By the time puberty is over in the middle to late teens, when adult height and full reproductive capacity have been achieved, the body is at its peak--the strongest, swiftest and healthiest it will ever be.
But the brain lags behind, laboring to adapt to the most complex society that has existed.
This mismatch--between a fully grown body and an immature brain that is trying to cope with emotions, sexual urges, poor judgment, thrill seeking and risk taking--is a key factor making motor vehicle accidents the No. 1 cause of death among adolescents and young adults, followed by murder and suicide.
Using powerful new imaging technology to look inside the brain, scientists are beginning to unravel the biology behind this critical period of development. They are finding that an adolescent's brain undergoes a previously unsuspected biological makeover--a massive growth of synaptic connections between brain cells.
This spectacular surge kicks off an extensive renovation of the brain that is not complete until the mid-20s. Scientists say the resulting learning curve, when teens struggle to shed childish thoughts for adult ones, is why adolescence is such a prolonged and perilous journey for so many.
It helps explain not only why teens are more prone to crash a car than at any other time of life, but why they are more likely to engage in risky sex, drug abuse or delinquency. Although teens often can think as logically as an adult, the process can be easily derailed by flaring emotions or other distractions.
"The reason that kids take chances when they drive is not because they're ignorant," said Temple University psychologist Laurence Steinberg. "It's because other things undermine their better judgment."
The synaptic growth spurt that occurs in puberty is similar to the ones that occur after birth, when the brain first begins to learn. The early exposure to the outside world enables the brain to connect to the body, developing its capacity for processing sound, sight, smell, touch and taste, and to make sense of them.
Learning occurs only after excess synapses not stimulated by experience are eliminated, much like the pieces of marble that have to be chipped away to create a work of art.
Now scientists have found that a second wave of growth and pruning occurs in adolescence. Synapses that are not incorporated into neural networks for memory, decision-making and emotional control are eliminated to make way for a leaner, more efficient brain.
This late blossoming of synapses, it is thought, provides the brain with a new capacity for learning and allows the brain to make the transition from childhood to adulthood.
For frazzled parents, the findings may provide new understanding and patience as their teens navigate this increasingly rough passage. Science is finally beginning to see what's going on in the teen mind.
"We're able to actually visualize what the changes are that are happening in the brain and how the brain is adapting to its environment and changing to help it deal with all these challenges that are happening during adolescence," said Dr. Sanjiv Kumra of the Albert Einstein College of Medicine, New York.
Blooming and pruning
The discovery of the adolescent brain's synaptic blooming and pruning was first made in 1999 by a National Institute of Mental Health research team headed by Jay N. Giedd, chief of brain imaging at the institution's child psychiatry branch.
Puberty normally begins between the ages of 9 and 13 in girls and 10 and 16 in boys. Giedd's team found that synaptic growth reaches its peak in girls at about 11 years of age in girls and in boys at 14--a discovery that may provide a biological basis for why girls start maturing sooner than boys both physically and mentally.
After the growth peaks, the whittling away of unused synapses begins. This is also when the fibers connecting brain cells are wrapped in a thicker coat of myelin insulation to enhance their communication.
The changes in the brain are tied up with the changes associated with puberty, which prepares the body for sexual maturity. "Adolescence is a time where the most important function is really preparing you for mating," Kumra said. "All these brain changes are happening to prepare the organism to be able to carry out that central and important function."
Brain grows into mid-20s
Long after puberty is over, however, the brain is still developing--a process lasting into the mid-20s, researchers say.
"The notion that the brain wasn't done, was still under construction so late, was pretty surprising because by 18 you can vote, get married and go to war," Giedd said.
But the more Giedd thought about it, the more it made sense. The long period of maturation, he says, has made it easier for the brains of modern humans to adapt to an increasingly complex society.
"The same brain that was used in the past for hunting and gathering berries now programs a computer," he said. "The key to all of that is having the plasticity built into the brain."
But this long period of brain development also has a significant downside when teens get behind the wheel of a car.
