Great question: The Insanity Defense Reform Act of 1984 reads, in part:
"It is an affirmative defense to a prosecution under any federal statute that, at the time of the commission of the acts constituting the offense, the defendant as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts. Mental disease or defect does not otherwise constitute a defense."
This is a change from previous insanity defense law. And it's pretty tough to prove that a person was unable to appreciate the nature and quality or wrongfulness of his acts. Note that even if a defendant hears voices and honestly cannot resist complying with the voices because of mental illness, so long as the defendant knows his actions are wrong, then the mental illness is not a defense.
Mental competency of an accused to stand trial is a different matter - the pertinent statutory provisions under the Insanity Defense Reform Act of 1984 are 18 U.S.C. §§ 4241, 4246, and 4247.
In short, competency to stand trial is about whether a defendant knows who the players in the system are and what their roles are (i.e. the judge, jury, prosecutor, and defendant's attorney); can understand what he's being charged with and why; and can assist with the his own defense. Thus it makes sense, the law says, to try to restore a defendant's ability to appreciate theses issues when a court finds the defendant not presently competent to stand trial.
The Supreme Court decisions on competency to stand trial, Pate v. Robinson, 383 U.S. 375 (1966), and Drope v. Missouri, 420 U.S. 162 (1975), have little to do with the insanity defense, since the standards are quite different. See Dusky v. United States, 362 U.S. 402 (1960).
Competency to stand trial decisions focus on whether a defendant can appreciate what's going on in the courtroom and with his lawyers and what's at stake. This can change over time.
Insanity defense issues focuses on culpability for the criminal actions, which is in theory fixed at the time of the event.
Licensed in Washington. This is NOT legal advice and you would be foolish to rely upon it, so don't. The information is for general educational purposes. State laws vary and the specific facts of any situation are unique; a tiny detail may have a huge impact on your legal situation. If you have a legal problem, talk to an attorney licensed in your state. My answer does not create an attorney-client relationship.
Sanity and fitness are two separate issues.
I hope that my response was helpful and informative. However, my answer should not be considered or relied upon as a legal opinion, and no attorney-client relationship has been established. If you are interested in retaining an attorney for your matter, please contact me to discuss your situation further at www.MacNeilFirm.com