“What any Parent Knows” But the Supreme Court Misunderstands: Reassessing Neuroscience’S Role in Diminished Capacity Jurisprudence

In Miller v. Alabama, the Supreme Court appealed to neuroscience studies concerning the diminished capacities of adolescents to justify leniency in the sentencing of juvenile offenders. Reflecting on the recent proliferation of juvenile proportionality cases, the Court noted “[o]ur decisions rested not only on common sense—on what ‘any parent knows’—but on science and social science as well.” This Article casts a skeptical eye on the legal import of these scientific insights into the adolescent brain for normative evaluations of criminal culpability. Although the studies cited offer little probative value beyond the common sense wisdom about children that “any parent knows,” the Court’s efforts to employ psychiatric data to objectify mitigating criteria have distracted the Justices from analyzing the precise legal relationship between diminished capacity and diminished culpability, while intractably confusing the Eighth Amendment doctrine of proportionality. After analyzing the history of both proportionality review and the diminished capacity defense, this Article cautions that judges should not automatically equate factual findings of neurobiological abnormalities—that merely evidence diminished capacity—with a moral-legal conclusion of lessened culpability. Given the wide applicability of this defense, such reductionist interpretations contravene the principles of moral responsibility, which seek to differentiate culpability among individual offenders. As an alternative means of reconciling the burgeoning role of neuroscience with the established tenets of the criminal doctrine, this Article proposes a novel framework for assessing the mitigating effect of brain science that judges could equally apply to all classes of offenders, including juveniles.