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.

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 (Steinberg, 2009a). 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.”

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 (Steinberg, 2009b). 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.

What Properties of a Brain Deem It Mature?

In the neurodevelopmental literature, a given neural measurement is typically interpreted as mature when it matches (to a sufficient degree) an “adult” reference. However, brain maturation is a multi-layered process that does not map on to a single developmental timeline. On the gross structural level, the developing brain exhibits reductions in cortical gray matter and increases in the volume and anisotropy of white matter from childhood to adulthood (Giedd et al., 1999). Although the field continues to refine its understanding of the cellular-molecular mechanisms underlying gross changes observable with magnetic resonance imaging (MRI), these changes are broadly thought to reflect synaptic pruning, myelination, and increased connectivity across widely distributed brain circuitry.

Longitudinal studies have been particularly informative in charting trajectories and points of asymptote in neurodevelopment. They show that reductions of cortical gray matter and increases in white matter continue to actively change well into the twenties and that a point of stability emerges earlier in some brain structures than others. Generally, regions of association cortex including the prefrontal cortex show particularly late structural development, whereas subcortical and occipital regions asymptote substantially earlier (Ostby et al., 2009, Tamnes et al., 2010; see Figure 1A). However, structural development continues to progress for a surprisingly long time. One especially large study showed that for several brain regions, structural growth curves had not plateaued even by the age of 30, the oldest age in their sample (Tamnes et al., 2010; see Figure 1B).

Figure 1

Figure 1. Regional and Methodological Variance in Neurodevelopmental Indices(A) Trajectories of cortical gray matter volume adjusting for total brain volume. Trajectories are schematized from data reported in Ostby et al. (2009).(B) Ages of developmental asymptote for connectivity and structural data. Resting-state functional connectivity (rsfMRI) data from Dosenbach et al. (2010) and the other measures reflect data reported in Tamnes et al. (2010). Note that the operationalization of “asymptote” varies by study.

Other work focused on structural brain measures through adulthood show progressive volumetric changes from ages 15–90 that never “level off” and instead changed constantly throughout the adult phase of life (Walhovd et al., 2005). Thus, a key challenge to classifying maturity based on structural indices is that it is ambiguous when an adult reference reaches a steady set-point—it depends on the type of anatomical measurement and the lobe or brain region selected. Moreover, it is unclear whether there is even a steady set-point at all.

Another maturing feature of the brain is the intrinsic patterns of connectivity that comprise brain networks. Measures of widespread brain connectivity shift in complex ways from childhood to adulthood, characterized by reductions in local connections and rises in distributed connections. These connectivity-based shifts are thought to reflect a brain that is becoming more efficient in its in-network communication and more integrated in its cross-network communication (Fair et al., 2009).

Dosenbach and colleagues (2010) used data-driven classification algorithms to compute an estimated “brain age” of individual subjects 7 to 30 years of age based on widespread intrinsic connectivity patterns within and between brain networks, measured using resting-state functional connectivity. Their classification algorithms identified adolescence as a period of rapid and widespread increase in connectivity followed by a slowing rate of change until approximately age 22, which was identified mathematically as the point of asymptote. This work suggests that widespread network connectivity measures settle into a fairly consistent reference state in the early 20s. However, these data also illustrate the challenges of applying general patterns of neurodevelopment from group-based to individual inference, as there is substantial variance in brain network connectivity that is unrelated to age. For example, some 8-year-old brains exhibited a greater “maturation index” than some 25 year old brains.

This section has described the neurodevelopmental trends of just two (structure, intrinsic connectivity) of several levels of brain maturation. Other neurodevelopmental processes include neurochemical shifts in neurotransmitter availability and receptor density, brain metabolic efficiency, hormonal change, and excitatory/inhibitory balance. On one hand, there is partial convergence in structural change and intrinsic connectivity, in that the maturational asymptotes for both indices extend well past the age of 18 (the legal definition of adulthood in the United States). On the other hand, there is also strong divergence. One could ascribe maturity to a brain based on network connectivity a decade sooner than based on some structural indices (see Figure 1B). Further, demonstrations of constant change in structure throughout adult life challenge the very notion that the brain reaches a steady adult referent that we can concretely call “mature.”

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.

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, Cohen and colleagues (2016) 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.

