Monday's 5-4 ruling lets them decide how juries should weigh evidence in sentencing convicts.
In deciding whether a convicted murderer should be sentenced to death or receive life in prison, jurors are asked to perform a balancing test.
On one hand, they must consider evidence of the seriousness and brutality of the crime. On the other, they must weigh factors about the defendant that might somewhat excuse the killer's actions.
But what happens if jurors conclude that the so-called aggravating and mitigating evidence is equally balanced? Can the jury still return a death sentence?
In a 5-to-4 decision upholding the death penalty statute in Kansas, the US Supreme Court on Monday said that states are free to decide for themselves how jurors should weigh conflicting evidence in death penalty cases.
There is no constitutional requirement that aggravating factors presented by prosecutors must outweigh mitigating factors highlighted by defense lawyers, the high court ruled.
Instead, the US Constitution requires only a fair assessment by jurors of both aggravating and mitigating evidence before deciding whether a death sentence is the appropriate punishment.
"Weighing is not an end; it is merely a means to reaching a decision," writes Justice Clarence Thomas in the majority opinion. "The decision the jury must reach is whether life or death is the appropriate punishment."
In a dissent joined by three other justices, Justice David Souter writes that when a jury is unable to draw a clear distinction between aggravating and mitigating evidence, the Kansas law creates an impermissible tie breaker in favor of a death sentence.
"A law that requires execution when the case for aggravation has failed to convince the sentencing jury is morally absurd," Justice Souter writes. "The court's holding that the Constitution tolerates this moral irrationality defies decades of precedent aimed at eliminating freakish capital sentencing in the United States."