The right of a defendant to attempt to reduce the charges facing him or her by entering an insanity plea has its basis in the earliest known criminal laws. Greeks and Romans believed that an insanity plea moved a person outside of the normal jurisdiction of everyday criminal laws, although this is because those people perceived insanity as the result of divine action.
Early English common law also provided a framework for a defendant to file an insanity plea after criminal laws were found to be deficient in their handling of the mentally unstable. An attempted assassination of King George III resulted in the first modern criminal laws providing special legal standing for individuals entering an insanity plea.
In May of 1800, James Hadfield failed in an attempt to assassinate the King. Criminal laws in effect in England considered the attempted assassination of the King a case of treason and criminal laws afforded a treasonous defendant with special rights. Hadfield was allowed to retain two lawyers for the treason trial, though criminal laws governing other charges would have required him to defend himself.
Hadfield chose Thomas Erskine, who was the best lawyer in England, to be his chief counsel. Erskine's defense presented the earliest modern example of an insanity plea. Erskine successfully argued that Hadfield was operating under the delusion that he needed to die by another's hand and thought the best way to accomplish this goal was to try to kill the King. Hadfield was found not guilty.
Criminal laws in effect only provided for the detention of dangerous individuals until their next moment of lucidity. There was concern in the Parliament that Hadfield would be released when his delusions faded and that then he would attempt assassination at a later time. As a result of their concern, a bill was presented in Parliament just four days after Hadfield's trial concluded.
The Criminal Lunatics Act changed English criminal laws to provide for the detention of a person who appeared to commit a crime as the result of even temporary insanity, even if he or she did not enter an insanity plea.
Criminal laws in the United States share a similar foundation. Foucha v. Louisiana is a 1992 Supreme Court decision which provided some protection to an individual who is committed to a mental health facility as the result of entering an insanity plea. Prior to this ruling, criminal laws in Louisiana provided for the incarceration of a person who had entered an insanity plea or been found not guilty by reason of insanity in a psychiatric hospital until both the patient's psychosis faded and he or she was no longer a threat to others.
Criminal laws had allowed trial courts handling the appeals of people in psychiatric hospitals to be kept in these facilities, even if these people had been medically deemed sane, if they could be considered a danger to themselves or others by the court. Terry Foucha, the petitioner in this case, had been committed on the grounds that he had a mental illness and was dangerous.
After treatment for the psychosis induced by his dependency on drugs, he continued to be hospitalized on the grounds that he had an anti-social personality. Anti-social personality disorders, however, are not considered mental illnesses nor are they treatable by psychiatrists. The Court ruled that an individual acquitted due to an insanity plea cannot be held for reason that would not also hold an individual who entered a guilty plea.
There are several other laws and rulings that address the rights of an individual who enters an insanity plea. Wainwright v. Greenfield ruled that a defendant invoking his or her Miranda rights does not invalidate his or her ability to enter an insanity plea.
The most broad reaching law governing individuals seeking to enter an insanity plea is the Insanity Defense Reform Act of 1984. This law overturned many of the criminal laws which defined the circumstances under which an insanity plea could be filed by a defendant.
It establishes that the insanity must be severe, and made it the responsibility of the defense to prove that there are grounds for the judge to allow an insanity plea instead of the prosecution's responsibility to prove that the defendant is sane.