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32 Scientific American, June 2015
© 2015 Scientific American
June 2015, ScientificAmerican.com 33
amazing
A mismatch in the maturation of brain networks
leaves adolescents open to risky behavior but also
allows for leaps in cognition and adaptability
By Jay N. Giedd
IN BRIEF
MRI studies show that the teenage brain is not an
old child brain or a half-baked adult brain; it is a
unique entity characterized by change ability and an
increase in networking among brain regions.
The limbic system, which drives emotions, intensi-
es at puberty, but the prefrontal cortex, which con-
trols impulses, does not mature until the 20s. This
mismatch makes teens prone to risk taking but also
allows them to adapt readily to their environment.
Earlier onset of puberty in children worldwide is ex-
panding the years during which the mismatch occurs.
Greater understanding of the teen brain should help
parents and society better distinguish typical behav-
ior from mental illness while helping teens become
the people they want to be.
Jay N. Giedd is chair of the division of child and
adolescent psychiatry at the University of California,
San Diego, and a professor at the Johns Hopkins
Bloomberg School of Public Health. He is also editor
in chief of the journal Mind, Brain, and Education.
NEUROSCIENCE
Illustration by Harry Campbell, Photograph by Ethan Hill
© 2015 Scientific American© 2015 Scientific American
34 Scientific American, June 2015
PAGE 32: GROOMING BY TINNA EMPERA Cloutier/Remix; STYLING BY MONICA COTTO;
PHOTOGRAPH FOR ILLUSTRATION PURPOSES ONLY
Neuroscientists have explained the risky, aggressive or just plain
baing behavior of teenagers as the product of a brain that is
somehow compromised. Groundbreaking research in the past
10years, however, shows that this view is wrong. The teen brain
is not defective. It is not a half-baked adult brain, either. It has
been forged by evolution to function dierently from that of a
child or an adult.
Foremost among the teen brain’s features is its ability to
change in response to the environment by modifying the com-
munications networks that connect brain regions. This special
changeability, or plasticity, is a double-edged sword. It allows
teenagers to make enormous strides in thinking and socializa-
tion. But the morphing landscape also makes them vulnerable
to dangerous behaviors and serious mental disorders.
The most recent studies indicate that the riskiest behaviors
arise from a mismatch between the maturation of networks in
the limbic system, which drives emotions and becomes turbo-
boosted in puberty, and the maturation of networks in the pre-
frontal cortex, which occurs later and promotes sound judgment
and the control of impulses. Indeed, we now know that the pre-
frontal cortex continues to change prominently until well into a
person’s 20s. And yet puberty seems to be starting earlier, extend-
ing the “mismatch years.”
The plasticity of networks linking brain regions—and not the
growth of those regions, as previously thought—is key to even-
tually behaving like an adult. Understanding that, and knowing
that a widening gap between the development of emotional and
judgment networks is happening in young people today, can
help parents, teachers, counselors and teenagers themselves.
People will better see that behaviors such as risk taking, sensa-
tion seeking, and turning away from parents and toward peers
are not signs of cognitive or emotional problems. They are a nat-
ural result of brain development, a normal part of adolescents
learning how to negotiate a complex world.
The same understanding can also help adults decide when to
intervene. A 15-year-old girl’s departure from her parents’ tastes
in clothing, music or politics may be a source of consternation
for Mom and Dad but does not indicate mental illness. A 16-year-
old boy’s propensity to skateboard without a helmet or to accept
risky dares from friends is not trivial but is more likely a manifes-
tation of short-range thinking and peer pressure than a desire to
hurt himself. Other exploratory and aggressive actions might be
red flags, however. Knowing more about the unique teen brain
will help all of us learn how to separate unusual behavior that is
age-appropriate from that which might indicate illness. Such
awareness could help society reduce the rates of teen addiction,
The “teen bra in”
is often ridiculed as an
oxymoron—an example
of biology gone wrong.
sexually transmitted diseases, motor vehicle accidents, unwant-
ed pregnancy, homicide, depression and suicide.
