In Brumfield v. Cain, the U.S. Supreme Court ruled in favor of Kevan Brumfield, a Louisiana death row inmate who claimed he was intellectually disabled and therefore ineligible for execution under Atkins v. Virginia (2002). The state courts had denied his claim without holding an evidentiary hearing to assess his intellectual capacity. The Supreme Court held that this denial violated due process as it did not provide him with a fair opportunity to prove his claim before being sentenced to death. In a 5-4 decision, the court concluded that there was reasonable doubt about Brumfield's mental capabilities and remanded the case back down for further proceedings consistent with their opinion.

In the dissenting opinion for Brumfield v. Cain, Justice Thomas argued that the majority had overstepped its bounds by reevaluating factual findings made by state courts and ignoring federal laws requiring deference to those determinations. He contended that there was no reason to believe Brumfield's intellectual disability prevented him from understanding his actions or their consequences when he committed murder. Furthermore, Justice Thomas criticized the majority's reliance on a Supreme Court ruling (Atkins v. Virginia) which prohibited execution of intellectually disabled individuals but left it up to states to determine how this should be assessed - a decision he believed led directly to disagreements like this one about what evidence is necessary for such evaluations.