In 1882, Queen Victoria’s seventh and last assailant, Roderick Maclean, was acquitted on the grounds of insanity. The Queen was livid at the verdict, and, in an extremely unusal move for a constitutional monarch, demanded of her Prime Minister, William Gladstone, that the law be changed. Gladstone complied, and a year later the curious insanity verdict of “guilty, but insane” became law.
That law existed until 1964 in Britain, when acquittal on the grounds of insanity was restored. But in the United States, the verdict of “guilty, but insane” has been revived, and lives on.
After John Hinckley’s aquittal on the grounds of insanity in the 1981 shooting of Ronald Regan caused widespread outrage similar to Victoria’s own in 1882, states across the union tightened up their insanity laws. Five states—Montana, Idaho, Utah, Nevada, and Kansas—abolished the insanity defense altogether. (Nevada later restored it.) And while Kansas allows no insanity verdict at all, Montana, Idaho, and Utah do: “guilty, but insane.”
Source: “The Insanity Defense Among the States,” at FindLaw: http://criminal.findlaw.com/criminal-procedure/the-insanity-defense-among-the-states.html