Being a teenager is as infuriating as it is amazing. Caught between childhood and adulthood, adolescents often have fully developed bodies, but their brains are still under construction. Here's what neuroscience is learning about the remarkable teenage brain, and how it affects behaviour.
Image: Teenage Mutant Ninja Turtles
Humans go through two distinct developmental phases, childhood and adolescence, the latter being defined as the transition into adulthood, starting with puberty and ending (at least in theory) with the successful independence from the parent. Though many pubescent teens look like adults, their brains are still growing and changing. As hard as it is to believe, the maturation phase doesn't typically end until we reach our early 20s; we take a very long time to truly grow up.
Quite obviously, a teenager's behaviour is influenced by a bunch of factors, including genetics, childhood experiences, and socialisation. But as neuroscientists are increasingly learning, the dynamics of the developing brain plays a crucial role as well.
The Troubling Teen Years
Somewhat paradoxically, teenagers often exhibit poorer decision making skills than when they were children. Adding to the paradox is the observation that many teens, when compared to adults, are physically stronger, faster, and more resistant to disease. Some teens even exhibit higher reasoning capacities than adults.
At the same time, however, the teenage years are characterised by emotional drama, vulnerability, impulsivity, and risk. Somewhat strangely, adolescents are at an increased risk of mortality compared to children; the rate of death by injury for teens between the ages of 15 to 19 are about six times that of the rate when they're between 10 and 14. Crime rates are also highest among young males. It's been said, therefore, that teens are "all gasoline, no brakes, and no steering wheel."
Much of this, say scientists, has to do with the process of neuromaturation. Teens are participating in the world -- and having to survive and thrive within it -- by tapping into a brain that's still going through critical developmental stages. The frontal lobes in particular are among the last areas of the brain to mature -- the part of the brain responsible for executive functions like planning, attention, motivation, working memory, and impulse control.
As a result of ongoing neuromaturation (the details of which we'll get into in just a bit), the teenage brain exhibits a set of consistently identifiable tendencies (at the risk of overgeneralizing, of course) as they relate to the maturity of judgement. There are three in particular worth highlighting:
- Increased novelty seeking
- Increased risk taking
- Increased desire to affiliate and interact with peers
At the same time, there are three prevailing myths about teens that neuroscientists are starting to dismantle, namely that they're incapable of making optimal decisions, they have no prefrontal cortex worth talking about, and that they all experience similar degrees of drama and stress. These are obviously gross generalization that don't give teens enough credit, nor do they describe the finer points of adolescent behaviour.
For example, let's consider self-control. Studies do show that teens fare worse than adults -- but only when they have to make decision in the "heat of the moment." And it's obvious that teens have a prefrontal cortex -- it's just still coming together (specifically, regions in the neural circuitry responsible for reward and control processing).
Neuromaturation can be broken down into two broad processes: the late maturation of the frontal lobes and the increasing connectivity of the brain.
During our teen years, the cortical areas of our brains continue to thicken as the neural connections grow. Surprisingly, our grey matter growth -- the stuff in our brains made up of neural cell bodies, nerve fibres, and support cells -- peak when were only 11 (girls) and 12 (boys).
From that point onward, rarely used connections get selectively pruned away, a process that makes the brain increasingly integrated and efficient. The surviving neural connections become better at transmitting information through myelination -- a process that provides insulation for neural connections allowing nerve impulses to travel around the brain more quickly and efficiently. Surviving neural connections become better at transmitting information.
This process -- synaptic overproduction, pruning, and myelination -- improves the brain's ability to efficiently transfer information between regions. In turn, teens get increasingly better at executing functions that we characterise with maturity, things like impulse control, attention, and motivation.
As a study from 2010 noted, "Although young children can demonstrate impulse control skills, with age and neuro-maturation...comes the ability to consistently use these skills."
In addition, the neural connections between the amygdala (the part of the brain responsible for emotional processing, like fear, aggression, anxiety, and vigilance) and the cortices that make up the frontal lobes become denser. This is the period when so-called "emotional maturity develops."
Fascinatingly, but also troublingly, this is the time when a temporal gap develops between the development of the social and emotional systems of the brain (which develops relatively early in puberty) and the cognitive control system -- a misaligned combination that often manifests as risk-taking.
Rewards and Impulsivity
Indeed, as already noted, risk-taking is an indelible part of the teenage years, a trait that can lead to high rates of alcohol and drug use, reckless driving, and sexual promiscuity. Teens engage in these sorts of risk-taking behaviours because they have a heightened sensitivity to rewards -- they have got a neurocircuitry rich in dopamine and, in the case of males, testosterone.
At the same time, teens are very susceptible to the feedback they get from their peers. This trait, in conjunction with a heightened reward sensitivity, often leads them to focus on the short-term benefits of risky choices over safer ones. For example, a 2009 study found that young teens took twice as many risks in a driving simulation game when they were tested in front of their peers than when they were tested alone. Older adolescents' driving was about 50% riskier in the company of peers. It all has to do with the way the teen brain processes rewards. But as teens get older, they experience gradual growth in their capacity to resist peer influence.
A related study showed that teens are more self-conscious than younger children or adults when they think peers are watching -- a result of their preoccupation with social evaluation.
Teens also make risky decisions in 'hot' contexts -- that is, where emotions are at stake or peers are present and social cognition is involved. Part of this has to do with their hyper-responsive dopaminergic reward system and the prefrontal systems related to impulse control.
Indeed, teens can also be impulsive. A recent study showed that teenagers made about 15% more errors than adults children when attempting to stop themselves from pressing a button when they saw the threatening facial expression (i.e. the "Go/No-Go" game). And males performed worse than females.
But as for those teens who did manage to restrain themselves, brain scans showed significantly higher activity in the ventromedial prefrontal cortex (vmPFC), the part of the brain responsible for top-down control behaviour.
In other words, maturity.
Additional reporting by Joseph Bennington-Castro.
Sources: "Adolescent Maturity and the Brain: The Promise and Pitfalls of Neuroscience Research in Adolescent Health Policy," "The Teen Brain: Still Under Construction," "Sex Differences in the Adolescent Brain," "The Teenage Brain: Self Control," The Teenage Brain: Sensitivity to Rewards," "The Teenage Brain: Altered Fear in Humans and Mice," "Peer Influences on Adolescent Decision Making," "Decision Making in the Adolescent Brain,"