Legal and medical insanity are not the same thing. One can have a medical diagnosis of some sort of mental illness, but be legally sane.
The modern insanity defense is based on the M'Naughton rules, which were formulated by Parliament in 1843 after Daniel M'Naughton was acquitted of the murder of Edward Drummond (he thought he was shooting at Robert Peel, the prime minister). It also is based on the fact that insanity was reconsituted medical condition in the early to mid-1800s. You had medical experts testifying about criminal responsibility, which was something new. What wasn't new was the need of judge and/or jury to determine the defendant's intent. The law was (and is) concerned with whether the accused disturbance precluded the capacity to form intent: the "will to do harm." This has to do with having a justice system philosophically designed to punish wrongful will; only intentional behaviour can therefore be punished. Thus a perpetrator who cannot determine the wrongfulness of an act is not fit for punishment. Prior to the nineteenth century, this was characterised in criminal trials by such terms as "derangement," "delirium," a "disordered intellect." In its essence, the stark, raving madman who "didn't know what he was about."
M'Naughton appeared otherwise sane, but claimed that he suffered from monomania on the subject of Robert Peel's government being out to get him. Mental science in the mid-nineteenth century claimed that monomania was a species of mental disease where the individual suffers a pathological preoccupation on one subject only, but appears otherwise of sound mind. This was also represented by the term "moral insanity," a notion developed by a psychiatrist called James Cowles Prichard and was an answer to the motiveless crime. His theory was that the will itself could be diseased: "some ruling passion seems to have entire possession of the mind." The appearance of monomania is also interesting, representing another shift in the way madness was conceptualised. A form of insanity could be recognised by its irrationality, the fact that the act committed had no discernable purpose and the defendant might not appear insane, except, of course, to a trained expert. This time period also correlates to the professionalisation of psychiatry and the rise of forensic psychiatry. Foucault has suggested (not uncontroversially) that the courtroom is really where psychiatrists argued for the importance of psychiatry. They could detect medical conditions that were dangerous to society, but unnoticeable by the lay person, such as monomania and moral insanity. I digress...
In any case, after M'Naughton was acquitted, Parliament tried to limit the scope of the insanity defense. The new criteria for judging insanity was: "at the time of commiting 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 was doing, or as not to know that what he was doing was wrong." US jurisprudence used the M'Naughton rules up until the 1950s. Between the 1950s and the 1980s, the insanity defense went through a few different criterion, which expanded its scope to include concepts like the "irrisistable impulse." The the American Legal Institute became the standard, which said, quite broadly: "if at the time of such conduct as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law." However, after John Hinckley was acquitted of his attempted assassination of Reagan, Congress passed the Insanity Defense Reform Act of 1984, which limited the application of the insanity defense and essentially brought it back to the stricter M'Naughton rules.
(My PhD is on the history of psychiatry and it started life as looking specifically at criminal responsibility and insanity, although it became more expansive than just that)
Last edited by thesilverspear; 07-22-2012 at 12:02 PM.