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Criminal | Defences

Insanity: Criteria

Revision Note | A Level

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  • D must prove, on the balance of probabilities, he was insane at the time of the offence
  • there is a presumption of sanity, so if prosecution wish to raise insanity, must prove D insane beyond a reasonable doubt
  • defence of insanity available for all offences were mens rea is required, excluded for strict liability
  • if defence of insanity successfully argued a special verdict of not guilty by reason of insanity given
  • judge will then make an appropriate order, depending on D’s mental state
  • range available including detention in a hospital, a guardianship order, a supervision and treatment order or an absolute discharge (Criminal Procedure (Insanity and Unfitness to Plead) Act 1991)

M’Naghten rules

  • defence of insanity is derived from a specific case

    M’Naghten (1853)

    • D tried to assassinate Prime Minister, but instead killed V, a civil servant
    • D suffered from extreme paranoia, thought government was persecuting him, D found not guilty of murder due to his mental state
    • D committed to a hospital, not as a result of the verdict, public outcry no law to deal with such situations, House of Lords considered case
  • ruling requires three elements to be proved: must be a defect of reason, as a result of a disease of the mind, which causes D not to know the nature and quality of his act or not to know what he was doing was wrong
  • these are known as the M’Naghten rules

Defect of reason

  • defect of reasoning means D’s powers of reasoning must be impaired
  • based on an inability to use powers of reasoning not merely a failure to do so
  • temporary confusion or absent mindedness does not amount to a defect of reason

    Clarke (1972)

    • D placed some items into her bag in supermarket and left without paying, D said she had no recollection
    • evidence showed D suffering from clinical depression and was diabetic, leading her to be absent minded
    • trial judge ruled that this raised insanity so D plead guilty to theft
    • Court of Appeal found she was merely temporarily absent minded, not insane, not guilty as she did not have the required mens rea

Disease of the mind

  • term disease of the mind is a legal not a medical one
  • law is concerned with the specific question of whether D can be held liable for his act, not his medical condition
  • disease of mind must be a physical one, not brought on by external factors, eg drugs
  • no express definition, case law has developed meaning


  • temporary insanity may be sufficient

    Kemp (1957)

    • D, during a blackout, attacked V, his wife with a hammer causing her grievous bodily harm
    • evidence showed D suffered from arterialsclerosis, which restricted the flow of blood to the brain causing temporary lapses of consciousness
    • court found this was insanity not automatism, held no distinction between diseases of the mind and diseases of the body affecting the operation of the mind, also whether a condition was permanent was irrelevant
    • Devlin: .. temporary insanity is sufficient to satisfy them [M’Naghten rules]...

Source of disease

  • source of disease is irrelevant

    Sullivan (1984)

    • D attacked V, his friend, D charged with GBH
    • evidence showed he was epileptic, having seizure when injury was caused, D argued automatism but trial judge ruled he would direct jury to find not guilty by reason of insanity
    • D plead guilty to ABH, D appealed
    • House of Lords confirmed conviction, source of disease was irrelevant

Reoccurring violence

  • reoccurring violent tendencies could amount to insanity

    Bratty (1963)

    • D strangled V in his car
    • evidence showed D suffered a psychomotor epileptic seizure at the time of the killing, condition means someone can carry out a purposeful act whilst in an unconscious state
    • court found this type of seizure could amount to insanity, it provided a distinction between insanity and automatism
    • Lord Denning: ..It seems to me that any mental disorder which has manifested itself in violence and is prone to recur is a disease of the mind...

Sleep walking

  • sleep disorders may amount to insanity

    Burgess (1991)

    • D wounded V, a friend, when sleepwalking
    • evidence showed D suffered from a sleep disorder and no evidence of an external cause for the sleep walking
    • trial judge ruled evidence sufficient for a finding of not guilty by reason of insanity, Court of Appeal upheld

Defendant’s knowledge

  • under M’Naghten rules, D’s disease of mind must cause him not to know the nature and quality of his act or not to know what he was doing was wrong

Nature and quality

  • term nature and quality refers to physical character of the act
  • D can argue he did not know nature and quality of his act if he can prove that he did not know what he was doing, or appreciate the consequences or circumstances in which he was acting
  • if D can prove he did not know nature and quality of his act, he lacks sufficient mens rea
  • a verdict of not guilty by reason of insanity is given, not an acquittal


  • D may know nature and quality of his act, but not realise it is wrong
  • D may have the necessary mens rea, but because of insanity may not realise it was wrong
  • in these cases D can use the defence of insanity
  • if D knew it was unlawful to kill defence unavailable

    Codere (1916)

    • D murdered V, his wife, by cutting her throat
    • D thought it was a loaf of bread, D argued insanity
    • found he was not able to rely on defence of insanity, because at the time D knew it was unlawful to kill
    • Lord Reading: .. [ If D knows] that his act is morally wrong according to the ordinary standard adopted by reasonable men or that it is legally wrong then it cannot be said that he does not know he was doing what was wrong...

    Windle (1952)

    • D killed V, his wife, by giving her 100 painkillers, V had spoken of her wish to commit suicide and was suffering from a mental illness
    • D gave himself up to police stating.. I suppose they will hang me for this...
    • D argued insanity, defence not accepted, D’s statement suggested he understood act was wrong
    • word wrong means contrary to the law
  • the difference between legally and morally wrong is not recognised

    Johnson (2007)

    • D stabbed V, a neighbour, after forcing his way into his flat
    • evidence stated D suffering from paranoid schizophrenia and hallucinations, two psychiatrists agreed he knew the nature and quality of his act and that it was legally wrong
    • one psychiatrist stated D did not consider his act morally wrong
    • judge ruled the defence of insanity was not available, D guilty of wounding with intent
    • Court of Appeal upheld the ruling, obliged to follow Windle (1952), that wrong means legal wrong
    • court noted an Australian decision which found D could be acquitted even if he knew his act was legally wrong, but believed his act to be morally right
    • .. [court case contained] illuminating passages indicating the difficulties and internal inconsistencies which can arise from the application of the M’Naghten rules if the decision in Windle is correct...
  • definition of wrong remians legal wrong but may be developed through future case law
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