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
TRIBUNE SPECIAL REPORT: TEENS AT THE WHEEL
Teens driven to distraction
Research shows why a teen brain capable of reasoning like an adult's is hijacked by emotions and impulses
We've upgraded our reader commenting system. Learn more about the new features.
The Baltimore Sun encourages civil dialogue related to our stories; you must register and log-in to our site in order to participate. We reserve the right to remove any user and to delete comments that violate our Terms of Service. By commenting, you agree to these terms. Please flag inappropriate comments.