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This lesson covers the two partial defences to murder that, if successful, reduce a conviction from murder to voluntary manslaughter: diminished responsibility (s.2 Homicide Act 1957, as amended by the Coroners and Justice Act 2009) and loss of control (ss.54–56 Coroners and Justice Act 2009). These defences are crucial because they allow the judge sentencing discretion rather than being bound by the mandatory life sentence for murder.
| Term | Definition |
|---|---|
| Voluntary manslaughter | Where the defendant has the actus reus and mens rea for murder but a partial defence applies, reducing the conviction to manslaughter |
| Partial defence | A defence that does not result in acquittal but reduces murder to voluntary manslaughter |
| Diminished responsibility | A partial defence based on the defendant's abnormality of mental functioning arising from a recognised medical condition |
| Loss of control | A partial defence based on the defendant losing self-control due to a qualifying trigger |
| Qualifying trigger | A circumstance (fear of serious violence or circumstances of an extremely grave character) that causes the loss of control |
Diminished responsibility is defined by s.2 Homicide Act 1957, as amended by s.52 Coroners and Justice Act 2009. The amended s.2(1) provides:
A person ("D") who kills or is a party to the killing of another is not to be convicted of murder if D was suffering from an abnormality of mental functioning which: (a) arose from a recognised medical condition, (b) substantially impaired D's ability to do one or more of the things mentioned in subsection (1A), and (c) provides an explanation for D's acts and omissions in doing or being a party to the killing.
| Requirement | Explanation |
|---|---|
| 1. Abnormality of mental functioning | The defendant's mental functioning must be abnormal — departing from what a reasonable person would regard as normal |
| 2. Arising from a recognised medical condition | The abnormality must be caused by a condition recognised by medical science (e.g., as listed in the WHO International Classification of Diseases or DSM-5) |
| 3. Substantially impaired D's ability to: (a) understand the nature of their conduct, (b) form a rational judgment, or (c) exercise self-control | The impairment must be more than trivial but need not be total |
| 4. Provides an explanation for the killing | The abnormality must be a cause of the killing (or at least a significant contributory factor) |
Byrne was a sexual psychopath who strangled a young woman and then mutilated her body. The Court of Criminal Appeal defined "abnormality of mind" (under the original s.2) as:
"A state of mind so different from that of ordinary human beings that the reasonable man would term it abnormal."
The court held that Byrne's inability to control his perverted desires amounted to an abnormality of mind. His conviction for murder was reduced to manslaughter.
Principle: Abnormality of mental functioning includes not only the inability to understand what one is doing but also the inability to exercise self-control. The Byrne definition remains authoritative under the amended s.2.
Dietschmann killed a man while extremely intoxicated. He was also suffering from an adjustment disorder (a recognised medical condition) following a bereavement. The House of Lords held that the defendant did not need to show that the recognised medical condition alone (without the effects of alcohol) would have substantially impaired his mental responsibility. The question was whether, despite the effects of alcohol, the abnormality of mental functioning arising from the recognised medical condition substantially impaired his abilities.
Principle: Intoxication does not automatically prevent a diminished responsibility defence. The jury should consider whether the recognised medical condition substantially impaired D's abilities, even taking into account the effects of alcohol.
The Supreme Court considered the meaning of "substantially impaired." Lord Hughes held that "substantially" should be given its ordinary meaning — it means something more than trivial but less than total. The court should not routinely direct the jury on the meaning of "substantially" unless a specific issue arises.
Principle: "Substantially impaired" bears its ordinary everyday meaning. It is a question of degree for the jury.
Diminished responsibility is the only defence in English criminal law where the burden of proof is on the defendant (on the balance of probabilities). The prosecution bears the initial burden of proving murder; the defendant must then prove diminished responsibility on the balance of probabilities (s.2(2) Homicide Act 1957).
flowchart TD
A[D is charged with murder] --> B{Was D suffering from an abnormality of mental functioning?}
B -->|No| C[Defence fails — murder conviction]
B -->|Yes| D{Did it arise from a recognised medical condition?}
D -->|No| C
D -->|Yes| E{Did it substantially impair D's ability to: understand conduct / form rational judgment / exercise self-control?}
E -->|No| C
E -->|Yes| F{Does it provide an explanation for the killing?}
F -->|No| C
F -->|Yes| G[Diminished responsibility established — voluntary manslaughter]
Loss of control replaced the old common law defence of provocation and is defined by ss.54–56 Coroners and Justice Act 2009. Section 54(1) provides:
Where a person ("D") kills or is a party to the killing of another ("V"), D is not to be convicted of murder if: (a) D's acts and omissions in doing or being a party to the killing resulted from D's loss of self-control, (b) the loss of self-control had a qualifying trigger, and (c) a person of D's sex and age, with a normal degree of tolerance and self-restraint and in the circumstances of D, might have reacted in the same or in a similar way to D.
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