The Mystery of the Teenage Brain

Natalie, a 14-year-old honour student who normally juggles babysitting requests with her after-school job, got caught driving her brother’s car without a licence. Isaac, a quiet, withdrawn and sometimes sullen-looking young man of 15, surprised his father by becoming the lead singer of a heavy metal rock band. He completed his new look by getting his eyebrow and lower lip pierced.

Were these decisions accompanied by thoughtful consideration? Could parents have anticipated such actions beforehand? Did the teens engaging in these behaviours consider the long-term consequences? The answer to all these questions is likely no.

On the flip side, Marshal, a 12-year-old young man known for incessant babbling and manic behaviour, surprised his mother by organizing a surprise birthday party for her complete with all the trimmings. Toryn, a 14-year-old who changes her make-up and hair color as often as her socks, shovelled snow for her elderly neighbour early one blustery Saturday morning—without being asked!

Such mercurial behaviour has puzzled adults for hundreds of years. The great philosopher Aristotle said that teenagers appeared “fickle in their desires,” which are as “transitory as they are vehement.” In fact, he dismissed their brains as “boiled” (Hine 1999, 37). Impulsive behaviour, poor judgment, hostile outbursts, emotional polarity, lack of self-control and motivation, risk-taking, rebellion, few inhibitions and social awkwardness. This list could be compiled by anyone who spends time with a teenager or a youngster about to become one.

In the past, adults tended to blame these frustrating and sometimes wondrous teenage characteristics on hormones. But recent scientific research has shown that the adolescent brain is very different than the adult brain we assumed it resembled. For the first time, scientists are looking beyond hormones to explain teenage behaviour. In the past, explaining the frustrating and sometimes dangerous behaviour that accompanies adolescence was the job of social scientists, psychologists, psychiatrists, educators and, perhaps, an exorcist or two—but not neuroscientists.

Jay Giedd, an American child psychologist and neuroscientist at the National Institute of Health, used functional magnetic resonance imaging to study young brains. His studies have helped him view the structure of the brain and establish connections between the physical changes taking place and the resulting mental functions. If a single word could be attributed to the development of the human brain, that word would be plasticity.

Simply stated, the human brain changes, grows and adapts. During adolescence, brain development undergoes so many crucial changes that neuroscientists now believe these adolescent changes rival early childhood as a critical period of brain development. The teenage brain is a work in progress, says Giedd.

Giedd points out that not all parts of the human brain mature at the same time. Areas involved in the experience and expression of emotions—the limbic system—mature earlier than those involved in judgment, organization and reasoning. This difference between expressing feeling and evaluating accounts for many teen behaviours that can be so confounding.

Especially active is an adolescent’s amygdala, an almond-shaped knot in the middle of the brain responsible for instinctual reactions such as “fight or flight,” displays of anger and unexplained outbursts of temper or even glee. When adults and adolescents were shown pictures of faces and asked to identify expressions, adolescents typically activated the amygdala while adults activated their frontal lobes. Such research suggests that activating different portions of the brain to perform the same task helps explain behavioural differences between adolescents and adults. Neuroscientists now know that teenagers process emotions in different parts of their brain than do adults, who typically use their prefrontal cortex—the region responsible for the so-called executive functions. This different brain activity is an important distinction between adolescents and adults. In a nutshell, the teenage brain is not yet fully “wired.”

 In fact, the strength of the brain’s ability to balance emotion and reason is a distinguishing feature of the teen’s journey. Adolescents are vulnerable, impressionable and raw—even in the deep regions of their tangled dendrites, writes Barbara Strauch, author of The Primal Teen. She sees adolescence as one of the most necessary and crucial steps in human development—one to be endured, indulged and even celebrated. As it turns out, teenagers may be a little “crazy,” but it is according to a very primal blueprint: they are crazy by design!

More information about how the brain develops is available on the ATA’s website:


Giedd, J N, J Blumenthal, N O Jeffries, et al. 1999. “Brain Development During Childhood and Adolescence: A Longitudinal MRI Study.” Nature Neuroscience 2, no 10: 861–63.

Hine, T. 1999. The Rise and Fall of the American Teenager. New York: Avon Books.

Restak, R M. 2001. The Secret Life of the Brain. Washington, D C: Joseph Henry Press.

Strauch, B. 2003. The Primal Teen. New York: Anchor Books.