

    • Some U.S. states add a “control” limb; CMIA retains the traditional rule.

  • Real-world example: Delusional belief that an act is commanded by God may satisfy the “wrongfulness” limb if the person genuinely believes a higher moral law overrides human law.

9. Links to Prior & Future Topics

  • Unfitness to be tried (also within CMIA) – Deals with competence at trial not at offence time.

  • Automatism – Separate defence where conduct is involuntary (e.g. sleepwalking); may overlap clinically but uses a different doctrinal path.

  • Diminished responsibility – Partial excuse in some jurisdictions (not Victoria for murder) that reduces murder to manslaughter.

10. Key Take-Away Mnemonic

Remember “NQW” → Nature, Quality, Wrongness.
If any leg fails due to a genuine mental impairment at the moment of the act, criminal responsibility dissolves.