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Citation: Byrne (1990) 171 CLR 30.

This information can be found in the Textbook: Brown et al, Criminal Laws: Materials and Commentary on Criminal Law and Process in New South Wales, (5th edition, Federation Press, 2011), pp. 548-9.


Background facts

  • Byrne admitted strangling a woman and mutilating her body.
  • He pleaded diminished responsibility, after submitting uniform psychiatric evidence which described him as a ‘sexual psychopath’, although he had normal phases.
  • He was thought not to be insane in the M’Naghten sense, but his sexual psychopathy was described as ‘partial insanity’.

Legal issues


  • Abnormality of mind is defined as 'a state of mind so different from that of ordinary human beings that the reasonable man would term it abnormal' (ie, an objective test).
    • It covers a wide range of activities (which might be abnormal): perception of physical acts and matters, the ability to form a rational judgment as to whether an act is right or wrong, and most importantly the ability to exercise will power to control physical acts in accordance with that rational judgment (ie, the ability to control oneself).
    • This question is determined using medical evidence.
  • If an abnormality is established, it must then be asked if the abnormality is such that it substantially impaired his mental responsibility for his act in the killing?
    • This is a question for the jury (did the abnormality operate in a way which diminishes the responsibility of the accused?).
    • A big emphasis is placed on whether the abnormality operated in a way so that the accused could not (as opposed to 'did not') stop himself from preforming the act.
  • In this case, there was indeed an abnormality (the medical evidence all agreed) and the jury determined the abnormality was so substantial that it diminished his responsibility in this case.
  • Charge was reduced to manslaughter.


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