GREATER CONNECTIVITY
will be surprised to hear that the brain
of a 16-year-old is dierent from the brain of an eight-year-old.
Yet researchers have had diculty pinning down these dier-
ences in a scientific way. Wrapped in a tough, leathery mem-
brane, surrounded by a protective moat of fluid and completely
encased in bone, the brain is well protected from falls, attacks
from predators—and the curiosity of scientists.
The invention of imaging technologies such as computerized
tomography and positron-emission tomography has o ered some
progress, but because these techniques emit ionizing radiation,
it was unethical to use them for exhaustive studies of youth. The
advent of magnetic resonance imaging (MRI) finally provided a
way to lift the veil, oering a safe and accurate way to study the
anatomy and physiology of the brain in people of all ages. Ongo-
ing studies are tracking thousands of twins and single individu-
als throughout their lives. The consistent theme that is emerging
is that the adolescent brain does not mature by getting larger; it
matures by having its dierent components become more inter-
connected and by becoming more specialized.
In MRI scans, the increase in connectivity among brain re -
gions is indicated as greater volumes of white matter. The “white”
in white matter comes from a fatty substance called my elin,
which wraps and insulates the long wire, or axon, that ex tends
from a neuron’s body. Myelination—the formation of this fatty
sheath—takes place from childhood through adulthood and
significantly speeds up the conduction of nerve impulses among
neurons. Myelinated axons transmit signals up to 100 times fast-
er than unmyelinated ones.
Myelination also accelerates the brain’s information pro-
cessing by helping axons recover quickly after they fire so that
they are ready to send another message. Quicker recovery time
allows up to a 30-fold increase in the frequency with which a
given neuron can transmit information. The combination of
faster transmission and shorter recovery time provides a 3,000-
fold increase in the brain’s computational bandwidth between
infancy and adulthood, permitting extensive and elaborate net-
working among brain regions.
Recent investigations are revealing another, more nuanced
© 2015 Scientific American
June 2015, ScientificAmerican.com 35Illustration by David Killpack (brains) and Jen Christiansen (nodal diagrams)
SOURCE: “DEVELOPMENT OF BRAIN STRUCTURAL CONNECTIVITY BETWEEN AGES 12 AND 30,”
BY EMILY L. DENNIS ET AL., IN NEUROIMAGE, VOL. 64; JANUARY 1, 2013 (SUPPLEMENTARY VIDEO 2)
role for myelin. Neurons integrate information from other neu-
rons but only fire to pass it on if the incoming input exceeds a
certain electrical threshold. If the neuron fires, that action initi-
ates a series of molecular changes that strengthens the synapses,
or connections, between that neuron and the input neurons.
This strengthening of connections forms the basis for learn-
ing. What researchers themselves are now learning is that for
input from nearby and distant neurons to arrive simultaneous-
ly at a given neuron, the transmission must be exquisitely
timed, and myelin is intimately involved in the fine-tuning of
this timing. As children become teenagers, the rapid expansion
of myelin increasingly joins and coordinates activities in dier-
ent parts of the brain on a variety of cognitive tasks.
Scientists can now measure this changing interconnectivity
by applying graph theory, a type of mathematics that quantifies
the relation between “nodes” and “edges” in a network. Nodes
can be any object or detectable entity, such as a neuron or a brain
structure like the hippocampus or a larger region such as the
prefrontal cortex. Edges can be any connections among nodes,
from a physical connection such as a synapse between neurons
to a statistical correlation such as when two parts of the brain
are activated similarly during a cognitive task.
Graph theory has helped me and others to measure how dif-
ferent brain regions develop and become interconnected to one
another and to correlate such features with changes in behavior
and cognition. Brain changes are not confined to adolescence.
Most brain circuits develop in the womb, and many continue to
change throughout life, well beyond the teen years. It turns out,
however, that during that period there is a dramatic increase in
connectivity among brain regions involved in judgment, getting
along with others and long-range planning—abilities that pro-
foundly influence the remainder of a person’s life.
TIME TO SPECIALIZE
along neurons is developing with age in
adolescents, another change is taking place. Brain development,
like other complex processes in nature, proceeds by a one-two
punch of overproduction, followed by selective elimination.