Pinpointing what neural signals track shifting behavior is a complex and important topic that is addressed elsewhere (Poldrack, 2015). 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

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., Somerville et al., 2011). 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.

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 (Casey et al., 2016). 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 (Somerville et al., 2011). 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.

Concluding Recommendations

It is exciting that dialogue about neuroscience is infiltrating policy considerations for youth. Likewise, neuroscientists can consider how to improve the translatability of their basic research. New large, multimodal brain imaging studies (such as the Adolescent Brain Cognitive Development study http://abcdstudy.org and the Human Connectome Project in Development) will bring forth unprecedented opportunity to pinpoint the timing of healthy brain development. These studies will provide test-beds for establishing intricate models of the pacing and interrelationships between brain structural, functional, and network development across several functional domains. In time, these large datasets could allow for the creation of multimodal “growth curves” which can be linked to behavioral profiles of interest to policy.

What can be done in the meantime? For one, many studies comparing adolescents to young adults frequently use an age of 18 as a cut-point for comparison between “adolescents” and “adults,” an approach that could obscure or even mask continued developmental change. Researchers could instead avail themselves of the nonlinear and growth curve modeling methods that allow for observation of full trajectories of change. Further, developmental studies frequently truncate “adult” samples at age 22 or even younger—typically too early to document points of asymptote in a particular neural process. Studies that do this might fail to capture the “leveling off” pattern that is thought to characterize mature brain function. Finally, given that behavior arises from complex circuit interactions in the brain, measures of functional brain activity in single brain regions should be supplemented with measures of brain functional connectivity and multimodal methods to identify interrelationships between brain structure, network organization, and function. These approaches will provide a more comprehensive view of the complex suite of mechanisms underlying brain maturation.

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Neurobiological Maturity: A Complex and Moving Target

The study of brain development, a field encompassing structural, functional, and network-level alterations across the lifespan, has witnessed significant advancements. Notably, substantial evidence highlights the complex differences between the brains of children, adolescents, and adults despite their comparable sizes. These findings have ignited interest from scientists and the public alike, prompting inquiries into the pace, timing, and psychological ramifications of human neurodevelopment.

Increasingly, legal and policy discussions incorporate neuroscientific evidence of ongoing brain development in adolescents. Arguments concerning culpability for criminal actions and competency in healthcare decision-making now consider the implications of continued neurodevelopment (Steinberg, 2009a). This recognition of ongoing development, often extending into early adulthood, suggests enduring neural plasticity and supports developmentally tailored interventions for health-risk behaviors. The integration of basic neuroscience into public discourse to guide such policies and interventions is a promising development.

Arguments advocating for developmentally informed policies often rely on the assertion that youths' brains are "still maturing," implying a significant distinction from a mature, adult state. However, the intricate nature of neurodevelopment presents challenges in defining this adult reference point for brain maturation. Further complicating matters is the lack of consensus among scientists regarding the specific brain properties indicative of maturity. This discrepancy likely stems from the research focus on the "journey" of developmental processes rather than on pinpointing a specific developmental "destination."

While the challenge of defining maturity extends beyond neuroscience, neuroscientific evidence of ongoing brain development carries substantial weight in legal and policy realms, sometimes disproportionately so (Steinberg, 2009b). The influence of such evidence is exemplified in its use in the Supreme Court case Roper v. Simmons, where neuroscientific data on continued brain maturation during adolescence contributed to the categorical overturn of the death penalty for juveniles. Given the increasing reliance on neuroscientific evidence in shaping developmentally informed policy, critically examining the concept of "brain maturity" and enhancing the translatability of basic research findings become essential for neuroscientists.

What Properties of a Brain Deem It Mature?

Neuroscientific research often equates neural maturity with the attainment of an "adult" reference point. However, brain maturation is a multifaceted process that defies a singular developmental timeline.

Structurally, the brain undergoes a decrease in cortical gray matter and an increase in the volume and anisotropy of white matter from childhood to adulthood (Giedd et al., 1999). These macroscopic changes, observed through magnetic resonance imaging (MRI), are generally attributed to synaptic pruning, myelination, and enhanced connectivity across brain circuitry.

