In charting the brain's normal development through adolescence, scientists discovered the brain's anatomy apparently keep on evolving far more than ever thought, which may help explain why can teens act the way they do.

Teenager Bryan Wright is one of hundreds of volunteers taking part in the research at the Child Psychiatry Branch of the National Institute of Mental Health in Bethesda, Md.

On summer evenings, Wright, 17, allows government researchers to peer into the very deepest recesses of his brain, just as they've been doing since he was 13.

Using magnetic resonance imaging, or MRI, government researchers can now journey through the brain, from top to bottom, left to right, back to front, and most important, from year to year.

"Recent advances in imaging technology, even over the last 18 months to two years, allow us to really zero in on specific brain parts," said Dr. Jay Giedd, who directs the project as chief of brain imaging at the research center.

Giedd, like most of his colleagues, had thought that since the brain has reached 90 percent of its full size by the age of 6, that it was virtually fully developed. Not so.

"What's new is that the brain's anatomy in the teen years is changing far more dynamically than we would ever have guessed," said Giedd.

In other words, the "makeup" of the brain, the individual elements of the brain, continue to evolve and become more efficient throughout the teenage years, even into a person's 20s.

New Findings Explain Mood Swings

Giedd's images show a thickening in gray matter on the outer part of the brain peaks at age 11 in girls and at age 12 in boys. Then, during the teen years, the brain trims back excess cells and connections so what's left is more efficient.

The cells and connections used the most survive and flourish, and those not used will whither and die, said Giedd.

The images show one of the last parts of the brain to complete this maturation process is the prefrontal cortex, the part of the brain responsible for planning, judgment and self-control.

"[Adolescents] are capable of very strong emotions and very strong passions, but their prefrontal cortex hasn't caught up with them yet. It's as though they don't have the brakes that allow them to slow those emotions down," said Charles Nelson, a child psychologist at the University of Minnesota.

Researchers say this may help explain the often irrational behavior of teenagers: the mood swings, and the risks they're often too willing to take.

And psychologists believe this new understanding of the adolescent brain — and its limitations — can help parents recognize there are some behaviors teenagers cannot easily control.

"If I walk into a class of kids who are 14 or 15," said Nelson, "those kids have a level of brain maturity that just does not map onto the kinds of emotional decision-making that a lot of those kids are being asked to make by teachers and parents."

Added Nelson: "The more teachers and the more parents that understand that there is a biological limitation to the child's ability to control and regulate emotion, [the more] they might be able to back off a little and be a bit more understanding."