In the 2019 case of McKinney v. Arizona, James Erin McKinney was appealing his death sentence for two counts of first-degree murder committed in 1991. The main issue at hand was whether a judge or jury must consider mitigating evidence related to post-traumatic stress disorder (PTSD) during sentencing. In this case, the trial court did not allow PTSD as a mitigating factor because it didn't directly cause the crime. However, on appeal in 2015, an en banc Ninth Circuit found that Arizona courts had been applying an unconstitutional "causal nexus" test between mitigation evidence and the crime committed when determining sentences. The Supreme Court ruled against McKinney by holding that state law does not require re-sentencing before a jury despite errors made in original sentencing proceedings regarding consideration of mitigating circumstances like PTSD; instead, such issues can be corrected on appeal without necessitating new sentencing trials.

In the dissenting opinion for McKinney v. Arizona, Justice Ruth Bader Ginsburg argued that when a state court makes an error under federal law and then later reweighs the mitigating and aggravating evidence as per Clemons v. Mississippi (1990), it must apply current federal law in doing so. In this case, she believed that meant considering McKinney's post-traumatic stress disorder as a mitigating factor during sentencing, which was not done originally because at the time of his trial in 1993, Arizona courts did not consider such factors unless they had some bearing on the crime committed. She also disagreed with her colleagues' view that there was no significant difference between independent appellate review and judicial fact-finding at sentencing; to her mind, only a jury could weigh aggravation against mitigation in capital cases due to their factual nature.