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Substantial impairment by abnormality of mind

Substantial impairment by abnormality of mind is a partial defence to murder. An ‘abnormality of mind’ refers to a state of mind so different from that of ordinary human beings that a reasonable person would deem it to be abnormal. However, the abnormality must arise from various specified causes.

The test

For the partial defence of substantial impairment by abnormality of mind to success, the following must be established by the accused on the balance of probabilities:

  • That at the time of the act causing death, the accused capacity either to:
    (i) understand events, or

    (ii) judge whether their actions were right or wrong, or
    (iii) control themselves

    was substantially impaired by an abnormality of mind arising from an underlying condition, and

  • That the impairment was so substantial as to warrant the accused liability for murder being reduced to manslaughter. This is a value judgment applying community standards.

The onus of proof

The accused bears the onus of establishing, on the balance of probabilities, the existence of this partial defence. Therefore, the accused must prove that it is more probable than not that the accused was suffering from Substantial impairment by abnormality of mind at the time of the alleged offence.

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