Longitudinal studies have provided valuable insights into the trajectories and asymptotic points of neurodevelopment. These studies reveal that reductions in cortical gray matter and increases in white matter persist into the twenties, with different brain structures reaching stability at varying rates. Generally, association cortices, including the prefrontal cortex, exhibit protracted structural development, whereas subcortical and occipital regions reach asymptote earlier (Ostby et al., 2009; Tamnes et al., 2010; see Figure 1A). Notably, structural development continues for a surprisingly extended period. One large-scale study demonstrated that structural growth curves for some brain regions had not plateaued even by the age of 30, the oldest age included in their sample (Tamnes et al., 2010; see Figure 1B).

Figure 1. Regional and Methodological Variance in Neurodevelopmental Indices(A) Trajectories of cortical gray matter volume adjusting for total brain volume. Trajectories are schematized from data reported in Ostby et al. (2009).(B) Ages of developmental asymptote for connectivity and structural data. Resting-state functional connectivity (rsfMRI) data from Dosenbach et al. (2010) and the other measures reflect data reported in Tamnes et al. (2010). Note that the operationalization of “asymptote” varies by study.

Research focusing on structural brain measures throughout adulthood reveals continuous volumetric alterations from ages 15 to 90, indicating ongoing changes rather than a stable endpoint (Walhovd et al., 2005). This lack of a clear plateau in certain structural measures presents a challenge for defining maturity based solely on structural indices. The ambiguity regarding the existence and timing of a steady set-point further complicates this task.

Beyond structural changes, intrinsic connectivity patterns within brain networks also undergo significant development. From childhood to adulthood, these patterns shift, characterized by reductions in local connections and increases in distributed connections. These connectivity-based shifts are thought to reflect a brain that is optimizing in-network communication efficiency and enhancing cross-network integration (Fair et al., 2009).

Using data-driven classification algorithms, Dosenbach et al. (2010) estimated the "brain age" of individuals aged 7 to 30 based on resting-state functional connectivity patterns. Their findings highlighted adolescence as a period of rapid connectivity increases, followed by a gradual slowing of change until approximately age 22, mathematically identified as the asymptote. While this research suggests a relatively stable connectivity reference state emerging in the early twenties, it also underscores the challenges of applying group-level developmental patterns to individual cases. Significant variability in brain network connectivity exists that is unrelated to age, as demonstrated by the observation that some 8-year-olds exhibited higher "maturation indices" than some 25-year-olds.

The preceding discussion highlights only two facets of brain maturation: structure and intrinsic connectivity. Numerous other neurodevelopmental processes, including neurochemical shifts, metabolic efficiency changes, hormonal fluctuations, and excitatory/inhibitory balance adjustments, contribute to overall brain development. While partial convergence exists between structural and connectivity maturation, with both extending beyond the legal age of adulthood in many societies, there is also notable divergence. Defining maturity based solely on network connectivity might result in assigning maturity a decade earlier than when using certain structural indices (see Figure 1B). Moreover, evidence of ongoing structural changes throughout adulthood challenges the very concept of a static adult reference point for "maturity."

How Does a Mature Brain Function?

Central to claims regarding maturity is the brain's capacity to process information and regulate behavior. Throughout childhood and adolescence, psychological competencies relevant to policy considerations, such as abstract reasoning, higher-order cognitive skills, and reward sensitivity, undergo significant development. These advancements are intertwined with the maturation of functional brain networks, as evidenced by age-related changes in brain-behavior relationships.

Recent research has focused on the brain function of "emerging adults," individuals aged 18–22 who are often treated as adults despite incomplete neurobiological maturation. Cohen et al. (2016) investigated the extent to which the brains of 18–21-year-olds functioned more similarly to adolescents or adults during a regulatory task involving threat cues and contexts. Their findings revealed that during threat conditions, brain activity in key areas, such as the dorsolateral prefrontal cortex, in the 18–21-year-old group resembled that of the 13–17-year-old group more closely than the 22–25-year-old group. This research provides compelling evidence for continued neurodevelopment during this transitional period.

As with structural data, functional data can be evaluated against an adult reference point. However, developmental changes in brain function encompass a broader range of patterns beyond simply increased or decreased activation compared to adults. For instance, when examining neural responses during a complex decision-making task, differences between adolescents and adults may manifest in various ways. Adolescents may require more time and extended neural computations to reach the same decision as adults, arrive at a different decision using similar neural processes, or employ entirely different strategies and neural processes leading to either the same or a different choice. Each of these developmental variations can be linked to distinct neurodevelopmental patterns in functional data.

