The U.S. Supreme Court's recent death penalty jurisprudence displays the Court's willingness to invalidate the death penalty for certain offenses or classes of offenders, including those with mental retardation and those who were under eighteen at the time of the offense. The Court has noted that the death penalty in these cases constitutes a disproportionate punishment because it fails to adequately serve the two primary goals of the Cruel and Unusual Punishments Clause: retribution and deterrence. Because the cognitive and volitional impairments caused by severe mental illness result in a parallel diminution in culpability and deterrability, severe mental illness is an appropriate next frontier at which to apply the Court's emerging concept of proportionality. Social attitudes have only recently begun to shift toward opposing the death penalty for those with severe mental illness at the time of the offense. Nonetheless, the Court's recent death penalty cases teach that the Court may independently determine that execution of these offenders is a disproportionate punishment if it concludes that executing such offenders does not adequately serve the goals of retribution and deterrence.