Evidence of continued neurobiological maturation through adolescence is increasingly invoked in discussions of youth-focused policies. This should motivate neuroscientists to grapple with core issues such as the definition of brain maturation, how to quantify it, and how to precisely translate this knowledge to broader audiences.

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The study of brain development encompasses evaluation of the structural, functional, and network-level changes that occur across the lifespan, along with the mechanisms that propel these changes (e.g., hormonal influence, experience, and so on). Over the past two decades, there has been an explosion of evidence revealing that despite being roughly equal in size, the brains of human children, adolescents, and adults differ in complex ways. Questions about the pace, timing, and psychological consequences of human neurodevelopment have thus fascinated basic scientists, clinical and applied scientists, and the general public.

Steinberg, 2009a Steinberg L. Discussions in legal and policy communities have also begun to incorporate neuroscientific evidence of immaturity into their arguments. Continued neurodevelopment has been cited in developmentally informed legal considerations such as culpability for criminal behavior and determinations of competence for health-related decision making (). Continued neurodevelopment also implies continuing plasticity, a tenet that supports developmentally timed interventions for health-risk behaviors. It is exciting that basic neuroscience is infiltrating public discourse to guide developmentally informed policies and treatment of youths.

Arguments for (neuro)developmentally informed policy rest on a foundational claim that youths’ brains are “still-maturing,” implying that they differ in some key way from a mature, adult point of reference. However, the complex nature of neurodevelopment itself poses challenges to establishing a point of reference that would indicate when a brain is mature. To complicate things further, there is little agreement among basic scientists on what properties of a brain should be evaluated when judging whether a brain is mature. This lack of consensus could reflect the fact that most neuroscientists are typically focused on the “journey”—the temporal unfolding of a particular development process—more than when a brain reaches a particular “destination.”

Steinberg, 2009b Steinberg L. The challenge of pinpointing the fuzzy concept of maturity is hardly constrained to neuroscience. There is widespread lack of agreement on the age at which individuals should be considered adults (with the associate rights and protections) based on psychological indicators of maturity as well. However, neuroscientific-based evidence of continued maturity is especially (and perhaps excessively) persuasive in shaping thinking in legal and policy spheres (). For example, neuroscientific data indicating continued brain maturation through adolescence was cited in a brief for the Supreme Court case Roper v Simmons, which categorically overturned the death penalty for juveniles. Because neuroscientific evidence is used to promote developmentally informed policy with increased frequency, it has become important for basic neuroscientists to critically examine the concept of “brain maturity” and to consider ways for basic science to improve its translatability on this issue.

How Does a Mature Brain Function? How the brain processes information and orchestrates behavior is central to claims about maturity. Children’s and adolescents’ psychological competencies are changing in a host of functional domains relevant to policy, such as improvements in abstract reasoning and higher-order cognitive skills, and non-linear peaks in reward sensitivity during adolescence. These competencies scaffold on the brain’s developing functional networks, evident in studies demonstrating changes in brain-behavior relationships with age. Cohen and colleagues (2016) Cohen A.O.

Breiner K.

Steinberg L.

Bonnie R.J.

Scott E.S.

Taylor-Thompson K.A.

Rudolph M.D.

Chein J.

Richeson J.A.

Heller A.S.

et al. There has been a recent surge of interest in the brain function of “emerging adults,” individuals approximately 18–22 years old who most societies treat as adults but for whom neurobiological maturation is incomplete by almost any metric. Recently,tested the degree to which the brains of 18–21 year olds functioned more similarly to adolescents or adults while engaging in a regulatory task including threatening cues and threatening contexts. Results showed that in the functioning of key brain areas such as the dorsolateral prefrontal cortex, the 18–21 year olds’ brain activity during threat conditions was more similar to a 13–17 year old reference group than a 22–25 year old reference group. These findings provide convergent evidence for continued neurodevelopment during the 18- to 21-year-old window. Like structural data, functional data can be evaluated relative to an adult reference point. However, developmental changes in brain function can differ from adult brain function in a host of ways that extend beyond whether there is more or less activation in a particular brain region relative to adults. Take for instance neural responses during a complex decision making task. An adolescent group could differ from an adult group in a variety of ways. They could take longer (and require temporally extended neural computations) to arrive at the same choice, they could make a different choice but use the same general neural processes to arrive at that choice, or their decision making could employ an entirely different suite of strategies and neural processes to arrive at either the same or a different choice. Each of these underlying sources of developmental difference could be linked to a different neurodevelopmental pattern in functional data. Poldrack, 2015 Poldrack R.A. Pinpointing what neural signals track shifting behavior is a complex and important topic that is addressed elsewhere (). For the current discussion, the key point is that there is no single progression that encompasses functional maturation. Neural activity intensifies and reduces, varies quantitatively and qualitatively, in linear and nonlinear ways that are both linked to—and independent of—behavioral differences across development. Each of these patterns reflects developmental progress, but the wide range of “journeys” prohibits a simple definition of what emerging brain functional maturity looks like.

