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.

“It’s a use it or lose it principle; those synapses that are used will survive and flourish and those that are not will wither and die,” says Jay N. Giedd, chief of the Unit on Brain Imaging in the Child Psychiatry Branch of the National Institute of Mental Health. This paring away remodels the brain to make it more specialized, based on how someone spends her time, and alterations in the frontal lobe of the brain, where impulse control, judgment and long-range planning occur, are particularly extensive during the teenage years. “From puberty to age 20, adolescents lose 1% of gray matter volume each year,” says Giedd, who has compiled the world’s largest library of pediatric neural images, with 7,000 brain scans of 2,500 people younger than age 30.

Yet, while gray matter is being lost, tracts of white matter appear to increase in size and density and become better organized. That may be because the diameter of the axons is increasing, or because there is a buildup of the fatty substance myelin, which sheathes the axons. Either way, with more white matter, nerve impulses travel faster. “You can combine input from different parts of the brain more quickly,” says Giedd, “but by choosing to speed up one path of information processing, you may lose some of the flexibility to find alternate routes.”

The problem for teens is that the white matter doesn’t reach maximum density in the prefrontal cortex—the locus of judgment, decision-making, long-range planning and impulse control—until age 25 or 30. At that stage, with more, quicker connections to other parts of the brain, the prefrontal cortex—the so-called executive part of the brain—is better able to moderate the impulses, emotions and rewards that reside elsewhere. But that’s much more difficult in an adolescent’s brain, and its lack of white matter is one reason teenagers are more likely to engage in immature, risky behaviors. “The way the prefrontal cortex talks to the rest of the brain is becoming more refined during adolescence, but it doesn’t fully collaborate with other brain regions until adulthood,” says Beatriz Luna, associate professor of psychiatry at the University of Pittsburgh School of Medicine. (Indeed, adults haven’t so much lost their vulnerability to poor judgment as they have strengthened their ability to override acting on irrational impulses.)

IN THE LABORATORY, LUNA USES A SIMPLE TASK TO DEMONSTRATE how the immature prefrontal cortex in adolescents can’t always control more impulsive parts of the brain, thus thwarting a teen’s best intentions to behave a certain way. She tells subjects who are inside a functional MRI—a brain scanner that measures blood flow to specific regions of the brain as they are activated by a task being performed—to focus on a cross in the middle of a screen in front of them but not to look at a light that will randomly appear on the screen when the cross disappears. “It’s dark and boring inside the fMRI, and our natural reflexive tendency is to look at that light,” says Luna. “But the part of your brain that controls your voluntary movements is telling you to look away from the light.”

Luna has subjects repeat the test 50 to 60 times. Children perform dismally, making errors on more than 50% of the trials. Teens successfully complete the task 60% to 70% of the time, whereas adults do it correctly 80% to 90% of the time. “Adolescents understand the instructions, but they can’t consistently engage the prefrontal cortex to suppress the impulsive tendency,” says Luna. “They have less voluntary control of their behavior than adults do.”

Nor can teens actively and consistently engage the anterior cingulate cortex, the brain area involved in detecting errors and monitoring performance. When adults make a mistake on a task they are performing in an fMRI, there is significant neural activity in the anterior cingulate cortex. “That region is saying, ‘Oh shoot, I messed up and I don’t want to make that mistake again, so what do I have to do differently?’?” says Luna. Teenagers will detect and immediately correct their errors. “But because the anterior cingulate cortex isn’t strongly activated, they aren’t using the part of the brain that says, ‘Let’s use this information to avoid making the same mistake next time,’” says Luna. So teens tend to repeat their errors, whereas adults can more carefully monitor their performance and learn from their mistakes.

Teens also suffer from immature working memory, a component of cognitive function that allows people to keep relevant information “online” to help them respond to each new situation. When asked to move their eyes to an area of a dark screen in which there had been a momentary flash of light, adolescents can’t precisely target the spot; that ability doesn’t develop until the early twenties. “Teens are not as accurate as adults at guiding their behavior by using their working memory,” Luna says.

