When an otherwise reasonable adolescent takes the car out for a predawn spin, doesn't crack a book until the night before the exam or guzzles beer until she gets sick on the driveway, parents typically blame raging hormones. But scientists have hit upon a new theory.
While studying brain images of normal teenagers, neuroscientist Jay Giedd of the National Institute of Mental Health and neurologist Paul Thompson of the University of California, Los Angeles, recently made a surprising discovery.
They found that adolescents undergo dramatic changes in the frontal lobe, or prefrontal cortex, the part of the brain critical to judgment, reason, self-control and planning.
"The surprise is how late very, very drastic brain growth is taking place," said Thompson, who directs UCLA's Laboratory of Neuro Imaging. "In late teens, the prefrontal cortex is the area that's changing the fastest, and we know that prefrontal cortex is responsible for inhibitions, suppressing risky behavior and controlling impulses."
The two scientists suggest that stereotypical teenage behavior, such as rebelliousness and risk-taking, could be caused by the rapid changes in the teenage brain.
"Teens often change in personality through those years, from being very emotional risk-takers to being very self-controlled by the mid-20s," Thompson said. "At the same time, the prefrontal cortex, which suppresses risky impulses and is involved in self-control, is growing the fastest. Wouldn't it be an intriguing thing if the brain changes mirror to some degree the change in behavior?"
If their theory proves true, it might point to a greater need for parental involvement during the teen years, Thompson said.
"It really enforces the need for good family support," Thompson said. "You need a lot of guidance through those years."
Giedd has gone a step further, suggesting that the types of activities teenagers engage in, from playing video games to watching MTV, could hurt their brain development. More constructive activities, such as playing sports or a musical instrument, could have positive repercussions for the brain.
Those inferences have drawn the ire of other experts in the field.
"Anything neuroscientists have to say about how the maturity of the frontal lobe relates to behavior is just speculation," said John Bruer, president of the James S. McDonnell Foundation in St. Louis, which funds research in cognitive neuroscience. "For every kid who's supposed to have unplugged frontal lobes, I can show you one who's playing the Mozart concerto."
Bruer, who is the author of "The Myth of the First Three Years" (Free Press, $25), which criticized the fervor about early brain growth, lambasted this latest theory as the next installment in pop psychology.
"I think people have a set of teenage behaviors that they don't like - take video games - and say if you do that while your frontal lobes are developing you'll be damaged forever, but if you play chess, you'll be a genius," Bruer said. "We really have to be careful in looking for purported biological explanations for very local social and cultural phenomena we may not like."
Meanwhile, other brain-development experts are hailing the newest theory as another drop in a deepening pool of evidence that indicates the frontal lobes may take two or more decades to fully develop.
"I think it's dead on," said Russell Barkley, professor of psychiatry and neurology at the University of Massachusetts Medical School.
Author of several books on attention deficit/hyperactivity disorder, Barkley has studied brain development in children and adolescents. He and many of his colleagues theorize that most people don't become fully self-aware until their early 30s.
"What we tend to do is look at teens as if they see things the same way we do, and of course they don't," Barkley said. "This is why all kids save their summer reading until Labor Day."
Giedd's and Thompson's observations help to explain why teenagers have a time horizon of one to three days, compared with the average 35-year-old, who has the capacity to plan 8 to 12 weeks ahead, Barkley said.
"It's that ability to plan ahead that is the essence of the frontal lobe," Barkley said. "What [Giedd and Thompson] have shown is that, of course, the frontal lobe is not mature yet."
As a result, Barkley said, kids don't always think things through. Typically, they don't fully anticipate the consequences of an act before they commit it.
"That's why the juvenile court system makes decisions differently than the adult court system," Barkley said. "It's founded on the idea that juveniles do not have the power that adults have to foresee the consequences of their actions."
It's the tough job of a parent to help teens grasp that there are consequences to their actions, said Kate Jacobs, a school social worker and mother of two teenagers in Oak Park, Ill.
"They just don't believe that any of the bad stuff that happens in the world can happen to them," Jacobs said.
And when it comes to planning, forget about it, Jacobs said.
"My son can't keep the week in focus, not without constant reminders," Jacobs said.
His first month of high school was about as relaxing as a fire drill.
"We had these frantic nights when he suddenly remembered he had something due the next day," Jacobs said. After weeks of panic, she got him a planning book.
Giving kids the tools to become independent is what parents should do , said Scott Hunter, a University of Chicago psychology professor and director of the university's pediatric neuropsychology service.
Whether the latest brain theory proves true, teenagers need parents who are involved but not overprotective, said Hunter, cautioning that parents could become too sheltering in response to the recent research.
"Part of being an adolescent and being an adult is making mistakes," Hunter said. "The nice part of making mistakes is being able to learn from them."
The brain can't develop without an interaction between experience and the brain's own biological processes, Hunter said.
"Growth is stimulated by experience and experience is what's enriching," Hunter said. "Sometimes parents do have to step in and make the right decision, but they need to help their adolescent develop the capacity for independence."
And, though teenagers generally are more likely to be impulsive and make inappropriate decisions, that tendency varies widely among individuals, making it unlikely that a rapidly sculpting prefrontal cortex is the only culprit, Hunter said.
"Brain development is probably not the only reason," Hunter said. "A lot of us have experiences when we're younger that help us make more mature decisions when we're teens."
Thompson agreed that a combination of factors, including a changing prefrontal cortex, might contribute to stereotypical teenage behavior.
"It needn't just be one or the other," Thompson said. "Now, we're well on the way toward piecing all these things together."