In the United States, variances in the insanity defense between states, and in the federal court system, are attributable to differences with respect to three key issues:

Availability: whether the jurisdiction allows a defendant to raise the insanity defense,

Definition: when the defense is available, what facts will support a finding of insanity, and

Burden of proof: whether the defendant has the duty of proving insanity or the prosecutor has the duty of disproving insanity, and by what standard of proof.

In Foucha v Louisiana (1992) the Supreme Court of the United States ruled that a person could not be held "indefinitely" for psychiatric treatment following a finding of not guilty by reason of insanity.

Availability.

In the United States, a criminal defendant may plead insanity in federal court, and in the state courts of every state except for Idaho, Kansas, Montana, and Utah. However, defendants in states that disallow the insanity defense may still be able to demonstrate that a defendant was not capable of forming intent to commit a crime as a result of mental illness.

In Kahler v Kansas (2020), the U.S. Supreme Court held, in a 6–3 ruling, that a state does not violate the Due Process Clause by abolishing an insanity defense based on a defendant's incapacity to distinguish right from wrong. The Court emphasized that state governments have broad discretion to choose laws defining "the precise relationship between criminal culpability and mental illness."

Definition.

Each state and the federal court system currently uses one of the following "tests" to define insanity for purposes of the insanity defense. Over its decades of use the definition of insanity has been modified by statute, with changes to the availability of the insanity defense, what constitutes legal insanity, whether the prosecutor or defendant has the burden of proof, the standard of proof required at trial, trial procedures, and to commitment and release procedures for defendants who have been acquitted based on a finding of insanity.

M'Naghten test.

The guidelines for the M'Naghten Rules, state, among other things, and evaluating the criminal responsibility for defendants claiming to be insane were settled in the British courts in the case of Daniel M'Naghten in 1843. M'Naghten was a Scottish woodcutter who killed the secretary to the prime minister, Edward Drummond, in a botched attempt to assassinate the prime minister himself. M'Naghten apparently believed that the prime minister was the architect of the myriad of personal and financial misfortunes that had befallen him. During his trial, nine witnesses testified to the fact that he was insane, and the jury acquitted him, finding him "not guilty by reason of insanity".

The House of Lords asked the judges of the common law courts to answer five questions on insanity as a criminal defense, and the formulation that emerged from their review—that a defendant should not be held responsible for their actions only if, as a result of their mental disease or defect, they (1) did not know that their act would be wrong; or (2) did not understand the nature and quality of their actions—became the basis of the law governing legal responsibility in cases of insanity in England. Under the rules, loss of control because of mental illness was no defense. The M'Naghten rule was embraced with almost no modification by American courts and legislatures for more than 100 years, until the mid-20th century.