Like Michelangelo’s David emerging from a block of marble,
many cognitive advances arise during a sculpting process in
which unused or maladaptive brain cell connections are pruned
away. Frequently used connections, meanwhile, are strength-
ened. Al though pruning and strengthening occur throughout
our lives, during adolescence the balance shifts to elimination,
as the brain tailors itself to the demands of its environment.
Specialization arises as unused connections among neurons are
eliminated, decreasing the brain’s gray matter. Gray matter consists
largely of unmyelinated structures such as neuron cell bodies, den-
drites (antennalike projections from the cells that receive informa-
tion from other neurons) and certain axons. Overall, gray matter
increases during childhood, reaches a maximum around age 10
and declines through adolescence. It levels o during adulthood
and declines somewhat further in senescence. The pattern also
holds for the density of receptor cells on neurons that respond to
neurotransmitters—molecules such as dopamine, serotonin and
glutamate that modulate communication among brain cells.
Although the raw amount of gray matter tops out around
puberty, full development of dierent brain regions occurs at dif-
ferent times. Gray matter, it turns out, peaks earliest in what are
called primary sensorimotor areas devoted to sensing and re -
sponding to sight, sound, smell, taste and touch. It peaks latest
in the prefrontal cortex, crucial to executive functioning, a term
that encompasses a broad array of abilities, including or -
ganization, decision making and planning, along with the regu-
lation of emotion.
Greater Networking Brings Maturity
The most signicant change taking place in an adolescent brain is
not the growth of brain regions but the increase in communica-
tions among groups of neurons. When an analytical technique
called graph theory is applied to data from MRI scans, it shows
that from ages 12 to 30, connections between certain brain regions
or neuron groups become stronger ( black lines that get thicker ). The
analysis also shows that certain regions and groups become more
widely connected ( green circles that get larger ). These changes ulti-
mately help the brain to specialize in everything from complex
thinking to being socially adept.
A NEW VIEW
Increasing Communications among Brain Regions over Time
Age 12 Age 30
Stronger connectionMore connections
© 2015 Scientific American © 2015 Scientific American
36 Scientific American, June 2015 Illustration by David Killpack (brain) and Jen Christiansen (graphic)
SOURCE: JAY N. GIEDD
An important feature of the prefrontal
cortex is the ability to create hypothetical
what-ifs by mental time travel—to consid-
er past, present and possible future out-
comes by running simulations in our
mind instead of subjecting ourselves to
potentially dangerous reality. As philoso-
pher Karl Popper phrased it, instead of
putting ourselves in harm’s way, “our the-
ories die in our stead.” As we mature cog-
nitively, our executive functioning also
makes us more likely to choose larger,
longer-term rewards over smaller, short-
er-term ones.
The prefrontal cortex is also a key
component of circuitry involved in social
cognition—our ability to navigate com-
plex social relationships, discern friend
from foe, find protection within groups
and carry out the prime directive of ado-
lescence: to attract a mate.
Adolescence is therefore marked by
changes in gray matter and in white mat-
ter that together transform the network-
ing among brain regions as the adult
brain takes shape. The prefrontal cortex
functions are not absent in teenagers;
they are just not as good as they are going
to get. Because they do not fully mature
until a person’s 20s, teens may have trou-
ble controlling impulses or judging risks
and rewards.
A MISMATCH IN MATURATION
, the hor-
mone-fueled limbic system undergoes
dramatic changes at the time of puberty,
which traditionally begins between ages
10 and 12. The system regulates emotion
and feelings of reward. It also interacts with the prefrontal cor-
tex during adolescence to promote novelty seeking, risk taking
and a shift toward interacting with peers. These behaviors,
deeply rooted in biology and found in all social mammals,
encourage tweens and young teens to separate from the com-
fort and safety of their families to explore new environments
and seek outside relationships. These behaviors diminish the
likelihood of inbreeding, creating a healthier genetic popula-
tion, but they can also pose substantial dangers, especially
when mixed with modern temptations such as easy access to
drugs, firearms and high-speed motor vehicles, unchecked by
sound judgment.