Identifying the specific neural signals underlying these behavioral shifts is a complex and crucial area of research discussed elsewhere (Poldrack, 2015). The key takeaway is that functional maturation does not adhere to a single, linear progression. Neural activity demonstrates dynamic fluctuations in intensity, quantity, and quality, both mirroring and diverging from behavioral differences observed across development. While all these patterns reflect developmental progress, their diversity precludes a simple definition of mature brain function.

Multiple Maturities

The concept of "multiple maturities" acknowledges the significant influence of contextual factors on adolescent and young adult behavior. For example, while adolescents and adults may exhibit similar performance levels on a cognitive task under specific conditions (e.g., minimal distractions, low emotional arousal), the introduction of reward cues can disproportionately impact adolescent performance (Somerville et al., 2011). This suggests that although adolescents may possess the baseline capability to achieve a certain performance level, their ability to express this capability consistently across different contexts may be limited. Behavioral research suggests that adolescents face greater challenges than adults in regulating their behavior in contexts involving emotions, social evaluation, and rewards.

The context-dependent nature of adolescent behavior suggests that maturity is not a singular threshold but rather a series of developmental milestones that influence the impact of various contexts on behavioral performance. Graduated driving laws exemplify context-sensitive policies. These laws initially restrict new drivers to highly controlled environments (e.g., daytime driving, no peer passengers) and progressively expand permissible driving contexts as drivers gain experience.

Neuroscience can provide valuable insights into understanding multiple maturities. As discussed earlier, different brain regions reach adult-like states at varying paces. The heightened influence of emotional and motivational contexts on adolescent behavior is thought to arise from normative, biased interactions between motivational and regulatory brain circuits (Casey et al., 2016). Neuroimaging studies suggest that in adolescents, the functioning of striatocortical circuitry, responsible for integrating signals of valuation, regulatory demands, and action, exhibits bias in contexts with high motivational salience (Somerville et al., 2011). Therefore, a relevant marker of brain maturity might be the ability to remain relatively impervious to contextual influences rather than a fixed pattern of neural activity or connectivity.

Narrowing in on Neurobiological Maturity

While the concept of defining a discrete point of "brain maturity" may be deemed arbitrary by some neuroscientists, particularly given the lifelong nature of brain changes, it is crucial to engage in a dialogue about this concept given its impact on policy decisions.

If we define brain maturity as the point at which every structural, functional, and connectivity index plateaus, the attainment of this state might occur somewhere between the thirties and never, a timeframe significantly later than the late teens and early twenties often scrutinized in discussions about maturity. It is essential to acknowledge that assertions of earlier brain maturation, for instance, in the early twenties, rely on a limited subset of available brain maturation indices.

A crucial question arises: Should certain indices of brain structure and function be prioritized over others when defining maturity? Considering the policy implications of deeming a brain "mature" can guide this prioritization. The use of brain imaging in policy largely serves to support behavioral science findings indicating that adolescents (and sometimes young adults) are still developing certain behavioral capabilities. Given this focus on psychological development, prioritizing measures of brain function as they relate to specific psychological domains seems reasonable. Focusing on brain function offers an advantage over other measures by allowing for estimations that account for the context dependencies inherent in adolescent behavior.

However, this approach requires abandoning the pursuit of a single age for brain maturity and instead recognizing multiple maturity points reflecting the development of different neural systems and associated behaviors. For instance, an individual might achieve "baseline cognitive maturity," the ability to engage in goal-directed behavior in neutral, non-distracting environments, significantly earlier than "cognitive-emotional maturity," the capacity to maintain goal-directed behavior in the presence of competing emotional cues.

Concluding Recommendations

The integration of neuroscientific perspectives into policy considerations for youth is a positive development. Neuroscientists, in turn, can contribute by enhancing the translatability of their research findings. Large-scale, multimodal brain imaging studies such as the Adolescent Brain Cognitive Development study (http://abcdstudy.org) and the Human Connectome Project in Development offer unprecedented opportunities to elucidate the timing and interplay between brain structure, function, and network development across various functional domains. These datasets hold the potential to generate multimodal "growth curves" that can be linked to behavioral profiles relevant to policy decisions.

