The United States Supreme Court has long determined what criminal punishments violate the Eighth Amendment by asking whether they fall short of the American people’s standard of decency. It has relied mostly on state legislation to reflect what people think is decent. In 1972, Justice Marshall suggested the Court should factor expert knowledge of the actual workings of death penalty systems into its analysis. The Author refers to this approach as “informed decency.” Marshall believed doing so would make the death penalty unconstitutional because the American people would reject it if better informed. This has come to be known as “the Marshall Hypothesis.” Some forty years later, in Hall v. Florida, the Court finally did something akin to what Marshall suggested with regard to a particular feature of the death penalty. The Court relied on the knowledge of professional psychological organizations to find unconstitutional the manner in which Florida determined ineligibility for the death penalty based on intellectual disability. In this Article, the Author explains that if such an informed decency is adopted on a larger scale, and applied to the death penalty itself, current views of experts in science and law would provide strong evidence to find the death penalty violates the American standard of decency and, as a result, the Eighth Amendment.