Under the Criminal Law Consolidation Act (SA) a defence of mental impairment exists. Under section 269H metal impairment has been defined to include:
- mental illness;
- intellectual disability;
- a disability or impairment of the mind resulting form senility.
However the definition does not include intoxication.
Intoxication, although not included in the defence for Mental impairment, can be regarded as a defence in some limited cases. Part 8 of the Criminal Law Consolidation Act provides that intoxication can be a defence in very limited circumstances, where the basis for the argument is that the defendant’s consciousness was so impaired to the point of criminal irresponsibility at the time. However self induced intoxication is not usually considered a defence and very strong evidence needs to exist for impairment by intoxication to succeed as a defence.
Whether a person is mentally competent to commit an offence or mentally competent to stand trial will be a question of fact.
A person will be mental incompetent to commit an offence if at the time of the alleged conduct a person was suffering from a mental impairment and because of this did not know the nature of their conduct; or did not know that the conduct was wrong; or was unable to control their conduct. Mental competence, however is presumed unless on investigation the contrary is found. Mental impairment will be determined based on the balance of probabilities.
Under section 269J the court is able to order an investigation if mental competency issues arise to establish if the defendant is mental fit to stand trial. The application can be made by either the Prosecution, the Defence or even the judge if they feel that it will prevent a possible miscarriage of justice.
A person will be mentally unfit to stand trial on a charge if:
(a) unable to understand, or to respond rationally to the charge or the allegation on which the charge is based.
(b) unable to exercise (or to give rational instructions about that exercise of) procedural right or
(c) unable to understand the nature of the proceedings or to follow the evidence.