Defence of Insanity

• 1. The rules on insanity based on the case of M'NAGHTEN. Where, D want to rely on the defence, three elements must be proved by D on a balance of probabilities:

  • that at the time of committing the act D had a defect of reason; and
  • this defect of reason was the result of a disease of the mind; and
  • this defect of reason caused D not to know the nature and quality of his act, or not know what he was doing was wrong.
• 2. Where D is found to be insane the special verdict is 'not guilty by reason of insanity.'
• 3. A defect of reason means that D was unable to reason at the time he acted. Temporary absent mindedness or confusion is not enough: CLARKE.
• 4. Ds defect of reason must be the result of a disease of the mind.

  • The disease can be a mental disease. E.g. schizophrenia, paranoia, manic depression.
  • The disease can be a physical disease which affects the mind. E.g. brain tumours, diabetes and sleep disorders.
  • Where Ds defect of reason was the result of an internal factor, then it is a disease of the mind. In HENNESSY failure to take insulin meant it was his diabetes which caused his defect of reason. This was an internal factor. The correct defence was insanity.
• 5. D must not know the nature and quality of his act, or not know he was doing is wrong.

  • Does not know what he did was legally wrong: WINDLE