Shortly before 5 p.m., the Manns head downstairs to the imaging floor to meet the magnet. Giedd, a trim, energetic man with a reddish beard, twinkly blue eyes & an impish sense of humor, greets Anthony & tells him what to expect.

He asks Anthony to remove his watch, his necklace & a high school ring, labeled keeper. Does Anthony have any metal in his body? Any piercings? Not this clean-cut, soccer-playing Mormon. Giedd tapes a vitamin E capsule onto Anthony's left cheek & one in each ear. He explains that the oil-filled capsules are opaque to the scanner & will define a plane on the images, as well as help researchers tell left from right.

The scanning will take about 15 minutes, during which Anthony must lie completely still. Dressed in a red sweat shirt, jeans & white K-Swiss sneakers, he stretches out on the examining table & slides his head into the machine's giant magnetic ring.

MRI, Giedd points out, "made studying healthy kids possible" because there's no radiation involved. (Before MRI, brain development was studied mostly by using cadavers.) Each of the Mann boys will be scanned 3 times.

The first scan is a quick survey that lasts 1 minute. The second lasts 2 minutes & looks for any damage or abnormality. The third is 10 minutes long & taken at maximum resolution. It's the money shot.

Giedd watches as Anthony's brain appears in cross section on a computer screen. The machine scans 124 slices, each as thin as a dime. It will take 20 hours of computer time to process the images, but the analysis is done by humans, says Giedd. "The human brain is still the best at pattern recognition," he marvels.

Some people get nervous as the MRI machine clangs noisily. Claustrophobes panic. Anthony, lying still in the soul of the machine, simply falls asleep.

CONSTRUCTION AHEAD
One reason scientists have been surprised by the ferment in the teenage brain is that the brain grows very little over the course of childhood.

By the time a child is 6, it's 90% to 95% of its adult size. As a matter of fact, we're born equipped with most of the neurons our brain will ever have - & that's fewer than we have in utero.

Humans achieve their maximum brain-cell density between the 3rd & 6th month of gestation - the culmination of an explosive period of prenatal neural growth.

During the final months before birth, our brains undergo a dramatic pruning in which unnecessary brain cells are eliminated. Many neuroscientists now believe that autism is the result of insufficient or abnormal prenatal pruning.

What Giedd's long-term studies have documented is that there's a second wave of proliferation & pruning that occurs later in childhood & that the final, critical part of this second wave, affecting some of our highest mental functions, occurs in the late teens.

Unlike the prenatal changes, this neural waxing & waning alters not the number of nerve cells but the number of connections, or synapses, between them. When a child is between the ages of 6 & 12, the neurons grow bushier, each making dozens of connections to other neurons & creating new pathways for nerve signals.

The thickening of all this gray matter - the neurons & their branchlike dendrites - peaks when girls are about 11 & boys 12½, at which point a serious round of pruning is under way.

Gray matter is thinned out at a rate of about 0.7% a year, tapering off in the early 20's. At the same time, the brain's white matter thickens. The white matter is composed of fatty myelin sheaths that encase axons & like insulation on a wire, make nerve-signal transmissions faster & more efficient.

With each passing year (maybe even up to age 40) myelin sheaths thicken, much like tree rings. During adolescence, says Giedd, summing up the process, "you get fewer but faster connections in the brain."

The brain becomes a more efficient machine, but there's a trade-off: it's probably losing some of its raw potential for learning & its ability to recover from trauma.

Most scientists believe that the pruning is guided both by genetics & by a use-it-or-lose-it principle. Nobel prizewinning neuroscientist Gerald Edelman has described that process as "neural Darwinism" - survival of the fittest (or most used) synapses.

How you spend your time may be critical. Research shows, e.g., that practicing piano quickly thickens neurons in the brain regions that control the fingers.

Studies of London cab drivers, who must memorize all the city's streets, show that they have an unusually large hippocampus, a structure involved in memory. Giedd's research suggests that the cerebellum, an area that coordinates both physical & mental activities, is particularly responsive to experience, but he warns that it's too soon to know just what drives the buildup & pruning phases.

He's hoping his studies of twins will help answer such questions: "We're looking at what they eat, how they spend their time - is it video games or sports? Now the fun begins," he says.

No matter how a particular brain turns out, its development proceeds in stages, generally from back to front. Some of the brain regions that reach maturity earliest - thru proliferation & pruning - are those in the back of the brain that mediate direct contact with the environment by controlling such sensory functions as vision, hearing, touch & spatial processing.

Next are areas that coordinate those functions: the part of the brain that helps you know where the light switch is in your bathroom even if you can't see it in the middle of the night.

