Gregg v. Georgia, 428 U.S. 153 (1976) and its companion cases stressed the fact that all of the approved statues required meaningful appellate review. 428 U.S. at 153. The purpose of appellate review is to provide "a means to promote the evenhanded, rational, and consistent imposition of death sentences..." Jurek v. Texas, 428 U.S. 262, 276 (1976). Since then, the Court has reiterated "the crucial role of meaningful appellate review in ensuring that the death penalty is not imposed arbitrarily or irrationally." Parker v. Dugger, 498 U.S. 308 (1991). It is a "crucial protection." Id. While the Court has held that comparative proportionality review is not required by the Eighth Amendment, see Pulley v. Harris, 465 U.S. 37 (1984), some form of meaningful appellate review is required. See also Sochor v. Florida, 504 U.S. 527 (1992).
The Court has also required "close appellate scrutiny of the import and effect of invalid aggravating factors to implement the well-established Eighth Amendment requirement of individualized sentencing determinations in death penalty cases.” Stringer v. Black, 503 U.S. 222, 230 (1992). In cases involving invalid aggravating factors, the Court initially adopted a different analysis depending on whether the sentence had been imposed in a “weighing” or a “non-weighing” state. In a non-weighing state, the aggravating factor or factors that render a defendant eligible for the death penalty play no special part in the sentencing determination. In such a jurisdiction, an invalid aggravator would not taint the sentence unless: (1) it attached the label “aggravating” to factors that are constitutionally impermissible or totally irrelevant to the sentencing process or to conduct that actually should militate in favor of a lesser sentence; or (2) it allowed the jury to hear evidence that otherwise would not have been before it. Zant v. Stephens, 462 U.S. 862, 885-886 (1983); Tuggle v. Netherland, 516 U.S. 10, 12-14 (1995) (per curiam). A weighing state, in contrast, requires the sentencer to arrive at the penalty decision through the process of weighing specified aggravating factors against any mitigating circumstances. When an aggravator was found to be invalid in a weighing state, the death sentence had to be set aside unless consideration of the invalid factor was determined to be harmless beyond a reasonable doubt or the reviewing court independently reweighed the valid aggravator(s) against the mitigating circumstances and found that a sentence of death was still mandated. Clemons v. Mississippi, 494 U.S. 738, 750-51 (1990). See also Parker v. Dugger, supra,.498 U.S. 308 (The state supreme court's affirmance of the death sentence without affording the petitioner appropriate appellate review, either by reweighing all the evidence, or including the evidence of non-statutory mitigating factors, deprived the petitioner of his Eighth Amendment right to individualized sentencing); Sochor v. Florida, supra, 504 U.S. 527 (Florida Supreme Court, having found insufficient evidence to support the separate aggravating factor found by the sentencing judge that the murder was "committed in a cold, calculated and premeditated manner," failed to cure the error when it affirmed the death sentence without either independently reweighing aggravating against mitigating factors or engaging in harmless error analysis).
More recently, the Court found the weighing/non-weighing scheme to be “needlessly complex and incapable of providing for the full range of possible variations.” Brown v. Sanders, 546 U.S. 212, 219 (2006). It therefore announced it would henceforth be guided by the following rule: “An invalidated sentencing factor (whether an eligibility factor or not) will render the sentence unconstitutional by reason of its adding an improper element to the aggravation scale in the weighing process unless one of the other sentencing factors enables the sentencer to give aggravating weight to the same facts and circumstances.” 546 U.S. at 220 (emphasis in original) (footnote omitted).
In addition, the Court has acknowledged that it functions, in some ways, as a court of last resort in capital cases. In Kyles v. Whitley, 514 U.S. 419 (1995), the Court reaffirmed that its "duty to search for constitutional error with painstaking care is never more exacting that it is in a capital case." 514 U.S. at 422 (quoting Burger v. Kemp, 483 U.S. 776 (1987)). In his concurring opinion, Justice Stevens, joined by Justices Ginsburg and Breyer, noted that due to the "current popularity of capital punishment," the Supreme Court's "duty to administer justice occasionally requires busy judges to engage in a detailed review of the particular facts of a case, even though our labors may not provide posterity with newly minted rule of law." Id. at 455. In other words, even if a capital case does not present a "certworthy" issue, the Court may grant review to correct an incorrect lower court decision. See, e.g., Kyles, supra (finding that Court of Appeals erroneously determined that state's failure to disclose exculpatory information did not violate Brady); Yates v. Evatt, 500 U.S. 391 (1991) (Court granted certiorari to review state supreme court's determination that unconstitutional, burden-shifting presumption of malice was harmless); Porter v. McCollum, 130 S.Ct. 447 (2009) (per curiam) (ruling that Florida Supreme Court unreasonably applied clearly established federal law in finding that Porter was not prejudiced by trial counsel’s deficient performance at sentencing phase); Smith v. Cain, 132 S.Ct. 627 (2012) (finding suppressed evidence to be material under Brady).