Rhiannon Timson blogs on the Insanity Defence.

Insanity Defence: What it is and how it is used in practice.

In criminal defence, the defences can be made on justification or by excuse, insanity is by excuse highlighting the idea that the defendant cannot be held criminally responsible for their conduct under the circumstances. It is one of the most controversial defences as it can be used to excuse the most abhorrent and immoral conduct and is known as the ‘perfect defence’ in many places as can result in acquittal, hence the US states of Idaho, Kansas, Montana, and Utah do not recognise the Insanity defence.

Despite being one of the most well-known defences in criminal law, the insanity plea is only used in less than 1% of all cases with a 26% success rate, and in 90% of these successful cases the defendant has been previously diagnosed with a mental illness. The reason behind the low usage and success rates is due to the difficulty in proving legal insanity. While many defendants do suffer from mental illnesses which is apparent in their conduct in the circumstances which can be established in evidence produced by a psychiatrist testimony, this differs from legal insanity which is more difficult to prove. The argument behind making a different standard for legal insanity is in order to achieve a criminal prosecution. The difference is that while the goal of a medical diagnosis is to aid the criminal with their recovery of the disorder they are battling, whereas the purpose of criminal law is that the defendant is punished and faces the appropriate consequences for their misconduct, therefore the conduct that the defendant exhibited cannot be excused by this defence if the defendant or society is benefitted by the punishment. The principle of the insanity defence is twofold, this means that the defendant must not have the ability to form criminal intent and does not have control over their own conduct. This is because the defendant does not have the mental capacity to acknowledge that by society standards their actions are unlawful or control their conduct and therefore will presumably repeat their actions, thus treatment is far more appropriate as punishment would not form any kind of deterrent. There a four types of insanity defence, irresistible impulse, M’Naghten, Durham, and substantial capacity.

M’Naghten Insanity Defence:

The M’Naghten defence is named after Daniel M’Naghten and was introduced in England,1843. It was after M’Naghten was under the paranoid delusion that Sir Robert Peel was trying to kill him, which led to the attempted murder of Sir Peel, however he accidentally killed Sir Peel’s secretary, Edward Drummond, and was put on trial. To the nation’s shock, M’Naghten was found not guilty by reason of insanity. Due to the public uproar that was a result of this verdict the House of Lords developed a test for insanity that makes up the foundation of the defence to this day.

The M’Naghten defence is cognitive and rather than focusing on the defendant’s ability to control their conduct, it primary focus is the awareness of the defendant. There are two elements that are required for the defence to apply to the case, firstly, when the defendant committed the felony, they must have been struggling with a ‘mental defect’ (could be known as ‘disease of the mind’ or a ‘defect of reason’). The second element is that the trier of fact must decide that defendant did not know that the nature or quality of the act was criminal or did not know that it was wrong due to the mental defect. The terms ‘disease of the mind’ and ‘defect of reason’, while having different interpreted definitions, they can be loosely held together under the umbrella idea that the defendant is impaired cognitively to the extent that they are cannot understand the nature or quality of the felony they committed, or the act is wrong. This is commonly seen in mental defect/disease cases such as paranoia, schizophrenia, and psychosis.

Different jurisdictions have different awareness levels that the defendant must obtain. The term that the jurisdiction uses determines what the trier of fact must examine, for example, the terms ‘know’, ‘understand’, and ‘appreciate’ are common terms used across the globe. If ‘know’ or ‘understand’ is used, then the trier of fact must be sure of the simple understanding under the attendant circumstances. Alternatively, when ‘appreciate’ is used, the trier of fact will scrutinize the defendant’s emotional state, they may also consider the personality and character traits of the defendant where relevant. The rarity of a case where the defendant is completely oblivious of their actions means that majority of cases submit in their claim that they were unaware that their misconduct was wrong. Again, different jurisdictions use different terms. Some use ‘legally wrong’ as their definition of wrong, meaning the defendant must be unaware that the act is condemned by the law. Whereas others, may use ‘legally and morally wrong’ which means that the defendant must also be unaware that the act is also condemned by societal values. When the defendant claims that the act was a ‘command by god’ is usually the only time ‘morally wrong’ is used autonomously. This is called a ‘deific defence’ and if any evidence is found that the defence is being used as a coverup or an attempt to bypass the law, it becomes apparent that the defendant knew the difference between right and wrong, consequently dismissing the claim of insanity under the M’Naghten case.

Irresistible Impulse Insanity Defence:

This defence has lost popularity in courts massively overtime, with many jurisdictions rejecting it. In some cases, it is easier to prove than the M’Naghten defence which means more mentally disordered defendants reached acquittal.

