In the 2003 case of Robert James Tennard v. Doug Dretke, Director, Texas Department of Criminal Justice, Correctional Institutions Division, the U.S Supreme Court ruled in favor of Tennard. The issue at hand was whether or not a defendant's low IQ should be considered as mitigating evidence during sentencing for capital punishment cases. In his appeal to the Supreme Court after being sentenced to death by a lower court in Texas for murder charges, Tennard argued that his significantly below-average intellectual functioning was not properly taken into account during sentencing due to an incorrect application of state law by the trial court. The Supreme Court agreed with him and held that any aspect of a defendant’s character or record and any circumstances surrounding an offense can serve as mitigating evidence “so long as the sentencer could reasonably find it warrants a sentence less than death.” This ruling clarified how mitigation is applied in capital punishment cases involving defendants with intellectual disabilities.

In the dissenting opinion for Tennard v. Dretke, Justice Clarence Thomas, joined by Chief Justice William Rehnquist and Justice Antonin Scalia, argued that the majority had misinterpreted precedent in deciding that a low IQ score was relevant mitigating evidence in capital sentencing proceedings. They contended that the Court's precedents did not establish such a broad rule but instead required a more specific connection between an offender’s mental condition and his crime before it could be considered as mitigation. The dissent also criticized the majority for failing to defer to state court judgments on federal habeas review unless they were unreasonable applications of clearly established law as determined by Supreme Court rulings. According to them, this approach undermined states' sovereignty over their criminal justice systems and led to unwarranted reversals of state court decisions.