In the case of Byron Keith Cooper v. Oklahoma, 1995, the U.S Supreme Court ruled that an Oklahoma statute setting a high standard for defendants to prove their incompetence was unconstitutional. The law required "clear and convincing evidence" of mental incompetence in order to halt criminal proceedings against a defendant. This ruling came after Byron Keith Cooper, who had been convicted of first-degree murder and sentenced to death in Oklahoma state court, argued he was denied his right under the Due Process Clause because he could not meet this burden of proof regarding his competency status during trial proceedings. The Supreme Court agreed with Cooper's argument by a vote of 7-2 stating that such requirement violated due process as it risked subjecting mentally incompetent defendants to trial which is fundamentally unfair.

In the dissenting opinion for Byron Keith Cooper v. Oklahoma, Justice Thomas argued that the majority's decision to strike down an Oklahoma statute requiring a higher standard of proof for competency to stand trial was incorrect. He contended that there is no historical basis or precedent in common law supporting this ruling, and thus it should not be considered a fundamental principle of justice protected by due process clause. Furthermore, he pointed out that many states have similar laws on their books without any apparent violation of defendants' rights. The dissent also criticized the majority's reliance on professional psychiatric standards as evidence against the statute's constitutionality; these standards are subject to change and do not necessarily reflect societal norms or legal principles.