What most determines teen behavior, then, is not so much
the late development of executive functioning or the early onset
of emotional behavior but a mismatch in the timing of the two
developments. If young teens are emotionally propelled by the
limbic system, yet prefrontal control is not as good as it is going
to get until, say, age 25, that leaves a decade of time during
which imbalances between emotional and contemplative think-
ing can reign. Furthermore, puberty starting at an earlier age, as
is the case worldwide, lengthens the gap of time between the
onset of increased risk taking and sensation seeking and the rise
of a strong, stabilizing prefrontal cortex.
The lengthening mismatch supports the growing notion that
the teen years are no longer synonymous with adolescence. Ad -
olescence, which society defines as the transition from child-
hood to adulthood, begins in biology with the onset of puberty
but ends in a social construct when a person achieves indepen-
dence and assumes adult roles. In the U.S., attainment of an
adult role—often characterized by such events as getting mar-
ried, having a child and owning a home—is occurring approxi-
mately five years later than in the 1970s.
The large influence of social factors in determining what
constitutes an adult has led some psychologists to suggest that
adolescence is less of a biological reality than a product of
changes in child rearing since the industrial revolution. Yet twin
studies, which examine the relative eects of genes and environ-
ment by following twins who have dierent experiences, refute
the view that social factors can substantially override the biolo-
gy. They show that the pace of biological maturation of white
Emotion vs. Control
Teenagers are more likely than children or adults to engage in risky behavior, in part
because of a mismatch between two major brain regions. Development of the hormone-
fueled limbic system ( purple ), which drives emotions, intensies as puberty begins (typi-
cally between ages 10 to 12), and the system matures over the next several years. But the
prefrontal cortex ( green ), which keeps a lid on impulsive actions, does not approach full
development until a decade later, leaving an imbalance during the interim years. Puberty
is starting earlier, too, boosting hormones when the prefrontal cortex is even less mature.
ROOTS OF RISK TAKING
Degree of Maturation
Limbic region
Prefrontal region
0 30 252015105Age:
Limbic region
Prefrontal cortex
Development mismatch
© 2015 Scientific American
and gray matter can be influenced somewhat by the environ-
ment but that the fundamental timing is under biological con-
trol. Sociologists see this, too; risk taking, sensation seeking
and a move toward peers happen in all cultures, although the
degree can vary.
VULNERABILITY AND OPPORTUNITY
, white matter and networking developments
detected by MRI underscore the observation that the most strik-
ing feature in teen brain development is the extensive changes
that occur. In general, this plasticity decreases throughout adult-
hood, and yet we humans still retain a level of plasticity far lon-
ger than any other species.
Protracted maturation and prolonged plasticity allow us to
“keep our options open” in the course of our own development,
as well as the entire species’ evolution. We can thrive every-
where from the frigid North Pole to hot islands on the equator.
With technologies developed by our brain, we can even live in
vessels orbiting our planet. Back 10,000 years ago—a blink of
an eye in evolutionary terms—we spent much of our time se -
curing food and shelter. Today many of us spend most of our
waking hours dealing with words and symbols—which is par-
ticularly noteworthy, given that reading is only 5,000 years old.
Prolonged plasticity has served our species well but creates
vulnerabilities in addition to opportunities. Adolescence is the
peak time of emergence for several types of mental illnesses,
including anxiety disorders, bipolar disorder, depression, eating
disorders, psychosis and substance abuse. Surprisingly, 50 per-
cent of the mental illnesses people experience emerge by age 14,
and 75 percent start by age 24.
The relation between typical adolescent brain changes and
the onset of psychopathology is complicated, but one underly-
ing theme may be that “moving parts get broken.” The idea is
that the extensive changes in white matter, gray matter and net-
working increase the chance for problems to arise. For example,
almost all the abnormal brain findings in adult schizophrenia
resemble the typical changes of adolescent brain development
gone too far.
In many other ways, adolescence is the healthiest time of life.
