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The Kids Can’t Help It
They say you never escape high school. And for better or worse, science is lending some credibility to that old saw. Thanks to sophisticated imaging technology and a number of longitudinal (纵向的) studies, we’re learning that the teen years are a period of crucial brain development subject to a host of environmental and genetic factors. This emerging research sheds light not only on why teenagers act the way they do, but how the experiences of adolescence—from rejection to binge drinking (酗酒)—can affect who we become as adults, how we handle stress, and the way we bond with others.
One of the most important discoveries in this area of study, says Dr. Frances Jensen, a neuroscientist at Harvard, is that our brains are not finished maturing by adolescence, as was previously thought. Adolescent brains "are only about 80 percent of the way to maturity," she said at the annual meeting of the Society for Neuroscience in November. It takes until the mid-20s, and possibly later, for a brain to become fully developed.
An excess of gray matter (the stuff that does the processing) at the beginning of adolescence makes us particularly brilliant at learning—the reason we’re so good at picking up new languages starting in early childhood—but also particularly sensitive to the influences of our environment, both emotional and physical. Our brains’ processing centers haven’t been fully linked yet, particularly the parts responsible for helping to check out impulses and considering the long-term results of our actions. "It’s like a brain that’s all cheered up not knowing where it needs to go," says Jensen.
It’s partially because of this developmental timeline that a teen can be so quick to think of a stinging remark, or a biting insult, and so uninhibited in firing it off at the nearest unfortunate target—a former friend, perhaps, or a bewildered parent. The impulse to hurl an insult is there, just as it may be for an adult in a stressful situation, but the brain regions that an adult might rely on to stop himself from saying something cruel just haven’t caught up.
In a paper published last year in the Journal of the American Academy of Child & Adolescent Psychiatry, Dr. Jay Giedd, a scientist at the Child Psychiatry Branch of the National Institutes of Mental Health, wrote that, according to brain scans conducted over several years, gray-matter volume peaks around or just before the beginning of puberty (青春期), and then continuously declines. In contrast, white matter (the stuff that helps connect areas of the brain) increases right up to, and beyond, the end of puberty.
These adolescent brain developments don’t happen to all parts of the brain at the same time. "The order in which this maturation of connection goes, is from the back of the brain to the front of the brain." says Jensen.
And one of the last parts to mature is the frontal lobe (大脑额叶), a large area responsible for modulating reward, planning, impulsiveness, attention, acceptable social behavior, and other roles that are known as executive functions.
It’s thanks in part to the frontal lobe that we are able to schedule our time with any sort of efficiency, plan in advance to arrange for a designated driver on a night out, and restrain ourselves from getting into fights any time we get involved in an argument. Unfortunately, it’s just these sorts of behaviors that teenage brains are not fully endowed to deal with—and the consequences are potentially fatal when it comes to high-risk behavior like drinking and driving.
This blast of teen-brain change is compounded by profound social and psychological shifts. Of particular importance is that adolescence is the time when we develop stronger social connections with our peers, and more independence from our parents.
"Before the transition to adolescence, kids’ interactions with one another, and the kinds of friendships that they have, are substantially different," explains Dr. Mitch Prinstein, professor and director of clinical psychology, at the University of North Carolina, Chapel Hill. "After adolescence they can really confide in friends, they turn to them as first sources of social support."
This cuts both ways. Healthy relationships have a positive effect on how an adolescent navigates through a noisy period of life. But at the same time, this reliance on friends makes young people susceptible to the influence of peer pressure, even when it is indirect.
"The most effective predictors of why adolescents engage in all kinds of health-risk behaviors—substance use, sexual behavior, even recently, self-cutting—is very much related to how much they perceive that their close friends are doing the same thing, or someone that they consider very cool and popular is doing the same thing," says Prinstein.
And, the latest research shows that some of these risky behaviors may have surprising lifelong consequences. Toni Pak, assistant professor in the department of cell and molecular physiology, Stritch School of Medicine, Loyola University, Chicago, recently demonstrated that rats exposed to binge drinking as adolescents developed some troubling issues as adults.
Meanwhile, a Harvard study found that kids who smoked pot before age 16 had more lifelong cognitive problems than those who started smoking after 16. The tests were done on subjects with an average age of 22, and those who smoked pot (大麻) earlier had problems remembering details, making decisions, and responding quickly when directions changed.
Another study found that peer rejection and public speaking create a greater chemical stress response in adolescents compared with children. The authors of the paper on this study noted that an increased stress response might be a biological strategy that allows adolescents to adapt to their environments, but that in high-risk individuals this upward shift in stress response "may tip the balance toward stress-response deregulation associated with depression and other psychopathology." As Kevin Beaver, a Florida State University researcher who studies adolescence and crime, says, "Stress can pull the trigger on the genetic gun."
The good news is that most of us make it out of adolescence just fine. And while a better understanding of the teenage brain may bring into focus dangers we hadn’t known existed, it may also allow us to identify who is at risk.
Through imaging technology and many studies, what factors do we learn that affect the brain development of the teen years