But an adolescent’s performance changes dramatically when a monetary reward is at stake. Environmental cues have much more of an effect on teenagers’ behavior than on adults’, and when young people are promised a $25 gift card for limiting mistakes during Luna’s experiments, their scores improve markedly, to match those of adults. “Adolescents can effectively engage the cognitive control part of the prefrontal cortex when they are motivated,” Luna observes, “but only after the reward regions of the brain are hyperactivated.” As connections and interactions between brain regions become stronger through young adulthood, older subjects may be able to distribute cognitive activity across several regions specialized for specific aspects of a task. That makes the response more rapid and accurate, and it also frees up the prefrontal cortex for more complex work, says Luna.


WHILE THE STRUCTURE OF A TEENAGE BRAIN MAY RESULT IN STARTLING GAPS in reasoning and decision-making, adolescents’ pursuit of risks, novelty and reward is more intense than it will ever be again. And that drive is geared toward one goal common to all mammals: to leave the nest and explore new territories in search of a mate. “In that regard, youth of today aren’t different from those in earlier generations or from other animals,” says Casey. The innate desire to take risks and to experiment is designed to motivate young people to learn the skills they need to live independently. What’s more, venturing out in the company of friends lets them seek potential mates. Even adolescents’ moodiness and sensitivity about perceived slights may serve an evolutionary purpose, keeping teens vigilant for threats while they are engaged in risky behavior, according to Casey.

Laurence Steinberg, distinguished professor of psychology at Temple University, has been intrigued by the question that has puzzled many parents of teenagers: “Why do intelligent kids do such stupid things when they are with their friends?” It’s a question that has guided his research for the past five years. In one experiment, he has teens and adults play a video game that awards points and monetary prizes for speeding a car through city streets. As in real life, the driver has to make decisions—Should I go through this yellow light? Can I safely drive faster?—and is penalized for crashing, which in the game results in points being deducted from a score. Steinberg has participants play the game alone and then in the presence of friends. He has found that being with peers—even if they’re only observing through a window from another room—activates the reward centers of adolescent brains but not those of adults. And when teens know friends are watching, they take twice as many driving risks.

Personality doesn’t seem to influence reckless driving, nor does gender. Teen girls, responding to questionnaires, say they engage in fewer risky behaviors than boys admit to—and parents may indeed monitor girls more closely—but those differences disappear when adolescents of either sex are sitting in front of a video screen, says Steinberg, author of several books on adolescent development.

The brains of human adolescents are also biologically programmed to send them looking for other people of their own age. Research has shown that teens derive much more pleasure from being with their peers than do adults, and social interaction is extremely important to adolescents. And the more time they spend with their friends, the more risks they take. “It is the pursuit of reward that drives risk-taking,” says Steinberg. “The brain activates the same circuits whether the reward is sex, money, food or social praise.”

ONE HYPOTHESIS ABOUT WHY THOSE MOTIVATIONS ARE SO IMPORTANT during adolescence focuses on dopamine, the “happiness chemical,” which is released at neural synapses and amplifies or inhibits electrical impulses in a way that tells the brain it has encountered a positive stimulus and that the stimulus should be pursued again in future. Dopamine may drive teens to pursue rewards even when they have to take inordinate risks to get them, and its effects may spike around the time of puberty, either because the brain is producing more dopamine or because the neurotransmitter finds an increased number of receptors, in various brain regions, to which it can bind. “We know that elevated dopamine intensifies pleasure, and it makes you want to seek out a reward again and again,” says Monica Luciana, professor of psychology and child development at the University of Minnesota.

A dopamine system in overdrive can be a lot for a teen to contend with. “It’s a double whammy,” says Luciana. Hypersensitivity to dopamine causes the parts of an adolescent brain that are tied to emotions to want immediate rewards, and because the prefrontal cortex, which could help control such impulses, is not yet fully developed, teenagers may not be able to help themselves when they see a chance for pleasure, she says. And there may be plenty of opportunities for teens, who can take advantage of unsupervised time to have sex or use illicit drugs. Indeed, sensitivity to dopamine leaves people susceptible to addiction, because drugs and alcohol flood the brain’s reward circuits with the chemical.

