MAY
8, 2004
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 debatethe burgeoning science
of brain developmentmay critically influence the high court's
final decision.
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.
MENTAL
MATURITY? New data on teens' unfinished brain development may aid
efforts to get rid of the juvenile death penalty in the United
States.
"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 2 like-minded colleaguesneuropsychologist Ruben Gur of the
University of Pennsylvania in Philadelphia and lawyer Stephen
Harper of the University of Miamispoke 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 volunteersall with active amygdalasincorrectly identified 1
in 4 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 tasksindicated by the time taken to answer them
correctlysuffers 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 expressionshappy, angry, sad,
or neutralafter 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 formationmeasured by
the total volume of white matter in the entire braindoesn'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.
source:
Science News (NCADP)
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