The very last part of the brain to be pruned & shaped to its adult dimensions is the prefrontal cortex, home of the so-called executive functions - planning, setting priorities, organizing thoughts, suppressing impulses, weighing the consequences of one's actions.

In other words, the final part of the brain to grow up is the part capable of deciding, I'll finish my homework & take out the garbage & then I'll IM my friends about seeing a movie.

"Scientists & the general public had attributed the bad decisions teens make to hormonal changes," says Elizabeth Sowell, a UCLA neuroscientist who has done seminal MRI work on the developing brain.

"But once we started mapping where & when the brain changes were happening, we could say, Aha, the part of the brain that makes teenagers more responsible isn't finished maturing yet."

RAGING HORMONES
Hormones, however, remain an important part of the teen-brain story. Right about the time the brain switches from proliferating to pruning, the body comes under the hormonal assault of puberty. (Research suggests that the 2 events aren't closely linked because brain development proceeds on schedule even when a child experiences early or late puberty.)

For years, psychologists attributed the intense, combustible emotions & unpredictable behavior of teens to this biochemical onslaught. And new research adds fresh support. At puberty, the ovaries & testes begin to pour estrogen & testosterone into the bloodstream, spurring the development of the reproductive system, causing hair to sprout in the armpits & groin, wreaking havoc with the skin & shaping the body to its adult contours.

At the same time, testosterone-like hormones released by the adrenal glands, located near the kidneys, begin to circulate. Recent discoveries show that these adrenal sex hormones are extremely active in the brain, attaching to receptors everywhere & exerting a direct influence on serotonin & other neurochemicals that regulate mood & excitability.

The sex hormones are especially active in the brain's emotional center - the limbic system. This creates a "tinderbox of emotions," says Dr. Ronald Dahl, a psychiatrist at the Univ. of Pittsburgh.

Not only do feelings reach a flash point more easily, but adolescents tend to seek out situations where they can allow their emotions & passions to run wild.

"Adolescents are actively looking for experiences to create intense feelings," says Dahl. "It's a very important hint that there's some particular hormone-brain relationship contributing to the appetite for thrills, strong sensations & excitement."

This thrill seeking may have evolved to promote exploration, an eagerness to leave the nest & seek one's own path & partner. But in a world where fast cars, illicit drugs, gangs & dangerous liaisons beckon, it also puts the teenager at risk.

That's especially so because the brain regions that put the brakes on risky, impulsive behavior are still under construction. "The parts of the brain responsible for things like sensation seeking are getting turned on in big ways around the time of puberty," says Temple Univ. psychologist Laurence Steinberg.

"But the parts for exercising judgment are still maturing throughout the course of adolescence. So you've got this time gap between when things impel kids toward taking risks early in adolescence & when things that allow people to think before they act come online. It's like turning on the engine of a car without a skilled driver at the wheel."

DUMB DECISIONS
Increasingly, psychologists like Steinberg are trying to connect the familiar patterns of adolescents' wacky behavior to the new findings about their evolving brain structure.

It's not always easy to do. "In all likelihood, the behavior is changing because the brain is changing," he says. "But that's still a bit of a leap." A critical tool in making that leap is functional magnetic resonance imaging (FMRI). While ordinary MRI reveals brain structure, FMRI actually shows brain activity while subjects are doing assigned tasks.

At McLean Hospital in Belmont, Mass., Harvard neuropsychologist Deborah Yurgelun-Todd did an elegant series of FMRI experiments in which both kids & adults were asked to identity the emotions displayed in photographs of faces.

"In doing these tasks," she says, "kids & young adolescents rely heavily on the amygdala, a structure in the temporal lobes associated with emotional & gut reactions. Adults rely less on the amygdala & more on the frontal lobe, a region associated with planning & judgment."

While adults make few errors in assessing the photos, kids under 14 tend to make mistakes. In particular, they identify fearful expressions as angry, confused or sad. By following the same kids year after year, Yurgelun-Todd has been able to watch their brain-activity pattern - & their judgment - mature.

Fledgling physiology, she believes, may explain why adolescents so frequently misread emotional signals, seeing anger & hostility where none exists. Teenage ranting ("That teacher hates me!") can be better understood in this light.

At Temple Univ., Steinberg has been studying another kind of judgment: risk assessment.

In an experiment using a driving-simulation game, he studies teens & adults as they decide whether to run a yellow light. Both sets of subjects, he found, make safe choices when playing alone. But in group play, teenagers start to take more risks in the presence of their friends, while those over age 20 don't show much change in their behavior.

"With this manipulation," says Steinberg, "we've shown that age differences in decision making & judgment may appear under conditions that are emotionally arousing or have high social impact." Most teen crimes, he says, are committed by kids in packs.

