Using magnetic resonance imaging (MRI) scanners to probe the brains of healthy teenagers and young adults, Elizabeth R. Sowell of the University of California, Los Angeles (UCLA) and her colleagues reported in 1999 that myelin, the fatty tissue around nerve fibers that fosters transmission of electrical signals, accumulates especially slowly in the frontal lobe.

The late phase of myelin formation, occurring in teenagers, provides a neural basis for assuming that teens are less blameworthy for criminal acts that adults are, Gur says. There's no way to say whether, for example, an individual 17-year-old possesses a fully mature brain. But the biological age of maturity generally falls around age 21 or 22, in Gur's view.

Although 18 years old represents an arbitrary cutoff age for receiving a capital sentence, it's preferable to 17, according to Gur.

"These brain data create reasonable doubt that a teenager can be held culpable for a crime to the same extent that an adult is," agrees neuroscientist J. Anthony Movshon of New York University.

Fear factor

Abigail A. Baird of Dartmouth College in Hanover, N.H., also suspects that delayed neural development undermines teens' judgment in ways that affect their legal standing. "There's no reason to say adulthood happens at age 18," Baird says. Unlike Gur, however, she estimates that the brain achieves maturity at age 25 or 26.

A 1999 investigation led by Baird and Deborah Yurgelun-Todd of Harvard Medical School in Boston raised the possibility that certain characteristics of teens' brains make it difficult for them to recognize when other people are scared. They tested 12 teenagers, ages 12 to 17. A functional magnetic resonance imaging (fMRI) scanner measured changes throughout participants' brains in blood flow, which studies have indicated reflect dips and rises in neural activity. As the teens briefly viewed and identified fear in pictures of people who had intentionally tried to look scared, the researchers observed marked increases in activity of an almond-shaped inner-brain structure called the amygdala.

Neuroscientists suspect that the amygdala is important for learning to attach emotional significance to facial expressions and other stimuli. However, the results of Baird and Yurgelun-Todd indicated that there may not be a simple relationship between amygdala activity and accurate face reading.

The teen volunteers\ufffdall with active amygdalas\ufffdincorrectly identified one in four fear expressions, usually labeling them as angry, sad, or confused.

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