In Mills v. Maryland (1987), the United States Supreme Court ruled that a death sentence cannot be imposed if jurors are required to unanimously agree on mitigating circumstances before considering them in their sentencing decision. The case involved John Mills, who was convicted of murder and sentenced to death by a Maryland court. His lawyers argued that the jury instructions were unconstitutional because they suggested that all 12 jurors had to agree on any particular circumstance before it could be considered as possibly justifying a life sentence instead of capital punishment. The Supreme Court agreed with this argument, ruling 5-4 in favor of Mills and remanding his case for resentencing. This landmark decision clarified how juries should weigh aggravating and mitigating factors during the penalty phase of capital cases, ensuring defendants' rights under the Eighth Amendment's prohibition against cruel and unusual punishment.

In the dissenting opinion for Mills v. Maryland, Justice Rehnquist disagreed with the majority's interpretation of Maryland's capital sentencing scheme and argued that it did not require unanimity among jurors on mitigating factors. He contended that the jury instructions were clear in stating that if a single juror found a particular circumstance to be mitigating, then it should be considered in their final decision-making process. Furthermore, he believed there was no constitutional requirement for such unanimity and criticized the majority for creating new constitutional rules without any historical or precedential support. Additionally, he expressed concern about potential implications this ruling could have on other states' death penalty laws which do not explicitly state whether they require unanimous agreement on mitigating circumstances.