The defence is similar to M’Naghten in the fact that the first element of the Irresistible Impulse defence is that the defendants has to be suffering with a mental defect. However, the second element brings in the notion of free choice (volition), this means that the defendant has no ability to control their actions even if they have acknowledged that the behaviour is illegal due to the mental defect. This is lighter than the M’Naghten defence, which does not exonerate defendants who are aware of misconduct. However, this defence, is more difficult for the trier of fact who must determine whether the action can be controlled or not for the irresistible impulse jurisdiction.

The Substantial Capacity Test:

The Substantial Capacity test was created in 1962 by the Model Penal code and was adopted by over half the US states and federal government by 1980. It became well known during the 1982 trial of John Hinckley who used the substantial capacity test to claim the insanity defence successfully during the federal trial for his attempted murder of President Ronald Reagan. The public outcry at this verdict led to many of the states, plus the federal government, moving to the more intransigent M’Naghten defence. Furthermore, the jurisdictions that shifted to the M’Naghten defence also switched over the responsibility of providing proof of insanity to the defendant.

Comparable to the other defences, the Substantial Capacity test also is constructed with two elements, the first being that the defendant must be experiencing a mental defect or disease. The second element is a combination of the M’Naghten defence with the Irresistible Impulse defence, where the cognitive standard is supplemented by the volitional idea. Its stated under the Model Penal code as “A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect he or she lacks substantial capacity to either appreciate the criminality of his or her conduct or conform his or her conduct to the requirements of the law.”.

Generally, under the substantial capacity test the establishment of insanity is easier to prove because the elements of the cognitive and volitional standards are scaled to much more flexible and accessible requirements. For example, it only requires the defendant to lack substantial capacity rather than total (M’Naghten). Furthermore, the classification of ‘wrong’ in the substantial capacity test is ‘criminality’, which justifies an act on being legally rather than morally wrong. The test also uses the term ‘appreciate’ rather than know which as previously stated, the jury takes in consideration the defendant’s character and whether it would be admissible to aid the success of the case.

The Durham Insanity Defence:

This form of insanity defence is only in practice in New Hampshire, US, and has been the adopted defence since the late 1800s. Also referred to as the Durham rule or product test, the defence was assumed by the Circuit Court of Appeals for the District of Columbia during the case of Monte Durham v. US, 1954 (linked case 1). It was recited in court as “[A]n accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect.” However, this defence is difficult to apply to other cases since the court failed to provide further detail and description for the definitions of product, mental disease, and mental defect. Consequently, the D.C Circuit rejecting the defence in the US v. Archie Brawner (linked case 2) and superseded by federal statute, title 18 USC (section 17 deals with insanity defence in the form of affirmative defence).

Due to the lack of definitions, the Durham insanity defence relies on proximate cause principles to reach a verdict. Again, like the other defences, Durham is twofold, the first element being the defendant must have a mental defect or mental disease. Although not stipulated, the language used in the judicial opinion suggests using unprejudiced psychological standards rather than zoning in on the defendant’s own personal cognition. The second element is focused on the causation of the act, if the misconduct is ‘caused’ by the mental disease or the mental defect, due to the conditions the act should be absolved.

Proving the Insanity of a Defendant

Just as courts must follow the rule of presumed innocence in order to exercise a fair trial, sanity is also presumed and consequently falls on the defendant to disprove this presumption by providing sufficient evidence to the court. Due to this some states in the US require the prosecution to provide further evidence suggesting the sanity of the defendant to a preponderance of evidence.

After the Hinckley case, many states have altered the Insanity defence to be known as the ‘Affirmative defence’. This requires the defendant to have the responsibility of proving their own insanity to the trier to the degree of preponderance of evidence, However, the federal government and some US states require the defendant to produce ‘clear and convincing evidence’ this is evidence to a higher degree of validity and standard than the preponderance of evidence.

Guilty but Mentally Ill

Another result of the Hinckley case led to some US states adopting a verdict of guilty but mentally ill. If the defendant is not found legally insane but can prove mental illness to preponderance of evidence at the time of the crime, the defendant will not be acquitted but can access mental health treatment simultaneously as being punished for their misconduct in prison.

