The following Supreme Court cases have a bearing on how mental competency issues are handled in state courts.

• Dusky v. United States, 362 U.S. 402 (1960)
  Test of defendant's competency to stand trial is whether he or she has sufficient present ability to consult with his or her lawyer with a reasonable degree of rational understanding and whether he or she has a rational as well as a factual understanding of proceeding against him or her; it is not enough that he or she is oriented to time and place and has some recollection of events.
• Pate v. Robinson, 383 U.S. 375 (1966)
  Test of defendant's competency to stand trial is whether he or she has sufficient present ability to consult with his or her lawyer with a reasonable degree of rational understanding and whether he or she has a rational as well as a factual understanding of proceeding against him or her; it is not enough that he or she is oriented to time and place and has some recollection of events.
• Jackson v. Indiana, 406 U.S. 715 (1972)
  Jackson was committed to pretrial commitment under a state statute that had a more lenient commitment standard, and a more stringent release standard, than those generally applicable to persons not charged with offenses. The Court found that the effect of the standards was to permanently institutionalize Jackson and held that he was deprived of the equal protection of the laws.
• Drope v. Missouri, 420 U.S. 162 (1975)
  The Missouri courts failed to accord proper weight to the evidence suggesting petitioner's incompetence. When considered together with the information available prior to trial and the testimony of petitioner's wife at trial, the information concerning petitioner's suicide attempt created a sufficient doubt of his competence to stand trial to require further inquiry. Whatever the relationship between mental illness and incompetence to stand trial, in this case the bearing of the former on the latter was sufficiently likely that, in light of the evidence of petitioner's behavior including his suicide attempt, and there being no opportunity without his presence to evaluate that bearing in fact, the correct course was to suspend the trial until such an evaluation could be made.
• Ford v. Wainwright, 477 U.S. 399 (1986)
  The Eighth Amendment prohibits states from inflicting the penalty of death upon a prisoner who is insane, and Florida's procedures for determining the sanity of a death row prisoner was not “adequate to afford a full and fair hearing” on the critical issue, and therefore the habeas petitioner was entitled to an evidentiary hearing in the district court, de novo, on the question of his competence to be executed.
• Godinez v. Moran, 509 U.S. 389 (1993)
  The standard of competency for pleading guilty or waiving right to counsel is the same as the competency standard for standing trial: whether the defendant has “sufficient present ability to consult with his lawyer with a reasonable degree of rational understanding” and a “rational as well as factual understanding of the proceedings against him,” Dusky v. United States, 362 U.S. 402 (1960) (per curiam).
• Estelle v. Smith, 451 U.S. 454 (1981)
  Where prior to in-custody court-ordered psychiatric examination to determine competency to stand trial defendant had not been warned that he had the right to remain silent and that any statement made could be used against him at capital sentencing proceeding, admission at penalty phase of capital felony trial of psychiatrist's damaging testimony on crucial issue of future dangerousness violated the Fifth Amendment privilege against compelled self-incrimination; because of lack of appraisal of rights and a knowing waiver thereof, the death sentence could not stand. The defendant’s Sixth Amendment right to counsel was violated as defense counsel was not notified in advance that the psychiatric examination would encompass issue of future dangerousness.
• Medina v. California, 505 U.S. 437 (1992)
  The Due Process Clause permits a State to require that a defendant claiming incompetence to stand trial bear the burden of proving so by a preponderance of the evidence.
• Riggins v. Nevada, 504 U.S. 127 (1992)
  Due process allows a mentally ill inmate to be treated involuntarily with antipsychotic drugs where there is a determination that the inmate is dangerous to himself and others and that the treatment is in his medical interest, but forcing antipsychotic drug on a convicted prisoner is impermissible absent a finding of overriding justification, and at least as much protection as also provided to persons detained for trial. It was error to order that defendant be administered antipsychotic drugs during the course of trial over his objection without findings that there were no less intrusive alternatives, that the medication was medically appropriate, and that it was essential for the sake of defendant's safety or the safety of others.
• Cooper v. Oklahoma, 517 U.S. 348 (1996)
  Oklahoma law presuming defendant is competent to stand trial unless he proves incompetence by clear and convincing evidence violates due process because the rule allows the State to try a defendant who is more likely than not incompetent.
• Sell v. United States, 539 U.S. 166 (2003)
  The Fifth Amendment Due Process Clause permits the government to involuntarily administer antipsychotic drugs to a mentally ill defendant facing serious criminal charges in order to render that defendant competent to stand trial, but only if the treatment is medically appropriate, substantially unlikely to have side effects that may undermine the fairness of the trial, and, taking account of less intrusive alternatives, necessary significantly to further important governmental trial-related interests.
• Indiana v. Edwards, 554 U.S. 164 (2008)
  The United States Constitution permits States to insist that those who are competent enough to stand trial but still suffer from severe mental illness to the point they are not competent to conduct trial proceedings by themselves be represented by counsel.