Parents and pediatric specialists have long believed the adolescent brain to be a world unto itself, free from the shackles of reason and absolutely unrelated to both its earlier, simpler incarnation and the well-balanced mechanism that it will eventually become. They knew that labeling 18 year-olds as legal "adults" was a counter-intuitive policy, and their views are being validated by a steady stream of neurological research. Why do some teens often seem so impervious to any form of logic? How can they appear both emotionally self-absorbed and insensitive to the emotions of others, and why do snap decisions that threaten the well-being of others sometime seem the norm?
The answers lie in watching the brain develop over time, and the largest of these research projects is an extended effort to do just that. Since its 1989 inception, the National Institute of Mental Health's Longitudinal Brain Imaging Project has used the MRI procedure to create regularly updated images of human neurology in its various stages of development. Researchers once believed that human mental capacity, along with physical performance abilities, plateaued at the end of one's teenaged years: the body, by 18 years of age, had already entered the period of its greatest efficiency. But the 2,000 participants in the NIMH project, aged 3 to 30, have helped to correct that misperception, illustrating the functional differences between the teenaged brain and its more mature incarnation.
The brain quickly grows larger and more complex in the period leading up to adolescence. But the connections that facilitate communication among its various tissues and the rest of the body are the most important factor in determining the brain's ultimate utility, and they simply aren't fully formed during the teenage years. Grey matter, the organic material making up the various lobes of the brain, reaches its peak mass in late childhood and actually begins to shrink during adolescence. Grey matter peaks reflect overdevelopment more than increased functionality, and by very gradually growing smaller, the brain acts to heighten its own efficiency. But white matter, the connective tissue responsible for establishing the interconnectivity of all neurological tissues, hasn't reached its functional peak, and this biological immaturity explains a great deal of the obstinate, insensitive behavior so common to those aged 13-20.
Spatial, linguistic, and sensory functions operate near their optimum capacity during adolescence; one's sense of sight and taste and the degree of control one has over the body will never be greater. But we now also know that the all-important prefrontal cortex, which serves as the final piece in the neurological development puzzle ultimately responsible for such crucial capacities as decision making, personal judgment, and impulse control, does not operate at maximum efficiency until the age of 25, because the white matter allowing it to perform its duties progressively grows thicker until middle age.
The larger point is that, where children communicate with their most basic needs in mind and adults tend to over-intellectualize the very decision-making process, the adolescent brain is emotionally impulsive, acting, as it were, from "the gut." And yet, while teens are very intimately familiar with their own desires, it's truly harder for them to identify with the same in others. This lack of empathy can be explained directly through the naturally arrested development of the prefrontal cortex, which simply doesn't see as much use in teens because the white matter so crucial to its inner workings is, at the time, insufficiently developed. These physiological trends do not in any way excuse the self-centered impulsivity in which so many teens indulge, but they do allow for a greater understanding of the intellectual limits of the adolescent brain. And with time they will help us better accomodate those imperfections.
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