In the interim, several recommendations can be implemented. First, researchers frequently employ age 18 as a cutoff point when comparing "adolescents" and "adults," potentially obscuring ongoing developmental changes. Utilizing nonlinear and growth curve modeling methods would allow for a more comprehensive examination of developmental trajectories. Second, truncating "adult" samples at age 22 or younger, as commonly done, may not capture the "leveling off" pattern indicative of mature brain function. Extending the age range of adult participants would address this limitation. Finally, considering that behavior arises from complex circuit interactions, incorporating measures of brain functional connectivity and employing multimodal methods that integrate brain structure, network organization, and function are essential. These approaches would provide a more comprehensive understanding of the multifaceted mechanisms underlying brain maturation.

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When is the Brain Mature? Policy Implications of Neurodevelopmental Research

Studying brain development involves looking at how the brain changes in terms of its structure, function, and networks throughout our lives. Researchers also investigate what drives these changes, like hormones and experiences. In recent years, we've learned that the brains of children, teenagers, and adults, despite being similar in size, are surprisingly different. This raises important questions about how our brains mature, when they reach full maturity, and how this affects us psychologically.

These questions are important not just for scientists but also for policymakers. Neuroscience research is increasingly used to inform laws and policies related to youth. For example, the idea that teenage brains are still developing has influenced discussions on criminal responsibility and the age of consent for medical decisions. However, the complexity of brain development makes it difficult to pinpoint exactly when a brain becomes mature. Scientists don't even agree on which aspects of the brain are most important for determining maturity.

What Makes a Brain Mature?

In neuroscience, we often compare a particular brain measurement to what's typical for adults to decide if it's "mature." But brain development doesn't follow a single timeline. Different parts of the brain mature at different rates.

For example, the brain's outer layer (the cortex) undergoes changes in gray matter (the thinking part) and white matter (the connections). Gray matter decreases while white matter increases from childhood to adulthood, likely due to processes like synaptic pruning (eliminating unnecessary connections) and myelination (improving the efficiency of connections).

Studies show that these structural changes in the brain continue well into our twenties. Some areas, like the prefrontal cortex (involved in planning and decision-making), develop much slower than others. Surprisingly, some studies even suggest that structural changes might continue past the age of 30!

Beyond structure, the brain's networks also change. Connections within and between different brain regions become more efficient and integrated as we age. Research suggests that these network changes might stabilize in the early twenties. However, there's still a lot of individual variation, meaning some people's brains might appear more "mature" at a younger age than others.

Importantly, structure and network development are just two aspects of brain maturation. Other factors like brain chemistry, metabolism, hormone levels, and the balance between excitation and inhibition within brain circuits are also important. This complexity makes it difficult to pinpoint a single definition for brain maturity.

How Does a Mature Brain Work?

The way our brains process information and control our behavior is central to discussions about maturity. Teenagers show improvements in abstract thinking and decision-making skills, but they can also be more sensitive to rewards and more easily influenced by emotions than adults.

Brain imaging studies show that brain activity patterns during tasks involving thinking, emotions, and rewards differ between teenagers and adults. For instance, research suggests that areas involved in controlling impulses and regulating emotions may not be as developed in teenagers, making them more susceptible to peer pressure or risky decisions when emotions are high.

The important takeaway is that there are many ways to define "mature" brain function. It's not just about how much activity we see in a brain region, but also about how that activity relates to behavior.

Multiple Milestones of Maturity

When psychologists talk about maturity in young people, they emphasize the role of context. Teenagers might perform just as well as adults on a task in a calm and controlled environment. However, their performance might drop significantly if the task becomes emotionally charged or if rewards are involved.

This suggests that there isn't one single point of maturity. Instead, we experience different "waves" of maturity, each making us less influenced by external factors.

Neuroscience can help us understand these multiple milestones. Different brain regions and their connections mature at different speeds. For example, areas involved in processing rewards and emotions mature earlier than those responsible for controlling impulses. This mismatch can explain why teenagers are more likely to engage in risky behavior when they are excited or around peers.

Toward a Better Understanding of Brain Maturity

Although the brain is always changing, it's still important for neuroscientists to provide input on what constitutes a "mature" brain, especially as this information can influence policies affecting young people.

