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Know brains from brawn

For thousands of years, bewildered parents have been asking the same question:

Why do our kids behave the way they do?

The children now love luxury; they have bad manners, contempt for authority; they show disrespect for elders and love chatter in place of exercise. Children are now tyrants, not the servants of their households. They no longer rise when elders enter the room. They contradict their parents, chatter before company, gobble up dainties at the table, cross their legs, and tyrannize their teachers.

Socrates, 5th century BC1

Adults have long struggled with understanding the thoughts, words and actions of the young. For centuries, we blamed the times or the new generation for the behavior of children and adolescents. In more recent decades, the finger has been pointed at raging hormones. Only in the last decade or so have scientific advances made it possible for us to see the real explanation lies in the normal development of the human brain.

Until recently, scientists held that the human brain was fully developed during early childhood. They believed there was a spurt of overproduction of gray matter during the first 18 months of life, followed by a steady decline as unused circuitry was discarded. Now, thanks to the internal picture-taking technology known as MRI (magnetic resonance imaging), we know brain growth during infancy is only the first wave of development.

We now know puberty starts in the brain.

In the late 1990s, Dr. Jay Giedd and colleagues at the National Institute of Mental Health in Bethesda, Maryland, discovered a second wave of overproduction of gray matter. This second wave takes place just prior to puberty, and is followed by major pruning of the brain’s synapses during adolescence. The brain’s structural maturation process then begins its final phase around age 16 or 17.2 And, brain growth isn’t complete until well into early adulthood, peaking at around age 25.

Time-Lapse Imaging Tracks Brain Maturation from ages 5 to 20
Source of brain images: 
Courtesy, Dr. Paul M. Thompson, Laboratory of Neuro Imaging

The brain images shown here were constructed from MRI scans of healthy kids. Red indicates more gray matter (the “working tissue” of the brain’s cortex), and blue indicates less gray matter. As the brain matures, gray matter declines in a back-to-front wave, reflecting normal neural pruning during adolescence.

In a young child, the first brain areas to mature are at the  extreme front and back of the brain. These are the areas responsible for the most basic functions, such as processing    the senses and movement. Areas involved in spatial      orientation and language (the parietal lobes) develop next.
Until children are about 11 years old, the part of their brains
that will eventually handle abstract thought – the cortex –
still isn’t fully developed. Their understanding of the world is limited to what they experience directly, including what they already know, see, touch and hear.

Areas with more advanced “executive” functions, including the prefrontal cortex, the brain's center of reasoning and problem solving – mature last.3 Importantly, the prefrontal cortex also controls working memory, inhibition and impulse control. So the delayed development of the prefrontal cortex means a lot of teenagers are simply not equipped to recognize the consequences of their actions. In adults, a fully developed frontal lobe curbs impulses coming from other parts of the brain. By contrast, the teen brain lacks brakes, and parents see the consequences.

No one makes it through the teenage years unscathed. Even the meekest, smartest, most obedient and sensible teenager will, at one point or another, find himself or herself facing the angry, disbelieving face of an adult who shouts, “What were you thinking?"

- Author and Psychologist, David Walsh.

So what does “neural pruning” mean? In neuroscience, pruning is a term used to describe the process in which unused synapses – tiny branches in the brain that transmit chemical signals between neurons – are cut back in normal development. Neurons are nerve cells that transmit information in the brain through electrical and chemical signals.

Babies are born with about 100 billion neurons, each of which has an average of ten thousand branches, or synapses. Only about 17 percent of the neurons are linked. In the weeks, months, years and decades that follow, all the rest of those billions of neurons get wired together.5 Within just the first few years of life, the number of neurons is roughly halved, as the brain prunes itself. Synapses are linked to make the transmission of information throughout the brain more efficient, and areas of the brain that are not used are left to wither.

The brain’s “use it or lose it” approach to neural connections explains why certain abilities, such as learning a foreign language without an accent, can generally be developed only before puberty. This means the teenage brain takes cognitive leaps, but there is also a gradual slowing down of other abilities.6

Adolescence is a time of substantial neurobiological and behavioral change, but the teen brain is not a broken or defective adult brain….These changes and the enormous plasticity of the teen brain make adolescence a time of great risk and great opportunity.7

- Jay Giedd, Chief, Brain Imaging Unit, Child Psychiatry, NIMH

Given the tremendous brain activity occurring during the teenage years, it’s no wonder we find our kids mystifying at this stage. Adolescents may have grown to the size of adults in body, but the interior infrastructure is far from complete. This is why prominent child psychiatrist Dr. Jean Clinton has given us two words to remember when relating to teens: “Under Construction.”

Higher-order brain centers don't fully develop until well after most kids have graduated from university. Gray matter is expanding until around age 24 for women and age 27 for men. So from toddler to young adult, their developing brains mean our children benefit from parental guidance for a long time. The question is: What form should our guidance take as our kids grow and change?


  1. Attributed by Plato, according to William L. Patty and Louise S. Johnson, Personality and Adjustment, (New York: McGraw-Hill, 1953), p. 277.
  2. Science Magazine, Editorial, (November 2004); Time Magazine, (May 10, 2004).
  3. Elizabeth R. Sowell, Paul M. Thompson, Kevin D. Tessner, and Arthur W. Toga, “Mapping Continued Brain Growth and Gray Matter Density Reduction in Dorsal Frontal Cortex: Inverse Relationships during Postadolescent Brain Maturation,” Journal of Neuroscience, (November, 2001).
  4. David Walsh, Why Do They Act That Way? A Survival Guide to the Adolescent Brain for You and Your Teen, (New York: Free Press, 2004), p. 55.
  5. Ibid., pp. 27-28
  6. Barbara Strauch, The Primal Teen: What the new Discoveries about the Teenage Brain Tell Us about Our Kids, (New York: Random House, Inc., 2003), p. 39.
  7. Jay Giedd, “The Teen Brain: Insights from Neuroimaging,” Journal of Adolescent Health, Volume 42, Issue 4 (April 2008), pp. 335-343.

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