Medical Insanity v Legal Insanity: How Section 84 of the Penal Code Works

A psychiatric diagnosis and a legal defence are not the same thing. The Federal Court said so in October 2024, and the distinction is back in public view after the Shah Alam High Court, on 14 September 2026, acquitted a teenager of murder on the ground of unsoundness of mind and ordered detention in a psychiatric hospital at the pleasure of the Sultan of Selangor.

That case remains open to appeal by the Public Prosecutor, and we make no comment on its merits. This article looks at the defence itself: what section 84 of the Penal Code requires, who must prove it, and what follows when it succeeds.

Three questions that are often confused

Mental illness can matter at three different points in a criminal case, and each has its own rule.

• Fitness to stand trial. Under section 342 of the Criminal Procedure Code (CPC), a court which suspects that an accused is of unsound mind, and so incapable of making a defence, must investigate, and may send the accused for observation in a psychiatric hospital. The question is about the accused’s condition now. If the accused is unfit, the trial is postponed and may resume once the accused is certified capable.

• Responsibility at the time of the act. This is section 84 of the Penal Code, the insanity defence proper. It looks back to the moment of the act.

• What happens afterwards. Sections 347 to 351 of the CPC govern the consequences of a successful defence.

A person can be fit to stand trial today and still have been legally insane when the act was done.

What section 84 asks

Section 84 relieves a person of criminal liability if, when the act was done and by reason of unsoundness of mind, that person was incapable of knowing the nature of the act, or that it was wrong or contrary to law. Three features stand out. The relevant time is the moment of the act, not the day of trial. The cause must be unsoundness of mind. And the effect must be an incapacity to know, either what one is doing or that it is wrong or unlawful. An illness which leaves that knowledge intact, however serious, does not satisfy the section.

The Federal Court’s two stages: medical, then legal

In PP v Mohd Rozani Yahaya [2025] 1 MLRA 203, decided on 21 October 2024, a Federal Court panel of Justices Zabariah Mohd Yusof, Abdul Karim Abdul Jalil and Hanipah Farikullah set out a two-stage approach. The first stage, medical insanity, is a matter for expert evidence. The second, legal insanity, is for the court alone: whether the incapacity described in section 84 has been shown. The court stressed that a person who is mentally ill is not, for that reason alone, exempt from criminal responsibility. In that case the prosecution’s appeal was allowed and the accused was ordered to enter a defence.

Who has to prove what

The prosecution must still prove that the accused did the act. Insanity is one of the general exceptions to criminal liability, and the burden of establishing it lies on the accused, on a balance of probabilities. Commentary on the Malaysian cases notes that judges weigh the expert evidence together with the accused’s conduct around the time of the event. Behaviour suggesting planning or escape can tell against the defence, while evidence of delusions or hallucinations at the time can support it. Each case turns on its own evidence.

Appeals can cut both ways

Insanity acquittals are uncommon, and the Public Prosecutor may challenge them on appeal. Rozani is one example of a prosecution appeal succeeding. Commentary also records PP v Shalima Bi, in which the Court of Appeal reversed a High Court acquittal on insanity grounds and the Federal Court, in 2016, restored it. The lesson is a modest one: appellate courts have both reversed and restored such findings, and the result depends on how the evidence measures up to section 84. None of this predicts the outcome of any pending matter.

After a successful defence: custody, not freedom

Section 347 of the CPC requires a court which acquits on this ground to state specifically whether the accused committed the act. If so, section 348 requires an order for safe custody and a report to the Ruler or the Yang di-Pertuan Agong, who may order confinement in a psychiatric hospital during the Ruler’s pleasure. Sections 350 and 351 deal with discharge on certification by the hospital’s visitors and medical officers, and with delivery to the care of a relative or friend on security. There is no fixed term, so detention may be shorter or longer than a prison sentence would have been.

The regime is not confined to homicide. In January 2024, in what was reported as the first prosecution for stalking under section 507A of the Penal Code, the accused was acquitted on the ground of unsound mind and ordered to be detained in a psychiatric hospital at the Ruler’s pleasure.

Practical takeaways for families and the public

• Fitness to stand trial and insanity at the time of the act are different questions, and each calls for its own psychiatric assessment.

• A diagnosis is only the starting point. The defence needs evidence about the accused’s capacity at the very time of the act.

• The burden is on the defence, on a balance of probabilities.

• Success leads to indefinite secure treatment, with release turning on medical certification and the Ruler’s decision. It is not an ordinary acquittal.

• The Public Prosecutor may appeal an insanity acquittal. Victims’ families have no separate right of appeal.

Where the Bandar Utama case stands

Our sympathy is with the victim’s family, who have publicly urged the Attorney General’s Chambers (AGC) to appeal; public figures have also asked the AGC to explain its position. As at 19 September 2026 no decision by the AGC on an appeal had been reported. Press reports put the period for filing a notice of appeal at 14 days, which would run to about 28 September 2026. We take no position on the verdict or on whether an appeal should be brought.

Authorities and sources

• PP v Mohd Rozani Yahaya [2025] 1 MLRA 203 (Federal Court, 21 October 2024)

• PP v Shalima Bi (Federal Court, 2016), as described in published commentary; no citation is given here

• Penal Code, sections 84 and 507A

• Criminal Procedure Code, sections 342 to 345 and 347 to 351

• Malay Mail, 21 January 2024 and 14 September 2026; The Star, 17 September 2026; The Vibes, September 2026

This article is general legal information only and is current as at 19 September 2026. It is not legal advice and does not create a solicitor-client relationship. Court decisions may be appealed and the law may change; please seek advice on your own circumstances. — Chris & Partners, Advocates & Solicitors