Presumption of Sanity in Criminal Law
The defence of insanity is one of the most complex and closely scrutinized defences in Nigerian criminal law. It operates as a complete defence, exonerating an accused person from criminal responsibility if successfully established. However, the law presumes that every person is of sound mind and responsible for their actions until proven otherwise.
Legal Foundation
Section 51 of the Penal Code (applicable in Northern Nigeria) and Section 28 of the Criminal Code(applicable in Southern Nigeria) provide that:
“Nothing is an offence which is done by a person who, at the time of doing it, by reason of unsoundness of mind, is incapable of knowing the nature of the act, or that he is doing what is either wrong or contrary to law.”
In MUSA v. STATE (2025) LPELR-81230(CA), the Court of Appeal clarified that insanity under Section 51 means unsoundness of mind which renders a person, at the time of the commission of the crime, incapable of knowing the nature or gravity of the act. The court reaffirmed that this defence only avails the accused where it can be shown that he was insane at the time the act was committed.
Burden and Standard of Proof for the Defence of Insanity
The Court of Appeal in DAVID v. STATE (2021) LPELR-53987(CA)clarified the burden and standard of proof required where an accused person raises the defence of insanity in a criminal trial.
The Court held that although the law presumes every person to be of sound mind, an accused who relies on insanity must rebut that presumption by establishing the defence.
Importantly, the standard required is not proof beyond reasonable doubt, but proof on the balance of probabilities.
The Court stated:
“An Accused Person who pleads insanity as a defence to an offence with which he was charged must prove that he was insane at the time of committing the act. The standard of proof required of the Accused is, however, proof on the balance of probabilities or preponderance of evidence and not proof beyond reasonable doubt. GUOBADIA VS. THE STATE (SUPRA); ONAKPOYA VS. THE QUEEN (1959) NSCC PG. 130. (1959) SNLR PG. 384.”
— Per UZO IFEYINWA NDUKWE-ANYANWU, JCA (P. 30, Paras A–C)
This principle aligns with Section 139 of the Evidence Act 2011, which places the burden on a person relying on a statutory exception or defence to prove its existence.
The Court in STATE v. KALIB (2021) LPELR-56084(CA)outlined the relevant evidence for establishing insanity as follows:
1. Evidence of the past history of the accused;
2. His conduct immediately before, during, and after the incident;
3. Testimony from prison officials who had custody of the accused;
4. Medical or psychiatric evidence;
5. Testimony of relatives on the accused’s behavior and reputation for sanity or insanity; and,
6. Evidence of hereditary or family history of insanity.
The courts have also noted that medical evidence, while desirable, is not indispensable. In MOHAMMED v. STATE (2024) LPELR-62507(SC), the Supreme Court emphasized that though medical evidence is the surest way to determine mental state, insanity may still be proven through credible circumstantial or eyewitness evidence.
Temporary and Self-Induced Insanity
The courts have recognized that insanity need not be permanent to constitute a defence. In MOHAMMED v. STATE (1997) LPELR-1902(SC), the Supreme Court held that temporary insanity could suffice if the accused was incapable of understanding the nature of his act at the material time.
However, ILU v. STATE (2025) LPELR-80702(SC) makes it clear that self-induced insanity, such as one caused by voluntary intoxication or consumption of prohibited substances, does not excuse criminal responsibility.
Distinguishing Insanity from Eccentricity
The courts have consistently differentiated between insanity and eccentricity. In ADDO v. STATE (2020) LPELR-55521(SC), the Supreme Court held that eccentric beliefs or unusual behavior do not amount to legal insanity. To qualify as insanity, the mental condition must be so severe as to deprive the accused of the ability to know right from wrong.
What the Court Considers
When the defence of insanity is raised, the court must evaluate the totality of the evidence — including the nature of the crime, the conduct of the accused before and after the act, and any medical or familial indicators of mental disorder. As stated in DAVID v. STATE (2021) LPELR-53987(CA), insanity must relate directly to the mental state of the accused at the time the offence was committed.
Legal Consequences Where Insanity Is Established
Where the defence of insanity is established, the accused person is not treated in the same manner as a person who receives an ordinary acquittal. The legal consequence of such a finding may include an order for the person’s continued custody in accordance with applicable law. The Court of Appeal recently clarified the consequence of a successful insanity defence in ABDULLAHI v. STATE (2026) LPELR-83007(CA).
Conclusion
The Nigerian courts have consistently held that while insanity can provide a valid defence to criminal liability, it must be clearly and convincingly established. The standard remains that of probability, and mere statements of being insane, unsubstantiated by evidence, will be disregarded. The jurisprudence underscores the delicate balance between protecting society and ensuring justice for individuals genuinely incapable of forming criminal intent due to mental illness.
Accused persons facing criminal charges may, depending on the circumstances of the case, also apply for bail while awaiting trial. Nigerian courts have explained the legal framework for bail applications in several decisions.
See:
Applying for Bail in Nigeria: Administrative vs Court Bail
How Nigerian Courts Decide Bail Applications.
Questions sometimes arise as to whether criminal proceedings should continue where related appeals or applications are pending. The Court of Appeal has clarified this issue in Can Criminal Proceedings Be Stayed Pending Appeal?
Litigation also requires active participation by parties. Courts have repeatedly warned that a party who fails to defend proceedings risks judgment being entered against them.
See:
When a Defendant Fails to Defend a Case: Lessons for Civil and Criminal Litigation
Disclaimer:
This article is for general informational purposes only and does not constitute legal advice. Readers seeking legal guidance on specific circumstances should consult a qualified legal practitioner.
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