The Mental Illness Defence in Criminal Trials

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In December 2012, elderly man Daniel Dunn killed his wife by hitting her with a hammer at their home.

The South Australian Supreme Court recently found that Mr Dunn was ‘mentally incompetent of murder’ as he was suffering from a major depressive disorder with psychotic features at the time of the attack.

Dunn was ordered to reside indefinitely at an aged care facility, and faces a lifelong psychiatric supervision order.

His case is an example of how the mental illness defence operates within the criminal law.

Though mental illness has become increasingly prevalent in contemporary society, the law in relation to mental illness has remained stagnant for some time.

In a report earlier this year, the Australian Law Reform Commission recommended a review of the mental illness defence.

What does the law currently say?

In New South Wales, section 38 of the Mental Health (Forensic Provisions) Act 1990 states that where evidence is given in court which shows that, at the time of the offence, a person was suffering from a mental illness and was not responsible for their actions, the jury must find them ‘not guilty’ by reason of mental illness.

Section 38 is based upon the long accepted premise that a person cannot be found guilty of a criminal act where they lack the mental capacity or reasoning to understand that what they were doing was wrong.

Interestingly, the Mental Health (Forensic Provisions) Act does not contain a definition of ‘mental illness.’

Instead, in determining whether or not a person suffered from a mental illness at the time of an offence, the court must turn to the very old case of M’Naghten.

M’Naghten, a case which originated in England in the mid-19th century, classified a mental illness as ‘a defect of reason, from disease of the mind, as not to know the nature and quality of the act [the defendant] was doing; or, if he did know it, that he did not know he was doing what was wrong.’

Courts have held that a ‘defect of reason, from disease of the mind’ refers to a ‘disease, disorder or disturbance.’

The condition may be ‘temporary or long standing, curable or incurable.’

Potential for reform

If you feel that the M’Naghten definition is complicated and outdated, you’re not alone.

Late last year, the New South Wales Law Reform Commission recommended an overhaul of the old M’Naghten test to bring it in line with modern understandings of mental illness.

The Commission also recommended that the new test be embodied in the Mental Health (Forensic Procedure) Act to promote clarity of the law and remove the ambiguities of the old test.

The test proposed by the Commission reads as follows:

‘A person is not criminally responsible for an offence if, when carrying out the conduct required for the offence, the person was suffering from a mental health impairment or a cognitive impairment that had the effect that the person:

(a) did not know the nature and quality of the conduct

(b) did not know that the conduct was wrong, that is, the person could not reason with a moderate degree of sense and composure about whether the conduct, as perceived by reasonable people, was wrong, or

(c) was unable to control the conduct.’

The proposed test rectifies several problems with the old M’Naghten test.

Firstly, it couches the test in modern and acceptable terms. A key problem with the old test was that it was phrased in a way that was ‘outdated and offensive’ in a modern context, and did not reflect scientific or medical definitions.

In particular, the phrase ‘disease of the mind’ could potentially be perceived as insulting and disparaging to people who suffer from mental health conditions.

There have been considerable advances in our understanding of mental illness since the case of M’Naghten was heard way back in 1843.

It’s therefore only fitting that modern legal principle should reflect contemporary understandings of mental illness.

A second recommendation was the establishment of a Forensic Working Group; composed of members of the Mental Health Review Tribunal,

Justice Health, Corrective Services, the Ministry of Health and other relevant organisations to support mentally ill persons in the system.

It was also recommended that the Group be made responsible for drafting an action plan for the detention, care and community support of mentally ill people.

Finally, the Commission made recommendations for the care and management of persons under the Act.

Under the current law, people who are found ‘not guilty by reason of mental illness’ may be detained indefinitely, or until the court is satisfied that members of the public will not be harmed by their release.

The potential for indefinite detention has arguably resulted in people being deterred from using the defence. In 2011/2012, for example, there were only 29 findings of ‘not guilty by reason of mental illness.’

The new recommendations are much more progressive, and propose a period of treatment of the mentally ill person, after which attempts would be made to reintegrate them into the community.

It is hoped that by making the test less restrictive, those who might otherwise be reluctant to rely on the defence may be more inclined to accept it and receive much-needed treatment for their illnesses – thereby reducing the likelihood of further offending.

While the recommendations have not yet been adopted, they represent a positive step towards further recognising the rights of mentally ill people in our society.

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About Ugur Nedim

Ugur Nedim is an Accredited Specialist Criminal Lawyer and Principal at Sydney Criminal Lawyers®, Sydney’s Leading Firm of Criminal & Traffic Defence Lawyers.


  1. a.rowley

    I was wondering if you know of any case with similar cases about people with mental illnesses?

  2. Crissy fultz

    My husband just got arrested for committing a crime in which at the time he was accused of the crime he was in a psychotic episode due to ptsd from past trauma that end to him trying to commit suicide on medication his doctor prescribed him for adhd. He tried overdosing to die because he struggles with deep depression and anxiety disorder and has ben showing signs for over a year now of b
    Having Schizophrenia . He was in a complete drug induced Psychosis. He absolutely at the time didn’t know right from wrong and has little memory of the incident. Do you know if he can get treatment in a hospital or drug program versus going to prison?

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