An Examination of the Plea of Insanity under the Nigerian Criminal Jurisprudence



In Nigeria and other Common Law jurisdictions, it is firmly established that some criminal defendants should not be held liable for their crimes due to a lack of mental capacity at the time of the commission of an alleged crime.

The plea of insanity presumes that the defendant committed the specific offence in question; nevertheless, because of the mental impairment brought on by insanity, the defendant is said not to be in control of his mind at the time of the offence. This means any mental disease severe enough to keep a person from having legal capacity and exempt them from criminal or civil liability is referred to as insanity.

This research paper examines the concept of insanity, the elements of the defence and reaches a conclusion regarding when this defence may be availed to a defendant at trial for it to be successful.


The term insanity as it relates to criminal defence is not susceptible to a precise universal definition. Various legal writers, jurists as well as Statutes have attempted to define the term. The Black’s Law Dictionary[1] describes insanity as;

“Any mental disorder severe enough that it prevents a person from having legal capacity and excuses the person from criminal or civil responsibility”. It is also a social and legal term rather than a medical one used to denote that degree of mental illness which negates the individual’s legal responsibility or capacity”

In the English case of Brathy V.A.G. for Northern Ireland[2], Lord Denning provided the following explanation of what constitutes insanity, among other things;

“A disease of the mind, as any disorder that has manifested itself in violence and is prone to reoccur at any rate, is a sort of disease for which a person should be detained in hospital, rather than being given an unqualified acquittal by the Court”

From a subjective view, insanity can be defined as

“as a deficiency in understanding that prevents a person from knowing whether what they are doing is right or wrong due to a physical or mental illness”.

Given the aforementioned definitions of insanity or mental illness by academics and renowned jurists, it is obvious that a person’s mental state at the time of the offence is relevant and essential in determining whether the defence will be available to him or not.


Historically, the case of Daniel M’Naghten decided in 1843 marked a turning point in the history of using “insanity” as a defence. The English case formulated the following criteria as the basis of the insanity plea;

  1. That everyone is presumed sane until the contrary is proved
  2. That it is defence for the accused to show that he is labouring such a defect of reason, due to disease of the mind as either not to know the nature or quality of his act or if he did know, not to know that he was doing wrong
  3. That if a man commits a criminal act under insane delusion, he is under the same degree of responsibility as he would have been on the facts as he imagined them to be.

Nigeria as a Common Law jurisdiction, has adopted this position with necessary modifications to suit local circumstances. Under Nigerian law, everyone is presumed sane and rational until it is proven otherwise.

Sections 27, 43, and 284 of the Criminal Code, Penal Code and The Administration of Criminal Justice Act, (ACJA), 2015 respectively, make provision for this legal presumption. The two Codes essential to the Nigerian criminal justice system are:

  1. The Criminal Code

The Criminal Code is used in the Southern part of Nigeria and Sections 27 and 28 of the Criminal Code lay out the provisions for insanity. Section 27 of the Code states the obvious and serves as a preamble to the defence of insanity in the following words:

Every person is presumed to be of sound mind and to have been of sound mind at any time which comes in question until the contrary is proved”.

Section 28 of the Code, which establishes the insanity plea, stipulates as follows:

“A person is not criminally responsible for an act or omission if at the time of doing the act or making the omission he is in such a state of mental disease or natural mental infirmity as to deprive him of capacity to understand what he is doing, or of capacity to control his actions, or of capacity to know that he ought not to do the act or make the omission”.

Section 28 of the Criminal Code was given judicial recognition by the Supreme Court in the case of Madjemu V State[3]. The brief fact of the case is that;

The Appellant was arraigned for the offence of murder of his wife. He pleaded not guilty to the charge. At the conclusion of the trial, the trial Court found him guilty as charged and sentenced him to death by hanging. He appealed to the Court of Appeal which dismissed the appeal. He further appealed to the Supreme Court which also dismissed the appeal and stated that to establish the defence of insanity, recourse could be had to the following relevant facts namely:

  • evidence as to past history of the accused person;
  • evidence as to the conduct of the accused immediately preceding the killing of the deceased;
  • evidence from prison officials who had custody of the accused person before and during his trial;
  • evidence of medical officers who examined the accused;
  • evidence of relatives about the general behaviour of the accused person and the reputation he enjoyed for sanity or insanity in the neighbourhood and
  • evidence showing that insanity runs in the family history of the accused;

and such other facts which will help the trial court come to the conclusion that the burden of proof placed by law on the defence has been dismissed but in this case, the Appellant has failed to do so.

  1. Penal Code

The Penal Code replaces the Criminal Code in the Northern part of Nigeria. The following provision is made about insanity by the Code. According to Section 51 of the Penal Code:

“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 he is doing what is either wrong or contrary to law”.

In the unreported case of State V Shittu,[4] the accused killed his mother by repeatedly beating her in the skull with a shovel, ostensibly in an effort to convert her to Islam. The trial judge, Gbadeyan J., found the defendant not guilty on the basis of insanity and made several comments, including:

“If in his lunacy the accused thought he was reciprocating the faithfulness of his loving mother or was correcting her by using a shovel to hit her several times believing up till his trial that she was loving, then it is crystal clear that he was denied by the unsoundness of mind the capacity to understand the nature of his act”.

