Teen Brains on Trial

The science of neural development tangles with the juvenile death penalty

Later this year, the U.S. Supreme Court will hear arguments about whether federal law should continue to permit executions of 16- and 17-year-olds convicted of murder. On this life-or-death issue, controversial legal and ethical views on teenagers’ capacity to control their behavior and obey the law will take center stage. However, a relative newcomer to the debate—the burgeoning science of brain development—may critically influence the high court’s final decision.

MENTAL MATURITY? New data on teens’ unfinished brain development may aid efforts to get rid of the juvenile death penalty in the United States. PhotoDisk

A coalition of psychiatric and legal organizations plans to submit a brief to the justices contending that teenagers often make poor decisions and act impulsively because their brains haven’t attained an adult level of organization. Consequently, the coalition argues, teenage killers are less culpable for their crimes than their adult counterparts are. Capital punishment of teens thus violates the constitutional amendment protecting citizens from cruel and unusual punishment.

“Our objection to the juvenile death penalty is rooted in the fact that adolescents’ brains function in fundamentally different ways than adults’ brains do,” says David Fassler, a psychiatrist at the University of Vermont in Burlington and a leader of the effort to infuse capital-crime laws with brain science.

Age-related brain differences pack a real-world wallop, in his view. “From a biological perspective,” Fassler asserts, “an anxious adolescent with a gun in a convenience store is more likely to perceive a threat and pull the trigger than is an anxious adult with a gun in the same store.”

Fassler and two like-minded colleagues—neuropsychologist Ruben Gur of the University of Pennsylvania in Philadelphia and lawyer Stephen Harper of the University of Miami—spoke in March at a Washington, D.C., press conference convened by groups that included the American Psychiatric Association and the American Bar Association.

Yet the zeal with which these organizations now wield brain studies to fight the juvenile death penalty masks a deep division among scientists about whether the data are ready for legal prime time.

Some researchers agree that capital-punishment laws should incorporate what’s known about teenagers’ incomplete brain development, even if the scientific story contains gaps. Don’t excuse criminal behavior, these scientists say, but acknowledge that adolescents who kill don’t deserve the ultimate punishment.

Members of another camp argue that brain science doesn’t belong in court because there’s no evidence linking specific characteristics of teens’ brains to any legally relevant condition, such as impaired moral judgment or an inability to control murderous impulses.

“Juvenile death sentences bother me, but this is an ethical issue,” remarks Harvard University psychologist Jerome Kagan. “The brain data don’t show that adolescents typically have reduced legal culpability for crimes.”

Frontal assault

Plans to apply brain science to balance the scales of justice come at a time when the juvenile death penalty is already on the defensive.

As of January 2004, 29 states prohibited capital punishment of juveniles.

Legislation to bar the death penalty for offenders under 18 years old is being considered in 14 additional states. Juvenile-death-penalty foes find this trend encouraging, since the Supreme Court justified its 2002 ruling against executing mentally retarded offenders by citing bans on that practice in 30 states.

Another heartening sign for opponents of the juvenile death penalty occurred in December 2003, when a Virginia jury decided to sentence 17-year-old Lee Malvo to life in prison for his participation in the D.C.-area sniper killings.

However, growing evidence that teenagers possess unfinished brains has received far more attention in the media than in the courts, Harper says. The legal system doesn’t appreciate that young people’s brains aren’t fully equipped for making long-term plans and reining in impulses, he contends.

Much of the concern about teen brains focuses on the frontal lobes. One way that scientists have learned about frontal lobe activity is by identifying associations between certain behaviors and increased frontal activity in healthy people. That work elaborated on previous studies of behavior changes in individuals who had suffered frontal-brain damage. Together, the findings implicate this neural region in regulating aggression, long-range planning, mental flexibility, abstract thinking, the capacity to hold in mind related pieces of information, and perhaps moral judgment.

Other investigations indicate that the number of brain cells and their connections surge just before puberty. But through late adolescence, pruning of excess neurons and their linkages produces substantial declines in the volume of the part of the brain, called the gray matter, that contains the cell bodies.

Therefore, the brain changes during adolescence mirror the initial wave of gray matter expansion in the womb and during the first 18 months of life, followed by a trimming-back period.

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—all with active amygdalas—incorrectly identified one in four fear expressions, usually labeling them as angry, sad, or confused.

In an ensuing fMRI study directed by Yurgelun-Todd, 16 participants ages 12 to 17 also erred frequently when labeling the emotion on fearful faces. Those less than 14 years old answered incorrectly about half the time and yet showed the most amygdala activity, while older teens made fewer errors and displayed less activity in the amygdala and more in the frontal lobes than the younger participants did.

Previous studies had found that, when given the same task, adults label most fearful expressions correctly and exhibit much more activity in the frontal lobes than in the amygdala.

The results in these small experiments remain preliminary. Even if the findings hold up, it’s not clear whether young teens’ difficulties in discerning fearful expressions stem from incomplete brain development or reflect unique duties assumed by the frontal lobes during adolescence. What’s more, teenagers and adults have yet to be similarly tested with faces displaying emotions other than fear.

Baird’s ongoing research suggests that the teen frontal brain indeed responds to spontaneous emotional expressions on the faces of friends and family members.

“Kids say that the posed expressions we show them look kind of weird,” Baird says.

Other evidence suggests that mental efficiency in solving emotion-related tasks—indicated by the time taken to answer them correctly—suffers with the arrival of puberty, when gray matter volume in the frontal lobes hits its peak, according to Robert F. McGivern of San Diego State University.

Response speed improves gradually after puberty and stabilizes at around age 15, a time when substantial neural pruning and myelin expansion in the frontal lobes have already occurred, McGivern and his colleagues reported in 2002.

