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Yes, their hormones are raging // But another culprit is the still-developing brain // Which makes impulses difficult to curb // Which makes risky behavior hard to resist.

The Teenage Brain

By Anita Slomski // Fall 2010
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juveniles, graffitti

Ilana Panich-Linsman/ VII Mentor Program

Terrance Graham did some terrible things as a teenager. At age 16, he robbed a restaurant in Jacksonville and stood by as an accomplice assaulted the manager with a steel bar; a year later, while on parole, he committed another armed robbery, holding a homeowner at gunpoint. But Graham was at the mercy of an immature brain when he committed his crimes, vulnerable to the corrupting influences of peers and “an underdeveloped sense of responsibility,” according to the U.S. Supreme Court in a May 17, 2010, decision that ruled it unconstitutional to impose life sentences on juveniles convicted of crimes in which no one was killed. Citing research that shows “fundamental differences between juvenile and adult minds,” the court opinion said that “a life without parole sentence improperly denies the juvenile offender a chance to demonstrate growth and maturity.”

That ruling, coming after a 2005 decision prohibiting death sentences for minors, seemed to acknowledge what most neuroscientists now believe—that having a brain that doesn’t reach full maturity until as late as age 30 may lead to startling lapses in judgment and decision-making—and, sometimes, extreme violence. Yet the profound changes young people undergo during the transition to adulthood can also be quite positive. “Adolescence is when people are often performing at their best,” says B. J. Casey, director of the Sackler Institute for Developmental Psychobiology at Weill Medical College of Cor­nell University. “They are stronger and quicker in their reflexes, and they can sometimes make faster, more efficient decisions than adults can. But when you alter their environment with an emotionally charged event, they act as if they weren’t thinking at all. Look at the crushes teenagers get, which can just completely hijack their brains.”

Whether to good effect or ill, all adolescents undergo the brain changes that are unique to this period of life. Yet there is tremendous variability in the behaviors that result. “Eighty percent of adolescents go through this process without any significant problems,” says pediatrician and adolescent researcher Ronald E. Dahl, professor of public health at the University of California, Berkeley. “They get along with teachers and parents and don’t get into much trouble. But there are also a lot of kids who fall off the track of doing well, and some who were already struggling get worse.” What’s more, the stakes are high, with adolescents at risk of injury or death from substance abuse, unprotected sex, crime, drunk driving or self-inflicted harm.

Yet it’s only during the past decade that adolescent neuroscience and related disciplines have attracted a surge of interest among researchers. Before then, most attention was focused on what happens much earlier, throughout the first few years of life. Because the brain of a six-year-old is virtually as large as an adult brain and looks surprisingly much like the grown-up version, scientists presumed that the most important part of development happened very early, says Dahl. Only during the 1990s, with the advent of more advanced, less dangerous brain imaging technology, did they begin to look seriously at what happens to the brain during the teenage years. In the past few years, advances have accelerated, revealing that something very interesting and very complicated happens once children hit puberty.

The brain’s gray matter is made up of neurons and the synapses that transmit electrical impulses through which neurons communicate. Neural signals travel throughout the brain along the fibrous bundles of axons, long projections of nerve cells, of which the brain’s white matter is composed. Throughout life, both gray and white matter (and hence the brain) are constantly changing, but during adolescence the proportions between the two shift markedly.

People are born with a nearly full complement of the brain’s neurons, but the number of synapses, the junctions that connect neurons, proliferate throughout childhood, reaching their peak during early adolescence—at about age 11 for girls and age 14 for boys. A neuron in the frontal lobe of an 11-year-old girl, for example, may have 20,000 connections to other neurons, whereas by the time she’s 50, each neuron could have as few as 10,000 synapses. This overproduction of neural connections in childhood is the hallmark of a brain primed for learning as new experiences create the foundation—and the potential—for the mind a person will have as an adult. But the size of the brain cavity and the metabolic resources for neural cells are finite, so some synapses must be pruned.

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Teenagers and Alcohol: Under the Influence

When vulnerable teenage brains are exposed to the effects of extreme alcohol consumption, the outcome may be bleak.


1. Brain and Cognition, February 2010, special issue edited by Monica Luciana. This is an excellent compilation of the latest research in adolescent brain and behavioral development by the field’s top scientists. Articles cover neuroanatomy, sex differences in the teenage brain, adolescent appetites for risk and sensation, and vulnerability to substance abuse and psychiatric disorders.

2. “Adolescent Development and Juvenile Justice,” by Laurence Steinberg, Annual Review of Clinical Psychology, Oct. 18, 2008. Long a critic of how juvenile offenders are handled in the United States, Steinberg argues that teens are less blameworthy than adults for the crimes they commit by virtue of their immature brains, and makes a case for a separate justice system for juveniles.

3. “Why Do Many Psychiatric Disorders Emerge During Adolescence?” by Tomáš Paus et al., Nature Reviews Neuroscience, December 2008. The authors examine how maturation changes in the adolescent brain may go awry and, in tandem with hormonal changes and psychosocial and environmental pressures, set the stage for the emergence of schizophrenia, anxiety, depression and substance abuse during the teenage years.

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