Brain scientists like to joke that car rental companies must have the best neuroscientists because they won't let a person rent a car until age 25. But the real reason is clear to any actuary: Every year between 5,000 and 6,000 teenagers are killed in motor vehicle accidents and 300,000 are injured.
Teen crashes are not just caused by showing off, substance abuse, aggression, thrill seeking or speeding, although they play a role, said Giedd.
Recent research suggests that an important culprit is the immaturity of the teenage brain and its lack of multitasking skills--especially in boys. The last part of the brain to mature is the prefrontal cortex, Giedd said, which may not fully develop until the mid-20s.
That's important, he explained, because this part of the brain controls decision-making, judgment and impulse control, all of which are involved in multitasking, or processing more than one thing at a time.
"The more multitasking that you do--talking on a cell phone, adjusting the volume of a stereo, talking to people in the car--the more trouble you're asking for," Giedd said. "And it fits into the sex differences: Women are better at multitasking than males at every age and they have a strikingly lower rate of car accidents."
Phones, friends and driving
Most teens multitask behind the wheel, a recent survey by the Allstate Foundation found. Sixty-five percent say they look at things other than the road, 56 percent make and answer phone calls, 44 percent say they drive with friends in the car and 47 percent find passengers sometimes distracting.
Researchers say the time it now takes for the brain to reach adulthood may help explain why modern adolescence lasts far longer than in traditional societies, where the time between going through puberty and becoming a breadwinner is two to four years.
True adulthood arrives not with sexual or physical maturity but with taking on a social role and being responsible for one's own actions, said pediatric psychiatrist Dr. Ronald Dahl of the University of Pittsburgh School of Medicine.
"If your adult task is to gather food, have babies or kill an animal with a spear, the interval between puberty and adulthood is much shorter," Dahl said. "Whereas if what you want to do requires finishing high school, four years of college and going to graduate school, it's going to take the brain a lot longer."
The National Institute of Mental Health team's newest discovery shows that the longer the brain takes to mature, the smarter it becomes.
"The later the peak [in synaptic growth and pruning] the higher the IQ, which is good news for late bloomers," Giedd said. "If you have the brain being more responsive to the environment for longer, then these changes can make it better suited to deal with the environment."
Adolescence has now become so extended that it runs to about age 25, experts say. "What sits in the middle of this stretched-out adolescence are incredible increases in behavioral and emotional health problems, and brain changes that take a long time and lots of practice to acquire necessary skills," Dahl said.
The brain's facility for early learning is remarkable: It's as good at reasoning by age 16 as it is in adulthood, Steinberg said. "So then the question is: `Well, if kids are as smart as adults, why do they do such dumb things?'" said Steinberg, who presented new findings this month at the Society for Research on Adolescence meeting.
Impulse control key
"We think the reason doesn't have to do with their basic intelligence. It has to do with ways in which emotional and social factors impair their judgment. This means that it takes longer than we probably thought for people to develop mature impulse control."
His study, which looked at 950 people of different racial and ethnic backgrounds between the ages of 10 and 30 in five countries, found that while reasoning powers mature early, things like impulse control, thinking of future consequences to behavior and resisting peer pressure take much longer. In fact, they slowly mature through the 20s, Steinberg said.
In a simulated driving study, Steinberg found that when teens were in the room by themselves their driving skills were the same as adults'. But when they tried to perform the same driving tasks with two friends in the room, the number of chances they took doubled. The presence of friends did not affect the driving of the adults.
Adolescence, Dahl said, is a time when passions can hijack the brain's ability to make decisions and control behavior, with potentially deadly results.
For some youngsters living in impoverished conditions, this is a particularly dangerous time. They reach adult body size but are being led by a brain that clings to childish impulses and passions--and might see nothing worthwhile in the future.
"The system is precarious, tipping on one side toward strong emotions and drives and on the other side not yet supported well enough by self-control," Dahl said.
"There's an important role for parents, coaches, teachers, other responsible adults and social systems to help support kids so that they can take some risks, do some experimenting, develop some ability for self-control, but not spiral into those terrible outcomes--death, disability, addictions, reckless sex, HIV and all the other problems that are so rampant in adolescence."