The immune system, resistance to cancer, tolerance to heat and
cold, and other traits are at their greatest. Despite physical robust-
ness, however, serious illness and death are 200 to 300 percent
higher for teens than for children. Motor vehicle accidents, the
number-one cause, account for about half of teen deaths. Homi-
cide and suicide rank second and third.
Unwanted teen preg nancy,
sexually transmitted diseases and behavior leading to incar-
ceration are also high, imposing tough, lifelong consequences.
So what can doctors, parents, teachers and teens themselves
do about these pitfalls? For clinicians, the paucity of novel med-
ications in psychiatry and the propensity of the adolescent brain
to respond to environmental challenges suggest that nonmedi-
cation interventions may be most fruitful—especially early in
teen development, when white matter, gray matter and net-
working are changing fast. Treatment of obsessive-compulsive
disorder is one example; behavioral interventions that trigger
the obsessive impulse but gradually modify a person’s response
may be highly eective and could prevent a lifetime of disability.
Appreciating that the brain is changeable throughout the teen
years obliterates the notion that a youth is a “lost cause.” It oers
optimism that interventions can change a teenager’s life course.
More study will help, too. The infrastructure for adolescent
research is not well developed, funding for this work is meager
and few neuroscientists specialize in this age group. The good
news is that as researchers clarify the mechanisms and influenc-
es of adolescent brain developments, more resources and scien-
tists are being drawn into the field, eager to minimize risks for
teenagers and harness the incredible plasticity of the teen brain.
Understanding that the adolescent brain is unique and rap-
idly changing can help parents, society and teens themselves to
better manage the risks and grasp the opportunities of the teen-
age years. Knowing that prefrontal executive functions are still
under construction, for example, may help parents to not over-
react when their daughter suddenly dyes her hair orange and
instead take solace in the notion that there is hope for better
judgment in the future. Plasticity also suggests that construc-
tive dialogue between parents and teens about issues such as
freedoms and responsibilities can influence development.
Adolescents’ inherent capacity to adapt raises questions
about the impact of one of the biggest environmental changes
in history: the digital revolution. Computers, video games, cell
phones and apps have in the past 20 years profoundly aected
the way teens learn, play and interact. Voluminous information
is available, but the quality varies greatly. The skill of the future
will not be to remember facts but to critically evaluate a vast
expanse of data, to discern signal from noise, to synthesize con-
tent and to apply that synthesis to real-world problem solving.
Educators should challenge the adolescent brain with these
tasks, to train its plasticity on the demands of the digital age.
Greater society has some compelling opportunities as well.
For one thing, it could be more focused on harnessing the pas-
sion, creativity and skills of the unique adolescent development
period. Society should also realize that the teen years are a turn-
ing point for a life of peaceful citizenship, aggression or, in rare
cases, radicalization. Across all cultures, adolescents are the
most vulnerable to being recruited as soldiers and terrorists, as
well as the most likely to be influenced to become teachers and
engineers. Greater understanding of the teen brain could also
help judges and jurors reach decisions in criminal trials.
For teens themselves, the new insights of adolescent neuro-
science should encourage them to challenge their brain with the
kinds of skills that they want to excel at for the remainder of
their lives. They have a marvelous opportunity to craft their own
identity and to optimize their brain according to their choosing
for a data-rich future that will be dramatically dierent from the
present lives of their parents.
MORE TO EXPLORE
The Primal Teen: What the New Discoveries about the Teenage Brain Tell Us
about Our Kids. Barbara Strauch. Doubleday, 2003.
Development of Brain Structural Connectivity between Ages 12 and 30:
A 4-Tesla Diusion Imaging Study in 439 Adolescents and Adults. Emily L.
Dennis et al. in NeuroImage, Vol. 64, pages 671–684; January 1, 2013.
Age of Opportunity: Lessons from the New Science of Adolescence. Laurence
Steinberg. Houghton Miin Harcourt, 2014.
FROM OUR ARCHIVES
The Myth of the Teen Brain. Robert Epstein; Scientic American Mind, April/May 2007.
scientificamerican.com/magazine/sa
For a review of the eectiveness of punishments for juvenile oenders, see ScienticAmerican.com/jun2015/giedd
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