Not all teens have a huge appetite for taking risks or exploring novel situations, of course, and individual differences in dopamine levels may explain why one teen gets a thrill driving 100 mph at midnight while another wouldn’t dream of violating a curfew. “The person who starts out with a very low level of dopamine may get a boost at adolescence but won’t demonstrate the same type of extreme behavior as someone whose already elevated dopamine system goes even higher,” says Luciana.

Another biological event that occurs during adolescence—the surge of reproductive hormones—further influences young brains, fueling a yearning for excitement and arousal. “The pull to try exciting new things, which can lead to risk-taking, tracks very closely with the onset of puberty,” says Dahl, who implicates testosterone, estrogen and perhaps androgens from the adrenal glands, which also are active during adolescence. This can be particularly challenging for children who begin puberty at earlier ages because they have less experience and less brain development, making it harder to deal with this upsurge of new feelings and motivations. With a later onset, in contrast, a child might have the added maturity to manage these sensation-seeking urges, says Dahl.

The effect of reproductive hormones on adolescents “may not drive behavior as much as tip the balance toward a tendency to take risks, which is very much about impressing peers to win their approval,” Dahl adds. In experiments with nonhuman primates, researchers have found that manipulating testosterone changes behavior only when the animals are living in an unstable social hierarchy, motivating them to work aggressively to increase their social status. Teens attending a small-town high school where everyone knows one another may not feel the need to take the same risks to establish their reputation in the way that kids do if their social groups are constantly changing. “Contemporary adolescence is filled with many ambiguous, uncertain social situations in which kids don’t know what people think of them,” says Dahl. “Some become highly motivated to prove themselves and be admired.”

While a perfectly functioning adolescent brain may encourage impulsive, often risky behavior, most people will literally outgrow such excesses. But that often isn’t the case for those afflicted with the many mental illnesses that may reveal themselves during this period. Anxiety disorders, bipolar disorder, depression, eating disorders and psychosis all commonly emerge during the teenage years, and they may result from anomalies or exaggerations of the typical brain maturation process, says Giedd.

Schizophrenia, for example, seems to stem in part from an abnormal overpruning of gray matter. “From age 12 to 19, you see a 7% reduction in gray matter in healthy kids, but a 28% reduction in kids with schizophrenia,” says Giedd. “During this time, when there is so much pruning, change and specializing going on in the brain, moving parts get broken.” Another possibility is that there may have been preexisting damage that only becomes obvious “when the biological clock of maturation forces someone to use a particular part of the brain, revealing abnormalities,” says Luna.

Yet although recent advances are helping scientists understand how brains are supposed to develop and how the process may go awry, they’re still a long way from being able to look at a brain scan and diagnose an anxiety disorder or schizophrenia; nor can they predict a healthy teen’s behavior based on neuroanatomy. “We have to look at parts of the brain as letters of the alphabet, while behaviors are like words, sentences or even paragraphs,” says Giedd. “We are just starting to understand that it is the combination of all parts of the brain that determine behavior.”

Culture, too, may play a role in how teens navigate the transition to adulthood. That’s what Margaret Mead, the cultural anthropologist, concluded during the 1920s in her groundbreaking fieldwork among Samoans, who seemed to sail through adolescence without the turmoil that most American youths tend to experience. Now, Laurence Steinberg, who last year won the inaugural $1 million Klaus J. Jacobs Research Prize, which recognizes scientific accomplishment in improving the lives of young people, plans to test the impulse control and sensation-seeking tendencies of 200 teenagers in each of nine countries—China, Colombia, India, Italy, Jordan, Kenya, the Philippines, Sweden and Thailand. The results could help Steinberg answer questions about which aspects of teenage behavior are universal and which are influenced by culture and parenting styles. His work, like that of the phalanx of other researchers now focusing on adolescent development, could help explain what happens, and why, during this often baffling period.