Multiple Maturities Somerville et al., 2011 Somerville L.H.

Hare T.

Casey B.J. A key principle that guides determinations about psychological maturity in adolescence and young adulthood is the degree to which contextual factors shape an individual’s behavior. For instance, an adolescent and an adult could achieve an identical level of performance on a cognitive task under certain conditions—say, when free of distraction and when the situation has low emotional arousal. However, if the context is shifted slightly by embedding reward cues in the cognitive task, adolescents’ performance disproportionally shifts compared to adults (e.g.,). Whereas adolescents might have the baseline capability of achieving a certain level of performance, they might not express that capability equivalently across situations. Behavioral research has indicated that adolescent regulatory behavior is challenged more than adults in contexts involving emotion, social evaluation, and reward. The contextual dependency of adolescent behavior implies that there is not one threshold of maturity—rather, there are waves of maturity that shape how influential different contexts are on behavioral performance. A prime example of context-sensitive policy is graduated driving laws. They initially constrain new drivers to highly regulated conditions (e.g., during the day, without peers in the car) and slowly broaden the range of driving contexts as new drivers gain experience. Casey et al., 2016 Casey B.J.

Galván A.

Somerville L.H. Somerville et al., 2011 Somerville L.H.

Hare T.

Casey B.J. How can neuroscience inform the concept of multiple maturities? As described earlier, different brain regions reach adult-like states at different paces and at different ages. The strong influence of emotional and motivational contexts on adolescent behavior is thought to emerge due to normative, biased circuit-level interactions between motivational and regulatory signaling in the brain (). For instance, neuroimaging evidence has accumulated to suggest that functioning of striatocortical circuitry, which integrates signals of valuation, regulatory demand, and action, is biased in adolescents in contexts in which motivational value is high (). As such, a relevant marker of a mature brain might actually be a relative imperviousness to context more than any static pattern of neural activation or connectivity.

Narrowing in on Neurobiological Maturity The work featured in this article highlights the challenges of operationalizing when a brain achieves “maturity.” Some neuroscientists may believe that the very notion of defining brain maturity is a misguided objective, as the brain never stops changing across the entire lifespan. However, seeing that neuroscientific claims are highly influential in shaping policy, neuroscientists’ voices should guide dialog on when a brain plateaus to an adult-like reference state. Let’s imagine considering a brain mature when every index of brain structure, function, and connectivity hits an asymptote. When would an average brain reach this threshold of maturity? From what I’ve reviewed above, the answer might lie sometime between “the 30s” and “never.” This range is remarkably late, given that arguments about reaching maturity tend to focus on the brains and behavioral profiles of individuals in their late teens and early twenties. It is important to acknowledge that claims that the brain reaches maturity earlier (in the early twenties, for instance) are based only on a subset of the available indices of brain maturation. An open question is whether some indices of brain structure and function should be prioritized over others in conversations about brain maturity. One way to answer this question would be to consider the goals of deeming a brain “mature” from a policy perspective. Brain imaging is primarily being used to corroborate evidence from behavioral science that adolescents (and sometimes young adults) are “on the journey” toward achieving a particular suite of behavioral capabilities. Given that these arguments center on psychological development, perhaps measures of brain function in relation to the corresponding psychological domains should be given priority. A focus on brain function would hold an advantage over other measures, because it would allow for estimates to reflect the context dependencies that also characterize adolescents’ behavior. However, one consequence of this framework would be the need to abandon the goal of identifying a single age-of-brain maturity. Rather, there would be a suite of maturity points that reflect different neural systems and different associated behaviors. For example, an individual could reach an age of “baseline cognitive maturity”—the capacity to engage in goal-directed behavior under neutral, non-distracted circumstances, substantially earlier than an age of “cognitive-emotional maturity”—the capacity to maintain goal-directed behavior in the face of competing emotional cues.