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Federal and state court decisions over the past year are reshaping the contours of juvenile justice litigation. At the federal level, the Supreme Court’s recent decision in Jones v. Mississippi left intact the Court’s current commitment to treating age 18 as the dividing line between youth and adult criminal sentencing. If a youth commits a crime at age 17 years, 364 days, 23 hours, 59 minutes, and 59 seconds old, that youth cannot be put to death or receive mandatory life without parole (LWOP). One second later, these constitutional protections disappear. Calling into question this line drawing, litigants across the country are actively leveraging neuroscientific research to argue that emerging adults ages 18 through early 20s should receive the same constitutional protections as those under 18. While federal courts have not been receptive to this argument, some state courts are. Groundbreaking recent cases in Washington, Illinois, and Massachusetts state courts may signal a potential path forward. In light of these many recent developments, this Essay provides the first empirical analysis of how courts are receiving the argument to raise the age for constitutional protections and introduces a publicly accessible, searchable database containing 494 such cases. The data suggest that at present, Eighth Amendment arguments to categorically extend federal Miller protections to those 18 and above are unlikely to win. At the same time, however, state constitutions and state-level policy advocacy provide a path to expand constitutional protections for emerging adults. We discuss the implications of these trends for the future use of neuroscientific evidence in litigation concerning the constitutionality of the death penalty and LWOP for emerging adults. As this litigation moves forward, we recommend further strengthening connections between litigants and the scientific and forensic communities. Whether at the state or federal level, and whether in courts or legislatures, the record should contain the most accurate and applicable neuroscience.

Because punishable guilt requires that bad thoughts accompany bad acts, the
Model Penal Code (MPC) typically requires that jurors infer the mental state of a
criminal defendant at the time the crime was committed. Specifically, jurors must
sort the defendant’s mental state into one of four specific categories—purposeful,
knowing, reckless, or negligent—which will in turn define both the nature of the
crime and the degree of the punishment. The MPC therefore assumes that ordinary
people naturally sort mental states into these four categories with a high degree of
accuracy, or at least that they can reliably do so when properly instructed. It also
assumes that ordinary people will order these categories of mental state, by
increasing amount of punishment, in the same severity hierarchy that the MPC
prescribes.
The MPC, now turning fifty years old, has previously escaped the scrutiny of comprehensive
empirical research on these assumptions underlying its culpability architecture.
Our new empirical studies, reported here, find that most of the mens rea
assumptions embedded in the MPC are reasonably accurate as a behavioral matter.
Even without the aid of the MPC definitions, subjects were able to distinguish regularly
and accurately among purposeful, negligent, and blameless conduct.
However, our subjects failed to distinguish reliably between knowing and reckless conduct. This failure can have significant sentencing consequences for certain
crimes, especially homicide.