There was an interesting note in the newspaper I read about just this issue. The initial idea that you had to be able to determine right from wrong was adjusted to the more broad idea that you not only had to know right from wrong, but must also be able to stop yourself from doing the wrong. Apparently several US states changed back to the stricter definition after the attempted assination of Ronald Reagan.

I feel that one reason for this backlash is the number of cases of people getting off using insanity or mental defect pleas. The twinkie killer in San Francisco comes to mind. In Calgary, a woman was declared not guilty by reason of mental defect and spent less than a year in hospital before being "cured". She didn't know or realise what she was doing. What she did was drive several hours south of Calgary, cross the border, buy a handgun and ammunition, smuggle it back into Canada, go home, phone her ex-husband, meet him in the underground parking garage at her condo building and shoot him five or six times. She was upset because he was an oil executive and divorced her for a younger woman and her lifestyle suffered. She only got 5 or 6 million dollars in the divorce settlement. Funny, if the tables were turned he would have been called a stalker who deliberately hunted down his ex-wife.