Why the Human Brain Takes Decades to Develop

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Physiologically, human adolescents are late bloomers on many fronts: learning to walk, securing and preparing their own food, and reproducing sexually. And that’s especially true for the development process inside our heads.  Even after a young adult has finished growing, perhaps married or claimed independence by most social standards, their brain typically continues developing — until they reach their mid- to late 20s. Adolescence, in fact, spans from the age of 10 until at least age 24, according to some definitions. Interestingly, the actual volume and size of the brain typically reaches its full mass in early adolescence. But key regions and connections need more time and experience to gradually form and mature. Read More: Compared to Neanderthals, Human Brain Growth is Slow and Steady The Prefrontal Cortex Develops Last Particularly the prefrontal cortex toward the front of the brain gets “remodeled” during adolescence, according to some brain experts. This thinking-center is responsible for judgements, reasoning and impulse control. Before that happens, younger adolescents tend to primarily navigate the world with the amygdala and limbic system, which develops earlier and is associated with emotional behavior and survival mechanisms such as fight-or-flight response. One neuroscientist, Sarah McKay, has described this as a sort of neurological “mismatch” in the developmental years. The adolescent period brings about “a significant neurological transition,” she said in blog post outlining key shifts that occur in the teenage brain. Namely, stronger connections gradually begin to form with the prefrontal cortex. Though, the exact timing and development process varies from person to person. This process explains why some childhood behaviors are often attributed to the still-developing brain. And that neurological reality correlates with societal age restrictions on things like drinking alcohol, smoking cigarettes and renting a car.  Matching the Brain with the External World Angela Griffin, a neuropsychologist at Southampton Children’s Hospital in the U.K., described the prefrontal cortex this way in a 2017 review in Healthcare: “One of the key goals of the pre-frontal cortex is to become skilled at reconciling internal emotional states with the demands of external reality.” Griffin’s review explains that during the teen years, brain connections between the hippocampus and frontal areas are strengthened, enabling youth to become gradually better at integrating memory and experience into their decision making. Or, as one 2010 study put it, cognitive control refines “the ability to resist temptation in favor of long-term goals.” That research, published in the Journal of the American Academy of Child and Adolescent Psychiatry, also highlighted how risk-taking behavior can be beneficial to brain development. The challenge, however, is to avoid risks and behavior that pose significant or long-term negative consequences — such as substance abuse and addiction. That’s where parental or caretaker guidance and influence can play an important role for children and teens. Read More: Substance Abuse in Adolescence Can Impact Brain Development Other research indicates that peer-to-peer social interaction is especially important for crucial functions in brain development, self-concept construction and mental health during the pre-teen and teenage years. Considering this fact, researchers at Stanford revealed in December that the COVID-19 pandemic appears to have led to poorer mental health, as well as accelerated brain aging in adolescents. Their preliminary findings, which are still pending peer-review, indicate that adolescents assessed after the pandemic shutdowns “not only had more severe internalizing mental health problems, but also had reduced cortical thickness, larger hippocampal and amygdala volume, and more advanced brain age.”  The exact implications and effects of that remain unclear, until further research is conducted. Decoding the Aging Brain As to the slow pace of development in the human brain, one researcher at Vanderbilt University, Suzana Herculano-Houzel, recently demonstrated why humans require more time than all other animals, including primates. It’s likely due to the excessive number of neurons the human brain contains in the cerebral cortex, according to Herculano-Houzel’s research. Her work has helped illustrate that across warm-blooded species, lifespan as well as length of time required for brain development correlates closely with the number of neurons in the cerebral cortex. For example, that means that while the elephant brain is larger than the human brain, and contains more neurons overall, the concentration of neurons in the cerebral cortex remains higher in humans. In general, that translates into a more complex brain system. “It makes sense that the more neurons you have in the cortex, the longer it should take a species to reach that point where it’s not only physiologically mature, but also mentally capable of being independent,” Herculano-Houzel explained in a Vanderbilt research news story. Incidentally, and surprising to many people, within a decade after fully developing, the human brain begins shrinking — often once a person reaches their 30s. And by the time they hit 60, the rate of shrinking has increased even more. Suffice to say, the brain develops and ages in dynamic ways that we are only beginning to grasp.

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