Insanity

Insanity

The defence of insanity derives from the M'Naghten Rules laid down in England in 1843, which remain part of the law of Hong Kong to this day. 

In law, every person is presumed to be sane and to possess a sufficient degree of reason to be responsible for the crimes he commits (the "presumption of sanity"). 

Under the M'Naghten Rules, however, that presumption may be rebutted. In brief, the defendant must prove that, at the time of the offence, his reason was impaired by a disease of the mind so as to result in one of the following: 

• that he did not understand the nature and quality of his act (that is, he did not know what he was doing); or 
• that (if he did understand the nature and quality of his act) he did not know that what he was doing was wrong. 

The word "reason" here refers generally to the intellectual faculties of intellect, memory and understanding. Thus, a defendant is suffering from "insanity" in the legal sense only if a disease of the mind has so impaired those faculties that he did not know what he was doing, or, if he did know what he was doing, did not know that it was wrong. 

Where a defence of insanity succeeds, the defendant is not acquitted outright but is found "not guilty by reason of insanity". The court may thereafter make such order as it thinks fit, including committing him to a mental hospital for detention and treatment rather than imprisonment. The burden of proof lies on the defence, which must prove insanity on the balance of probabilities (that is, on a more-likely-than-not basis). In general, the defence must support the plea with expert medical evidence; the defendant's own account alone is not sufficient.