  1. Administration of Criminal Justice Act, (ACJA) 2015

Pursuant to Section 284 ACJA, Where the defendant appears to be of unsound mind at the time of any remand or similar pre-trial proceedings before a court, and the issue of the state of soundness of mind of the defendant is in question, with regards to a defence to the main offence for which he is arrested relating to insanity or intoxication, the court shall proceed to deal with the defendant in accordance with sections 278 to 291 of this Act and shall not make any finding of fact in relation to such defence that the defendant is open to plead at his trial for the offence.


The fact that an accused person is suffering from a mental disease or natural infirmity ipso facto does not entitle him or relieve him of criminal responsibility. He must show that it was such as to deprive him of any one of the three different capacities listed in Section 28 of the Criminal Code:

  1. Capacity to understand what he is doing:

This defence is helpful once the accused can present convincing proof to the court that his insanity stopped him from understanding what he was doing. In the case of Guobadia V. State[5] the Apex Court held that “Evidence that an accused had a mental disorder which does not show that the disorder deprived him of the capacity to understand what he was doing and to know that he ought not have done the act called in question is no satisfactory evidence of defence of insanity under the law

  1. Capacity to know that he ought not to do the act or make the omission

By virtue of the decision in Guobadia’s Case[6] a defendant who seeks to rely on the defence of insanity under Section 28 of the Criminal Code, must demonstrate that, at the time the offense was committed, he lacked, among other things, the mental ability to know that he should not have performed the act or done the omission at the time the alleged crime was committed.

  1. Capacity to Control His Action

The addition of the words “capacity to control his action” in Section 28 of the Criminal Code is a notable feature. Modern psychiatry understands that a person may be conscious of the nature of his behaviour or even understand that it is bad, yet nonetheless, carry the act out of an uncontrollable impulse. An example is the case of R V. Echem,[7] where the accused was charged with murder and was found to be legally insane based on evidence adduced by a medical doctor that the accused had sustained a head injury previously which was likely to affect his mental balance, rendering him unable to control his action.


Every person is presumed to be of sound mind and to have been of sound mind at any time until the contrary is proven. The presumption of sanity, in effect, places the legal onus on the accused to satisfy the Court on credible evidence that he was insane at the time he committed the offence charged. This is codified in Section 139(3) (c) of the Evidence Act, 2011 and it has also been settled by a plethora of cases. For ease of reference, Section 139(3) (c) is reproduced below;

Nothing in sections 135 and 140 or in subsection (1) or (2) of this section shall affect the burden placed on a defendant to prove a defence of intoxication or insanity”.

This makes it essential to take into account the circumstances leading up to the alleged crime in every situation where legal insanity is raised as a defence. In the case of State V Omoniyi,[8] the accused was alleged to have killed his wife. The evidence adduced during the Court trial showed that he was found to be non-compos mentis (not of sound mind) at the time he was arrested. The Court held that based on the entirety of evidence before the Court including the medical report, there was no other conclusion but that the accused was most probably insane at the time he committed the offence.

Furthermore, the presumption of sanity should be noted, that it is applicable to anybody who exhibits periods of lucidity. Unless it is obvious that such a person acted in a bad state of mind, the law presumes that the crimes they commit were committed during their lucid moment.[9]


A successful insanity defence results in a special verdict of “not guilty by reason of insanity”, which typically results in the defendant’s confinement to a mental institution rather than an absolute acquittal. A determination of insanity absolves a person of criminal culpability in that they are not found guilty and by virtue of Section 438 of the Administration of Criminal Justice Act 2015 are instead detained in special custody pending the Governor’s pleasure. It is also pertinent to note that Section 289 of the Administration of Criminal Justice Act empowers the medical officer of an asylum to certify that a detainee is no longer a danger to himself or the society, but the Attorney General is not duty-bound to act on such report.

It has also been argued that the power of the Governor to keep the detainee at his pleasure should be regulated or limited as continuous detention of a person found not guilty by reason of insanity even after he has been declared by a medical officer to be cured and fit to relate with the society amounts to a violation of freedom of movement enshrined in the Constitution of the Federal Republic of Nigeria, 1999 (as amended)[10]


This research has extensively examined the plea of insanity under Nigerian Criminal law buttressing the fact that an act of an accused person must have been of unsound mind at the time the alleged offence was committed, which is clearly the foundation of this plea under criminal liability. Also, according to Section 28 of the Criminal Code, there are three capacities that must be proven to be absent in order for the defence to be successful.

  1. Garner, B., Black`s Law Dictionary (7th Edn, St. paul Minm: Thompson west, 1999)p.797
  2.  [1961] 3 All ER 523,
  3. (2001)FWLR (p52) 2210 SC.
  4. (Unreported) suit no1 KWS/1 /84 delivered on February 26, 1985.
  5. (2004) 6 NWLR (pt. 869) pg. 360
  6. Ibid
  7. (1952)1 WACA 158
  8. (171) N.M.L.R 269
  9. State V. Yusuf Idowu (1971) NNLR p 93
  10. Okonkwo and Naish, Criminal Law in Nigerian; Spectrum Book Limited, p. 154

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