This paper will discuss the insanity defense in American criminal law and how it is applied by juries. This first part of the paper will examine the background of the defense and the various standards which have been developed by courts to determine whether or not a defendant is criminally responsible for his actions. The second part of the paper will discuss the various factors which influence jurors in their decisions concerning the criminal responsibility of a defendant who has raised the defense of insanity.
The insanity defense goes back to 13th Century England, where a noted legal scholar explained that children and "madmen" could not be held guilty of crimes because they are innocent in their designs and have no will to do harm. A madman "knows not what he does and lacks mind and reason, and is not much removed from a brute." During these times, various legal definitions were used to describe under what conditions a madman could not be held guilty for acts he committed, but the most common definition was the abstract "knowledge of good and evil" test. By the late 17th Century, the insanity defense was a formal part of the law. Although given weight under common law, this particular rule was not always followed, especially since most trials where the defense was asserted were stopped when all parties agreed that the defendant was insane.
After the acquittal of a defendant who killed the the private secretary to the Prime Minister, the issue of an insanity rule was brought up by both Houses of Parliament, and the House of Lords asked fifteen judges to render an opinion as to what such a rule should be. The judges formulated a rule which stated that in order to establish a defense based upon insanity:
[I]t must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he w...