Given that the rules are completely foreign in an area of law where cultural differences are extremely important, the M'Naghten standard functions far better than might be expected. However, in a study of homicide cases from 1966 to 1972, it was found that for those who showed signs of mental disturbance, the presence of expert testimony at their trials seemed to be strongly related to the judicial outcome. The decision not to call for a court examination seemed to be highly prejudicial to the accused in that he had almost no chance of being acquitted as insane without the help of expert testimony. Yet given the small number of homicide cases which actually come to trial, this should not be an excessive burden to Western Nigeria's psychiatric profession. The Criminal Procedure Code could easily be amended to require mental examination. Those homicides in which witchcraft was involved were likely to result in convictions, although the Western Nigerian judges were not unwilling to acquit for insanity. While distinctive patterns of homicide occurred among those accused identified as mentally ill, they were processed through the courts more slowly and were more likely to be found guilty. Recommendations for change would include the issuance of more detailed reports on the behavior and appearance of the accused in custody, as well as the institution of a background investigation immediately after the crime, which would preserve useful facts for the defense. As long as traditional healers treat more disturbed patients than will ever see a mental hospital, then the current, routine exclusion of such testimony seems unnecessarily prejudicial to the accused. Such fine points as the formulation of standards for irresistible impulse and diminished responsibility must be set out in the courts' opinions so that Western Nigerian case law can develop. Finally, all references to juries should be removed from the official legislation, as juries are never used. Appendixes present case studies and descriptions of the research instruments. Footnotes are provided.