If we were to wait until every single aspect of the brain reached full maturity, we might be waiting until our 30s, or even indefinitely! Therefore, we need to think critically about which brain measures are most important for defining maturity in the context of specific policies or decisions.

Given that discussions about maturity often revolve around behavior and decision-making, focusing on how well brain functions support these abilities might be more useful than focusing solely on brain structure. This approach also acknowledges that a teenager might be "mature" in some contexts (e.g., planning for a school project) but not others (e.g., resisting peer pressure at a party).

Recommendations for Future Research

It's encouraging to see neuroscience research influencing policy decisions related to youth. However, neuroscientists can also improve how their research is applied in the real world.

• Use appropriate comparison groups: When studying brain development, researchers should be cautious about simply comparing "teenagers" to "adults" with an arbitrary cutoff age like 18. This approach might miss important developmental changes that happen in the late teens and early twenties.

• Study a wider age range: Including adults beyond their early twenties in studies is crucial to understand when brain development truly levels off.

• Look beyond single brain regions: Focusing on how different brain regions connect and interact with each other is important to understand complex behaviors and decision-making processes.

Large-scale brain imaging studies will be essential to create more detailed maps of brain development and its relationship to behavior. This will help us create more nuanced policies that consider the different "waves" of maturity and support healthy development in young people.

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When is a Brain "Mature"?

Scientists who study how the brain grows and changes throughout life are fascinated by how our brains, while similar in size, work differently in childhood, adolescence, and adulthood. This knowledge is important not just for scientists but also for making laws and policies that affect young people.

We often hear that young people's brains are "still maturing," suggesting they are different from adult brains. This idea has been used in discussions about how responsible young people are for their actions and their ability to make important decisions, like those related to healthcare. However, this raises a big question: when exactly does a brain become mature?

How Do We Decide if a Brain is Mature?

Scientists often compare young brains to "adult" brains to track development. However, the brain matures in many ways, not just all at once.

• Brain Structure: One way the brain matures is by changing its physical structure. As we grow, the outer layer of the brain (gray matter) actually shrinks while the connections between different brain areas (white matter) grow stronger and more organized. These changes happen at different speeds in different parts of the brain. For example, areas at the front of the brain, important for planning and decision-making, develop much later than areas at the back, responsible for vision. Surprisingly, some studies show that these structural changes continue into our 30s or even later, making it hard to pinpoint when this part of brain development is "done."

• Brain Networks: Another crucial aspect of brain maturation is how different brain areas communicate with each other. As we grow, these connections become more efficient and complex. Think of it like upgrading from a local road network to a superhighway system. While research suggests that these large-scale brain networks settle into a more stable pattern in our early twenties, there are still individual differences. Some young teenagers might show more "mature" brain networks than some people in their mid-twenties.

These are just two examples of how the brain matures. Other changes involve brain chemicals, energy use, hormones, and the balance of brain activity. What's important to remember is that different aspects of the brain mature at different speeds, making it tricky to define a single point of "maturity."

How Does a Mature Brain Work?

The way our brains process information and control behavior is central to the question of maturity. As children grow, they get better at things like abstract thinking, planning, and controlling impulses. This is connected to how different brain areas work together.

For example, young adults (18-21 years old), while legally adults in many places, still show brain activity patterns closer to teenagers than to older adults when dealing with threats or stressful situations. This suggests their brains are still developing the ability to regulate emotions and make decisions under pressure.

It's not just about how much brain activity there is, but also how it happens. For instance, when making a decision, a teenager might use different brain processes or take longer to reach the same conclusion as an adult. These differences in brain function, just like differences in structure, make it difficult to define a single pattern of a "mature" brain.

Different Types of Maturity

It's also important to remember that a person's behavior can change depending on the situation. For example, a teenager might perform just as well as an adult on a test in a quiet room, but struggle when there are distractions or rewards involved.

This means that "maturity" isn't a single finish line but rather a series of milestones influenced by both our biology and our experiences. Think of graduated driving laws: they recognize that new drivers need time and practice in different situations before handling more complex driving scenarios.

So, When Does the Brain Actually Mature?

