A Defence of Mental Incompetence

What used to be described in legal parlance, and is still so by some, as the defence of insanity is now broken into two separate limbs under South Australian Law.

The First limb is where a person was not mentally competent to commit the offence.  The second limb is unfitness to stand trial and is dealt with here.

This requires that the person suffers from a mental impairment as defined by the Criminal Law Consolidation Act 1935.

A mental impairment is defined as:

  • A mental illness (meaning a pathological infirmity of the mind, including a temporary one of short duration);
  • An intellectual disability; or
  • Disability or impairment of the mind resulting from senility.

The underlying feature of each criteria of mental impairment is that it is either permanent or prone to reoccurrence.  For example, bipolar disorder is considered a mental impairment for the purposes of the Act, whereas concussion is not.

It then requires that as a result of that mental impairment one of more of the following is true about the accused:

  • They did not know the nature or quality of the conduct
  • They were unable to understand that the conduct was wrong; that is, the person could not reason about whether the conduct, as perceived by reasonable people, is wrong; or
  • They were totally unable to control the conduct.

So there are two limbs to the defence:

  • the person must suffer from a mental impairment (for example schizophrenia); and
  • As a result of that impairment one of the above three criteria must be met (in the example of schizophrenia, a person in psychosis would likely meet the first criterion).

However, the defence contains a proviso that if a person is found to be mentally incompetent to commit an offence and the trial judge is satisfied, on the balance of probabilities, that the mental impairment at the time of the conduct alleged to give rise to the offence was substantially caused by self-induced intoxication (whether the intoxication occurred at the time of the relevant conduct or at any other time before the relevant conduct), the person may not be dealt with under the mental impairment legislation, but may (if appropriate) be dealt with under the legislation dealing with intoxication.

This proviso is somewhat challenging because on its plain reading it appears to rule out those who meet the first limb of the test through self-induced intoxication, rather than the ones who meet the second.  That is, if, for example, the schizophrenia is caused by self-induced intoxication with drugs, then the defence is not available even though this self-induced intoxication may have occurred years before the conduct.

Alternatively, it may be interpreted, as would appear a more logical goal of the legislation, that when the second limb of the test is met as a result of self-induced intoxication then the defence is not available.

The Judge has a discretion to allow the defence despite the mental impairment having been substantially caused by self-induced intoxication having considered:

  • the time and circumstances of when and how the intoxication caused the mental impairment; and
  • the interests of justice; and
  • whether the making of such an order would affect public confidence in the administration of justice.

The onus is on the Defendant to establish that they have a mental impairment defence available to them on the balance of probabilities.

If a person is found to have committed the conduct alleged against them but to have a mental impairment defence, then they are found not guilty by reason of mental impairment.

The person is not sentenced, but is declared liable to supervision.  This process is be dealt with in a separate post.



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