Other researchers are exploring how the adolescent propensity for uninhibited risk taking propels teens to experiment with drugs & alcohol. Traditionally, psychologists have attributed this experimentation to peer pressure, teenagers' attraction to novelty & their roaring interest in loosening sexual inhibitions.

But researchers have raised the possibility that rapid changes in dopamine-rich areas of the brain may be an additional factor in making teens vulnerable to the stimulating & addictive effects of drugs & alcohol. Dopamine, the brain chemical involved in motivation & in reinforcing behavior, is particularly abundant & active in the teen years.

Why is it so hard to get a teenager off the couch & working on that all important college essay? You might blame it on their immature nucleus accumbens, a region in the frontal cortex that directs motivation to seek rewards.

James Bjork at the National Institute on Alcohol Abuse & Alcoholism has been using FMRI to study motivation in a challenging gambling game. He found that teenagers have less activity in this region than adults do.

"If adolescents have a motivational deficit, it may mean that they're prone to engaging in behaviors that have either a really high excitement factor or a really low effort factor, or a combination of both." Sound familiar?

Bjork believes his work may hold valuable lessons for parents & society. "When presenting suggestions, anything parents can do to emphasize more immediate payoffs will be more effective," he says. To persuade a teen to quit drinking, e.g., he suggests stressing something immediate & tangible - the danger of getting kicked off the football team, say - rather than a future on skid row.

Persuading a teenager to go to bed & get up on a reasonable schedule is another matter entirely. This kind of decision making has less to do with the frontal lobe than with the pineal gland at the base of the brain. As nighttime approaches & daylight recedes, the pineal gland produces melatonin, a chemical that signals the body to begin shutting down for sleep.

Studies by Mary Carskadon at Brown Univ. have shown that it takes longer for melatonin levels to rise in teenagers than in younger kids or in adults, regardless of exposure to light or stimulating activities. "The brain's program for starting nighttime is later," she explains.

PRUNING PROBLEMS
The new discoveries about teenage brain development have prompted all sorts of questions & theories about the timing of childhood mental illness & cognitive disorders. Some scientists now believe that ADHD & Tourette's syndrome, which typically appear by the time a child reaches age 7, may be related to the brain proliferation period.

Though both disorders have genetic roots, the rapid growth of brain tissue in early childhood, especially in regions rich in dopamine, "may set the stage for the increase in motor activities & tics," says Dr. Martin Teicher, director of developmental biopsychiatry research at McLean Hospital. "When it starts to prune in adolescence, you often see symptoms recede."

Schizophrenia, on the other hand, makes its appearance at about the time the prefrontal cortex is getting pruned. "Many people have speculated that schizophrenia may be due to an abnormality in the pruning process," says Teicher. "Another hypothesis is that schizophrenia has a much earlier, prenatal origin, but as the brain prunes, it gets unmasked." MRI studies have shown that while the average teenager loses about 15% of his cortical gray matter, those who develop schizophrenia lose as much as 25%.

WHAT'S A PARENT TO DO?
Brain scientists tend to be reluctant to make the leap from the laboratory to real-life, hard-core teenagers. Some feel a little burned by the way earlier neurological discoveries resulted in Baby Einstein tapes & other marketing schemes that misapplied their science. It is clear, however, that there are implications in the new research for parents, educators and lawmakers.

In light of what has been learned, it seems almost arbitrary that our society has decided that a young American is ready to:

• drive a car at 16
• to vote & serve in the Army at 18 
• to drink alcohol at 21

Giedd says the best estimate for when the brain is truly mature is 25, the age at which you can rent a car. "Avis must have some pretty sophisticated neuroscientists," he jokes.

Now that we have scientific evidence that the adolescent brain is not quite up to scratch, some legal scholars & child advocates argue that minors should never be tried as adults & should be spared the death penalty.

Last year, in an official statement that summarized current research on the adolescent brain, the American Bar Association urged all state legislatures to ban the death penalty for juveniles. "For social & biological reasons," it read, "teens have increased difficulty making mature decisions & understanding the consequences of their actions."

Most parents, of course, know this instinctively. Still, it's useful to learn that teenage behavior isn't just a matter of willful pigheadedness or determination to drive you crazy - though these, too, can be factors.

"There's a debate over how much conscious control kids have," says Giedd, who has 4 "teenagers in training" of his own. "You can tell them to shape up or ship out, but making mistakes is part of how the brain optimally grows."

It might be more useful to help them make up for what their brain still lacks by providing structure, organizing their time, guiding them thru tough decisions (even when they resist) & applying those time-tested parental virtues:

• patience
• love

—With reporting by Alice Park/New York