Given all these complexities, it's hard to say exactly when a brain is "mature." If we wait until every single aspect of the brain reaches its final adult-like state, we might be waiting until someone's 30s or even later!

This means we need to decide what aspects of brain development are most important when we talk about "maturity," especially when making rules and policies for young people.

Instead of looking for a single brain-maturity age, perhaps we should focus on the brain functions most important for the decisions young people are making. For example, "baseline cognitive maturity" might develop earlier, allowing for basic planning and thinking in calm situations, while "cognitive-emotional maturity," which helps us control emotions and make good choices under pressure, might develop later.

Moving Forward

While we still have much to learn about the brain, it’s crucial to use this knowledge carefully when creating policies that affect young people.

Here are a few things scientists can do:

• Study a wider age range: Instead of just comparing teenagers to young adults (often stopping at age 22), studies should include older adults to understand how brain development continues over the lifespan.

• Look beyond single brain areas: Instead of focusing on one part of the brain at a time, scientists need to study how different brain regions connect and work together, giving a more complete picture of brain function.

• Connect brain changes to real-world behavior: It's essential to understand how different patterns of brain activity relate to how young people actually behave in different situations.

By improving how we study and understand brain development, we can make more informed decisions about how to support young people and create policies that are fair and effective.

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Growing Up: How Do Brains Know When We're Grown Up?

Scientists who study the brain look at how the brain looks, how it works, and how different parts of the brain talk to each other. Over the last twenty years, scientists have discovered that kids, teenagers, and adults have brains that work differently, even though they are about the same size! This has made scientists, doctors, and even regular people very curious about how our brains grow up.

Sometimes, people who make laws and rules about how we live use information about the brain to make decisions. For example, they might think about how a young person’s brain is still developing when deciding if they should be punished the same way as an adult who broke the law. They also think about this when making rules about whether teenagers can make decisions about their health. This is because our brains are still changing and learning new things even when we are teenagers!

What Makes a Brain Done Growing?

Scientists often compare the brains of young people to the brains of adults to figure out what changes as we age. They look for things like how much gray matter (the thinking part of the brain) there is and how much white matter (the part that connects different parts of the brain) there is.

Scientists have learned that our brains keep changing even when we are in our twenties! Some parts of the brain, like the part that helps us make plans and decisions, take longer to finish growing than other parts. This means there isn't one single moment when our brains are suddenly grown up.

Another way to study brain development is to look at how different parts of the brain talk to each other. This is like figuring out the brain's communication network. Scientists have discovered that as we get older, the brain becomes better at sending messages between different areas. However, just like with brain structure, these communication networks continue to develop well into our twenties.

How Does an Adult Brain Work?

Besides looking at how the brain changes, scientists also study how the brain works differently in children, teenagers, and adults. For example, teenagers are getting better at thinking abstractly and making decisions, but they are also more sensitive to rewards. Scientists can see these changes when they look at how the brain acts while someone is doing a task.

Interestingly, teenagers’ brains sometimes work more like the brains of children than adults, especially when they are feeling strong emotions. This shows us that brains don't just "grow up" all at once. Different parts of the brain mature at different speeds.

Growing Up in Stages

Think about it this way: You don't suddenly know how to do everything when you turn a certain age. You learn and grow in stages. The same is true for your brain. There isn't one "grown-up" brain. Brains develop different abilities at different times.

For instance, a teenager might be able to do a math problem just as well as an adult when they are calm. But, if you add something distracting or exciting, the teenager might have a harder time focusing. This is because the part of their brain that helps them control their impulses and emotions is still developing.

Figuring Out Brain Maturity

It's really hard to say exactly when a brain is "mature" because it's always changing throughout our lives. Some scientists believe it's not even possible to pick one age when the brain is fully developed.

One thing is for sure: just because someone is legally an adult (usually 18 years old) doesn't mean their brain is finished growing! Some scientists think we should pay more attention to how the brain works when deciding if it's "mature." This means looking at how well a person can control their impulses, make decisions, and handle their emotions in different situations.

New Discoveries and What's Next

Right now, scientists are doing huge studies to learn even more about how the brain develops. These studies are like taking pictures of the brain as it grows and changes.

In the future, this research will help us understand even more about how our brains become "mature" and how we can best support young people as they grow up! For now, it's important to remember that just like us, brains grow up gradually!

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