There are circumstances when insanity can be used as a defence to criminal charges, however, successful use of such a defence is rare. Below we explain the insanity defence in more detail, look at how it’s applied in court and address some of the misconceptions around this controversial legal defence.
Written by Jennifer McCloy. Edited by Grant Longstaff. Published 21 August 2026.
What is the insanity defence?
The insanity defence is a legal defence which can be used when the accused was suffering from a mental condition at the time they committed an offence. For such a defence to be used the mental state of the person needs to indicate they didn’t understand what they were doing was morally wrong, or didn’t understand what they were doing, at the time of the offence.
The rules behind this defence were established in the case of R v M’Naughten (1843) and brought into statute by the Trial of Lunatics Act 1883, which provides for a special verdict of not guilty by reason of insanity.
We spoke with Jennifer McCloy, Head of Law Undergraduate Programmes, who explains the insanity defence in more detail.
“Where the accused’s mental state at the time of committing the criminal act is in question, the insanity defence may be used. The court starts from an assumption of sanity. This means the burden of proof is on the defendant to prove on the balance of probabilities that at the time the criminal act was committed they were labouring “under a defect of reason, from a disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know that what he was doing was wrong.”
This has been held to cover three main things. Firstly, that there must have been a recognised mental disorder at the time of the offence. Secondly, the defendant must establish that they did not know that his act was against the law. Thirdly, they must establish that they did not know that the act was morally wrong, which has been held to mean wrong “by the standards of ordinary people”.”
The Trial of Lunatics Act 1883 was in place until it was replaced by the Criminal Procedure (Insanity) Act 1964.
Can someone be held responsible for an act they committed while unable to understand reality?
Jennifer said: “In order to be found guilty of a crime, an individual must be shown beyond reasonable doubt to have committed the act which is prohibited by law, known as the actus reus. This on its own is not enough. It must also be proven beyond reasonable doubt that the individual also had the intention to carry out the crime, which is known as the mens rea.
An insanity defence can be applied where it can be shown that the accused was suffering from a mental disorder that rendered them incapable of understanding the nature and quality of their actions or knowing that their actions were wrong. The outcome of the successful application of the defence is that someone who may have committed an act which is deemed to be an offence is nevertheless considered not to be criminally liable because they lacked the required mens rea.
If the insanity defence is successfully argued, the defendant will be found Not Guilty by Reason of Insanity. This means that they are held not to be criminally responsible for their actions. Instead of being sent to prison, it’s likely they will be made subject to a hospital order, which may involve them being detained in a secure setting for psychiatric treatment.”
Common myths surrounding the insanity defence
Is the insanity defence easy to prove?
“The defence is not often successfully used,” Jennifer explains. “Proving that they did not know that the criminal act they were committing was morally wrong places a high bar for defendants.”
A discussion paper by The Law Commission, which made provisional proposals for reform of the defences of insanity and automatism, supports this, reporting there are typically under 30 successful insanity pleas each year. They are also preparing to examine the defence of insanity as part of its 14th Programme of Law Reform.
Does a successful insanity defence mean the perpetrator “gets away with their crime”?
Jennifer said: “Even though the individual won’t go to prison, they are unlikely to walk away from the court. If their mental disorder persists at the time of trial and they are considered to be a danger to themselves or others, a hospital order would probably be used. This means that the individual would be detained in a secure setting for treatment until they no longer meet the criteria for detention. In practice, this can mean they are deprived of their liberty for much longer than might have occurred had they been sentenced to imprisonment.”
Challenges and controversies to the insanity defence
There are frequent legal and ethical debates about the balance between protection of society and the rights of the mentally ill. We asked Jennifer for her opinion on some of the questions which appear during such debates. For instance, what if the offender knows they are doing wrong, but believes they have no choice?
R v Keal [2022]
“The case of R v Keal [2022] involved a man who attacked his family members. While carrying out the attacks he said, “I know I’m sorry I don’t want to, I’m sorry dad” and while attacking his mother he said, “I’m sorry this isn’t me it’s the devil”. Following his arrest he was diagnosed with psychosis and delusions including a belief he was possessed by the devil.
At trial, the court held that the defence of insanity was not available to someone who knew what he was doing was wrong, but believed he had no choice but to commit the act. The court stated that the current law on insanity could not be interpreted as involving an element of choice. This would represent a significant change which could only be implemented by Parliament.”
Conflict with the UN Convention on the Rights of Persons with Disabilities
“The UN CRPD is a human rights treaty signed by the UK in 2009. The aim of the Convention is to ensure that people with disabilities have full and effective inclusion in society and are not subject to discrimination. It has been suggested that Article 12 of the Convention requires the abolition of any criminal defence which relieves or mitigates liability because of a disability. The key argument is that the existence of the defence fails to respect the choices of a person with a disability and deprives them of the right to be held accountable for their behaviour.”
Lack of a philosophical basis for the insanity defence
Jennifer references a recent article in Medicine, Science and the Law, the official journal of the British Academy for Forensic Sciences (BAFS), written by Nicholas Hallett as a source of discussion.
“Hallett argues that the insanity defence effectively extinguishes the accused’s moral responsibility for the crime alongside their criminal responsibility, and that there is no coherent philosophical basis for the defence. One suggestion to reform the defence is to add a volitional element to the defence, meaning that the defence could only apply where there was a near-total loss of control caused by the mental disorder.”
Despite being uncommon, the insanity defence highlights the complexities and intricacies of the legal world and makes for fascinating discussion.
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