Summary: Because different areas of the brain mature at different rates, neuroscientists warn that pinpointing a single moment when the brain “reaches maturity” is far more complicated than it often appears.
Source: Cell Press.
Neuroscientists cannot definitively say when your brain becomes a legal adult. Law must draw a line between adolescence and adulthood — a line that varies worldwide from roughly age 10 to the early 20s — but the brain does not mature all at once. Different regions follow different timetables, and many mature over long, overlapping periods. In an Opinion published December 21 in Neuron, Harvard psychologist Leah Somerville explains why current neuroscience tools are insufficient for defining a single, universal age of brain maturity. A critical first step, she notes, is agreeing on what criteria would reliably characterize a “mature” brain.
“In the last 10–15 years, neuroscientific evidence that the brain continues to mature into adolescence has become influential for policymakers, even at the level of the U.S. Supreme Court,” says Somerville, director of Harvard’s Affective Neuroscience & Development Lab. “Neuroscientists are cautious about assigning a single age of maturity because there are many conceptual and practical problems. But policy debates have pulled researchers into these discussions, and that makes it important to consider how to translate neuroscience findings responsibly.”
Research shows clear structural differences between adolescent and adult brains: typical patterns include reductions in cortical gray matter and increases in white matter integrity. However, these changes do not map to one uniform developmental timeline. Individual variability is substantial, and different neural systems mature at different rates. In one large study, several brain regions had not reached a developmental plateau even by age 30, illustrating the prolonged and region-specific nature of maturation.
The brain’s plasticity — its capacity to adapt to experience, form new connections, and reorganize across the lifespan — also means change continues throughout life. Simple measures such as white matter volume or static patterns of connectivity do not provide a fixed baseline that can distinguish maturity from immaturity. For example, one study observed that some 8-year-old brains showed more advanced connectivity measures than some 25-year-old brains, highlighting the substantial overlap and individual differences across ages.
“When people suggest using brain scans or a single number to diagnose whether an individual is mature or immature, neuroscientists are rightly concerned,” Somerville says. “Condensing the complexity of brain development into a single cutoff is problematic. There is extensive behavioral evidence that adolescents often differ from adults on average, but the legal age of 18 has no intrinsic biological ‘magic’ to it.”
Somerville participates in a working group on translating neuroscience at the Massachusetts General Hospital Center for Law, Brain, and Behavior. The group aims to clarify how findings about brain development might inform questions of responsibility, culpability, and legal policy, and to prepare materials that are accessible to legal audiences. The Center also organizes public events, lectures, and panel discussions to foster informed dialogue among researchers, clinicians, lawyers, and the public.
“Our goal is to help policymakers recognize that maturity cannot be treated as a static, one-time event,” Somerville says. “Brain organization and behavior continue to change across adolescence and into adulthood, but many of those changes are gradual and subtle rather than abrupt. Translating neuroscience to policy requires careful attention to variability, context, and the limits of current measurement tools.”
This work was supported in part by the National Science Foundation and the American Psychological Association F. J. McGuigan Early Career Investigator Research Prize for Understanding the Human Mind. The underlying opinion piece is titled “Searching for Signatures of Brain Maturity: What Are We Searching For?” by Leah H. Somerville, published in Neuron (December 21, 2016).
Key points:
- Different brain regions mature at different rates; there is no single neural timetable that marks full maturity for all systems.
- Structural markers such as gray and white matter change throughout development, but they do not provide a definitive cutoff age for maturity.
- Brain plasticity causes continuous change, and individual variability means chronological age is an imperfect proxy for neural development.
- Policy decisions that rely on neuroscience should account for complexity, overlap, and measurement limitations rather than seeking a single biological threshold.
Caputo, J. (Cell Press). “How To Define Maturity When the Brain Never Stops Changing.” NeuroscienceNews. Published December 21, 2016. Article summarizes the opinion piece by Leah H. Somerville in Neuron.
Abstract
Searching for Signatures of Brain Maturity: What Are We Searching For?
Evidence of continued neurobiological maturation through adolescence is increasingly cited in discussions of policies affecting young people. This trend should encourage neuroscientists to address foundational questions such as how to define brain maturation, how to measure it reliably, and how to communicate findings and limitations to broader, nontechnical audiences.