In the case of Scott Louis Panetti v. Nathaniel Quarterman, 2006, the U.S Supreme Court ruled that a death row inmate must have a rational understanding of why he is being executed in order to be considered competent for execution. The court held that it was not enough for an inmate to merely be aware of his impending execution and its reason; instead, he must rationally understand the connection between his crime and punishment. This ruling came after Panetti, who had been diagnosed with schizophrenia before committing murder in 1992, attempted to represent himself at trial while exhibiting clear signs of mental illness. He later appealed his death sentence on grounds that he did not fully comprehend why he was being put to death due to delusions caused by severe mental illness.

In the dissenting opinion for Scott Louis Panetti v. Nathaniel Quarterman, Justice Clarence Thomas, joined by Justice Antonin Scalia, argued that the majority had overstepped its bounds in interpreting federal law to create a new standard for competency in capital cases. They contended that Panetti's claim was procedurally defaulted and should not have been considered by the court at all. Furthermore, they disagreed with the majority's interpretation of Ford v. Wainwright (1986), arguing it only required a basic understanding of one’s impending execution and why one is being executed - criteria which Panetti met according to them despite his severe mental illness. The dissenters also criticized the majority for failing to provide clear guidance on how lower courts should apply this new standard going forward.