Redefining Adulthood: The Legal Debate Over Brain Science

Anisha Gandhi

In 2012, the Supreme Court decided the landmark case Miller v. Alabama, banning mandatory sentences of life without parole for juveniles. [1] The Court ruled that children have diminished culpability, heightened susceptibility to peer pressure, immature development, and an increased ability to change in comparison to adults. [2] They therefore considered life sentence sentencing for juveniles as “cruel and unusual punishment” and unconstitutional under the Eighth Amendment. [3] 

Now, over a decade later, the Massachusetts Supreme Judicial Court (SJC) has built on this ruling by adding a new age category: “emerging adults.” [4] In Commonwealth v. Mattis (2024), the court ruled that mandatory life sentences without parole for individuals aged 18 to 20 violate the Massachusetts Constitution. [5] The decision, grounded in neuroscience, [6] raises fundamental questions about the role of scientific evidence in shaping legal doctrine. While brain development research supports reconsidering sentencing for young adults, using it to set categorical constitutional rules introduces significant legal tensions.

This ruling reflects a trend visible across multiple domains of law. The Supreme Court has increasingly incorporated scientific research into its constitutional analysis—from Brown v. Board of Education (1954), which used psychological studies on segregation’s harmful effects, [7] to Atkins v. Virginia (2002), where neuropsychological evidence-informed prohibitions against executing intellectually disabled individuals. [8] In Roper v. Simmons, the Court incorporated research on the development of adolescent brains to ban the death penalty for juveniles, [9] creating a foundation for Miller and now Mattis. As Justice Kennedy noted in Graham v. Florida, courts’ constitutional analyses must be informed by “developments in psychology and brain science” that show “fundamental differences between juvenile and adult minds.” [10] Courts primarily rely on legal precedent, text, and history to determine constitutional protections. However, the Massachusetts Supreme Court’s approach in Mattis represents a significant evolution of this trend—moving beyond acknowledging scientific evidence as supplementary support to positioning neuroscience as the central justification for extending constitutional protections to a previously unprotected class of defendants. 

However, the Mattis decision highlights inconsistencies in how courts apply scientific evidence. While neuroscience has influenced juvenile sentencing, courts have been more hesitant elsewhere. In Kahler v. Kansas, for example, the Supreme Court ruled that Kansas’s prohibition of the insanity defense did not violate the Eighth Amendment. [11] This selective use of science implies courts prioritize preferred outcomes over consistent methods. As law professor David Faigman explains, courts cite science when it aligns with their views but dismiss it when it does not. [12] The court’s reliance on neuroscience in Mattis without clear guidelines risks setting a precedent that undermines judicial consistency.

The court’s majority opinion emphasized that individuals aged 18 to 20 share many neurological characteristics with younger juveniles, like underdeveloped impulse control. [13] This reasoning effectively applied the logic of Miller v. Alabama beyond the juvenile category, recognizing that the current cutoff age may be inadequate for determining full adult culpability.

The decision raises a critical question: if the law acknowledges that brain development continues till 20, why should we stop there? Research suggests that cognitive maturation extends into the mid-to-late 20s, [14] which challenges the court’s decision to draw a new categorical distinction. The court did not provide a rationale for selecting this age as the new boundary, other than that it represented a reasonable extension of prior juvenile justice principles. There are no guidelines or precedents for extending laws to new age groups, so the potential is unlimited. However, this trend raises concerns about how far the courts might extend these boundaries. Without clear guidelines, there is a risk of continuously pushing the age limit further until the line between juvenile and adult justice becomes indistinct.

Beyond its immediate impact on sentencing, Commonwealth v. Mattis’ ruling signals a shift in how courts balance scientific evidence with constitutional principles. The ruling may also place pressure on legislatures to revisit existing age-based policies. If people aged 18 to 20 lack full adult culpability in sentencing, should they also receive different treatment in areas like housing, employment, military service, or even electoral decision-making? A study on developmental justice and cognitive development from the Fordham Urban Law Journal suggests that the key brain regions involved in long-term reasoning and impulse control continue developing into the mid-20s, a fact that has previously been used to justify raising the minimum age for purchasing alcohol and other high-risk activities. [15] While the right to vote is a fundamental democratic principle, the Commonwealth v. Mattis decision sets a precedent for using scientific research to reshape age-related constitutional rules. If courts begin to reassess legal adulthood based on neuroscience, this could fuel debates over whether the voting age should be raised in light of emerging evidence about cognitive development, potentially influencing future electoral policies.

The Mattis decision marks a key shift in how neuroscience shapes constitutional law, but its approach raises concerns. By using brain development research to extend protections yet arbitrarily stopping at age 20, the court risks inconsistency and legal instability. Moving forward, courts must establish clearer frameworks for integrating science into legal decisions while maintaining judicial coherence. The challenge is not whether neuroscience should inform the law, but how to do so without creating unpredictability. Mattis highlights both the potential and pitfalls of science-driven legal change.