The researchers had studied 246 youngsters, ages 10 to 17, and 49 young adults, ages 18 to 22. In one trial, participants saw a series of faces with various posed expressions—happy, angry, sad, or neutral—after being told to answer “yes” if they saw a happy face and “no” for all others. Each face appeared for only a fraction of a second.

The participants then completed three additional trials in which they were told to answer “yes” for angry, sad, or neutral faces.

Girls responded to these problems more slowly at ages 11 and 12 than they did at age 10, while boys took longer to answer at age 12 than they did at ages 11 or 10. These declines closely corresponded to puberty’s onset in each sex, McGivern says.

Cycles of brain growth in boys and girls, which are timed differently during adolescence, sometimes aid and sometimes hinder mental dexterity in detecting various emotions, in McGivern’s view.

Risky business

Scientists are also beginning to probe the brain’s contributions to teenagers’ penchant for risky and impulsive behaviors, such as experimenting with illicit drugs. Preliminary data indicate that, while playing a simple game to win monetary prizes, adolescents exhibit weaker activity than young adults do in a brain region that scientists consider to be crucial for motivating efforts to obtain rewards or attain goals.

A team led by James M. Bjork of the National Institute on Alcohol Abuse and Alcoholism in Bethesda, Md., used fMRI to scan the brains of 24 people, half between ages 12 and 17 and the rest between 22 and 28. Brain measurements were taken as the participants decided whether to press a button upon seeing various visual cues, only one of which they had been told to respond to. On some trials, correct answers yielded prizes of 20 cents, 1 dollar, or 5 dollars. On others, correct answers prevented losses of those amounts.

The prospect of gaining or losing money elicited many common responses in the brains of teens and young adults, the scientists reported in the Feb. 25 Journal of Neuroscience. However, on potential moneymaking trials, teens displayed unusually weak activity in the right ventral striatum, a structure at the brain’s base that’s been implicated in fueling the motivation to acquire rewards.

This finding is consistent with the theory that the amount of stimulation that’s enough to give adults a motivational boost is insufficient to arouse teens. To get the same rewarding feeling, teens may seek the added lift that comes from risky behaviors. Bjork and his coworkers plan to conduct larger fMRI studies of teen motivation that include youngsters prone to delinquency and drug abuse.

There’s still a long way to go in untangling how brain development influences what teens do and why they do it, remarks Jay N. Giedd of the National Institute of Mental Health in Bethesda. Courts and legislatures grappling with the juvenile death penalty nonetheless need to consider the brain’s unfinished status during adolescence, especially in the frontal lobes, according to Giedd, a pioneer in research on brain development.

Adds neuroscientist Bruce McEwen of Rockefeller University in New York City, “There’s enough known about brain development to call for serious discussions between scientists and the legal community.”

Immature data

UCLA’s Elizabeth Sowell, another prominent brain-development researcher, takes a dim view of the movement to apply neuroscience to the law. Delayed frontal-lobe maturation may eventually be shown to affect teenagers’ capacity to make long-term plans and control their impulses, she says, but no current research connects specific brain traits of typical teenagers to any mental or behavioral problems.

“The scientific data aren’t ready to be used by the judicial system,” she remarks. “The hardest thing [for neuroscientists to do] is to bring brain research into real-life contexts.”

The ambiguities of science don’t mix with social and political causes, contends neuroscientist Bradley S. Peterson of the Columbia College of Physicians and Surgeons in New York City. For instance, it’s impossible to say at what age teenagers become biologically mature because the brain continues to develop in crucial ways well into adulthood, he argues.

A team led by Sowell and Peterson used an MRI scanner to probe the volume of white and gray matter throughout the brains of 176 healthy volunteers, ages 7 to 87. The researchers reported in the March 2003 Nature Neuroscience that myelin formation—measured by the total volume of white matter in the entire brain—doesn’t reach its peak until around age 45.

Although gray matter volume generally declines beginning around age 7, it steadily increases until age 30 in a temporal-lobe region associated with language comprehension.

Such findings underscore the lack of any sharp transition in brain development that signals maturity, according to neuroscientist William T. Greenough of the University of Illinois at Urbana-Champaign. Definitions of adulthood change depending on social circumstances, Greenough points out. Only 200 years ago, Western societies regarded 16-year-olds as adults.

“Brain science offers no simple take-home message about adolescents,” says B.J. Casey of Cornell University’s Weill Medical College in New York City. “It’s amazing how little we know about the developing brain.”

Brain-scanning techniques, including the popular fMRI, remain a “crude level of analysis,” Casey notes. At best, blood-flow measurements indirectly tap into brain-cell activity as people perform a task, such as identifying emotions in posed faces, that may superficially simulate a real-world endeavor. What’s more, many critical brain-cell responses are too fast for MRI to track.

Brain data, particularly those on delayed frontal-lobe growth in adolescents, also need to be put in a cultural and historical perspective, Harvard’s Kagan asserts. Frontal-lobe development presumably proceeds at roughly the same pace in teenagers everywhere. Yet current rates of teen violence and murder vary from remarkably low to alarmingly high from country to country, he notes.

“Something about cultural context must be critical here,” Kagan says. “Under the right conditions, 15-year-olds can control their impulses without having fully developed frontal lobes.”

If incomplete brains automatically reduce adolescents’ capacity to restrain their darker urges, “we should be having Columbine incidents every week,” he adds.

Several research teams have now undertaken the difficult task of searching for links between specific traits of teens’ brains and their real-life decisions and behaviors, says psychiatrist Ronald Dahl of the University of Pittsburgh Medical Center. “Brain data are eventually going to support reduced legal culpability for adolescents,” Dahl predicts “but we’re not quite there yet.”

It remains to be seen where the Supreme Court is.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.