Diminished Capacity

Alternatively, while only recognised in a minority of jurisdictions, Diminished Capacity differs slightly from Insanity defence and is an imperfect failure of proof defence. It means that for example, a first-degree murder could be reduced to a second-degree murder or manslaughter charge if the court decide the defendant lacks the mental capacity to form the criminal intentions to carry out a first-degree charge. This defence was abolished in California after a case (linked case 3)  known infamously called the ‘Twinkie Defence’ where a San Francisco supervisor, Dan White, successfully used it on his trial for the murders of Harvey Milk (The film called Milk is based on his life and his victories for the gay community of San Francisco) and George Muscone (Mayor of San Francisco). The jury convicted White of voluntary manslaughter instead of first-degree murder after finding evidence of a chemical imbalance in the brain of white due to his excessive diet of twinkies. The public uproar as a result of this highly publicised trial led California to abolish the usage of Diminished Capacity defence and limiting admissible evidence of diminished capacity to solely sentence proceedings.

Syndrome Defence


Very similar to Diminished Capacity, the Syndrome defence means the necessity of intent is invalidated so can function as failure of proof defence in a minority of jurisdictions. Successful use of the Syndrome defence is rare but has been used to excuse a defendants drunk driving and assault and battery of a police officer due to the defendant experiencing premenstrual syndrome.

Mental Capacity to Stand Trial

Different to the insanity defence, mental competence to stand trial is where if the defendant is analysed and reviewed before trial, if found that they are mentally incompetent to stand trial, their trial is put on hold until mental competency is reobtained. In simplified terms, a defendant put on trial must be able to understand his or her charges to be able to assist in his or her own defence. The Model Penal Code states that “[N]o person who as a result of mental disease or defect lacks capacity to understand the proceedings against him or to assist in his own defence shall be tried, convicted or sentenced for the commission of an offense so long as such incapacity endures”. A defendant who is incompetent at the time of the trial is subject to treatment and medication even involuntarily until competence is reobtained.

Disposition of the Legally Insane

Although being found not guilty by reasons of insanity means the defendant is devoid of any criminal record and absolved from any criminal responsibility, they are not allowed to freely return to society. Many states require the defendant to be automatically committed to a mental health treatment facility until sanity is restored. This is also what the Model Penal Code says “[w]hen a defendant is acquitted on the ground of mental disease or defect excluding responsibility, the Court shall order him to be committed to the custody of the Commissioner of Mental Hygiene [Public Health] to be placed in an appropriate institution for custody, care and treatment.”. Alternatively, other states review mental competency after the verdict is returned, if now found mentally sane they are free whereas if they are still found to be affected by the mental defect, they are committed to a treatment facility.

Temporary Insanity

Temporary Insanity is also recognised by many jurisdictions, and the only difference to permanent insanity is the timeframe the defendant is affected for. This means at the time of the offence one of the insanity defences; M’Naghten, Irresistible Impulse, Durham, or Substantial Capacity, must have been recognised, but if by the time the prosecution is concluded, the defendant has regained mental competence they are free to be released.

Links to cases mentioned:

  1. Durham v. US https://scholar.google.com/scholar_case?case=1244686235948852364&hl=en&as_sdt=2&as_vis=1&oi=scholarr
  2. US v. Brawner https://scholar.google.com/scholar_case?case=11189306675234843637&q=U.S.+v.+Brawner&hl=en&as_sdt=2,5&as_vis=1
  3. Dan White Murder Trial (Twinkie Defence)


Glossary of Legal Jargon mentioned:

  • Defendant – A person who has been accused of committing a crime
  • Conduct – The manner in which a person behaves.
  • Abhorrent – Inspiring disgust and loathing.
  • Acquittal – A release or deliverance from the charge of an offence by verdict of a jury, judgment of a court, or other legal process.
  • Testimony – A formal account used as evidence in a court of law.
  • Criminal Prosecution – The institution and conduct of legal proceedings against a defendant for criminal behaviour.
  • Criminal Intent – The intention of one to commit a crime.
  • Trier of fact – A person, or group of people, who determines facts in a legal proceeding, usually a trial.
  • Jurisdictions – A system of law courts.
  • Attendant Circumstances – The elements other than actus reus, mens rea and the result that define the crime, additional facts surrounding the circumstance of a crime.
  • Exonerate – To absolve someone from blame from a crime or wrongdoing.
  • Federal Statute – Laws enacted by Congress with (and in some circumstances without) the approval of the President.
  • Proximate Cause – An event sufficiently related to an injury that the courts deem the event to be the cause of that injury
  • Presumed Innocence – The legal principle that one is considered “innocent until proven guilty”
  • Preponderance of Evidence – the burden of proof is met when the party with the burden convinces the fact finder that there is a greater than 50% chance that the claim is true.
  • Battery – The intentional touching of another person without the consent of that person and without lawful excuse.