Mental Impairment

For criminal intent to be established, the ability of the judgement of the accused must not be impaired or diminished due to a ‘disease of the mind’.

The defence of mental impairment is a statutory defence, legislated under section 20 of the Crimes Act 1997 (Vic) and is used when an accused is afflicted by a mental disorder, disturbance or disease that causes them to not know what they are doing, or not know that what they are doing is wrong. When deciding a criminal case, courts assume that all people have the ability to make rational decisions and are aware of the outcomes of their actions. When raised, the defence of mental impairment acts to show this assumption does not apply.

To successfully rebut the presumption of sanity and show mental impairment beyond reasonable doubt, the accused my show that at the time of offending, their mental impairment caused them to:

  • Not know the nature of what they were doing or appreciate the consequences of their behaviour;

  • Not know that their conduct was wrong or unlawful.

Furthermore, it must be proven that this state was caused by a ‘disease of the mind’, which in Victorian law includes, but is not limited to:

  • Brain injuries

  • Tumours

  • Schizophrenia

  • Psychomotor epilepsy

  • Hyperglycaemia

  • Cerebral arteriosclerosis

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