14.3 Brain Development

Like the other parts of the body, different parts of the brain grow at different rates. Myelination and maturation occur in sequence, proceeding from the inner brain to the cortex and from back to front (Sowell et al., 2007). That means that the limbic system, including the amygdala, the site of intense fear and excitement, matures before the prefrontal cortex, where planning, emotional regulation, and impulse control occur.

Furthermore, pubertal hormones target the amygdala and other crucial parts of the HPA axis directly (Romeo, 2013), but full functioning of the cortex requires maturation beyond the teen years. For both of these reasons, the instinctual and emotional areas of the adolescent brain develop ahead of the reflective, analytic areas. Early puberty means emotional rushes, unchecked by caution.

A Need for Caution

Brain scans confirm that emotional control, revealed by fMRI studies, is not fully developed until adulthood, because the prefrontal cortex is limited in connections and engagement (Luna et al., 2013). When compared with the brains of emerging adults, adolescent brains show heightened arousal in the brain’s reward centers.

Same People, But Not the Same Brain These brain scans are part of a longitudinal study that repeatedly compared the proportion of gray matter from childhood through adolescence. (Gray matter refers to the cell bodies of neurons, which are less prominent with age as some neurons are unused.) Gray matter is reduced as white matter increases, in part because pruning during the teen years (the last two pairs of images here) allows intellectual connections to build. As the authors of one study that included this chart explained, teenagers may “look like an adult, but cognitively they are not there yet” (K. Powell, 2006, p. 865).

NITIN GOGTAY ET AL., 2004.

Teens seek excitement and pleasure, especially the social pleasure of a peer’s admiration (Galvan, 2013). In fact, when others are watching, teens find it thrilling to take dramatic risks that produce social acclaim, risks they would not dare take alone (Albert et al., 2013).

Many types of psychopathology increase at puberty, especially when puberty is early, bringing extreme stress before the prefrontal cortex matures. Two experts explain, “higher rates of psychopathology among early maturers are expected because their slow-developing neurocognitive systems are mismatched with the fast-approaching social and affective challenges at the onset of puberty” (Ge & Natsuaki, 2009, p. 329).

The fact that the frontal lobes (prefrontal cortex) are the last to mature may explain something that has long bewildered adults: Many adolescents are driven by the excitement of new experiences, sensations, and peers—forgetting the caution that their parents have tried to instill. The following is one example.

The normal sequence of brain maturation (limbic system at puberty, then prefrontal cortex sometime in the early 20s) combined with the early onset of puberty means that, for contemporary teenagers, emotions rule behavior for years. The limbic system, unchecked by the slower-maturing prefrontal cortex, makes powerful sensations—loud music, speeding cars, strong drugs—compelling.

It is not that the prefrontal cortex shuts down. Actually, it continues to mature throughout childhood and adolescence, and, when they think about it, adolescents are able to assess risks better than children are (Pfeiffer et al., 2011). It is the balance and coordination between the various parts of the brain that is off-kilter, not the brain itself (Casey et al., 2011).

When stress, arousal, passion, sensory bombardment, drug intoxication, or deprivation is extreme, the adolescent brain is flooded with impulses that might shame adults. Teenagers brag about being so drunk, they were “wasted,” “bombed,” “smashed”—a state most adults try to avoid and would not admit. [Lifespan Link: Abuse of alcohol and other drug abuse are discussed in Chapter 16.]

Also, unlike adults, some teenagers choose to spend a night without sleep, to eat nothing all day, to exercise in pain, or to risk parenthood or an STI by avoiding a condom. The parts of the brain dedicated to analysis are immature until years after the first hormonal rushes and sexual urges, while teenagers have access to fast cars, lethal weapons, and dangerous drugs. My friend said to his neighbor, who gave his son a red convertible for high school graduation, “Why didn’t you just give him a loaded gun?”

One example of the cautious part of the brain being overwhelmed by the emotions of the moment comes from teens sending text messages while they are driving. In one survey, 64 percent of U.S. 16- to 17-year-olds said they had been in a car when the driver was texting, as occurs in every state and nation even though it is illegal almost everywhere (Madden & Lenhart, 2009). More generally, despite quicker reflexes and better vision than at later ages, far more teenagers die in motor-vehicle accidents than adults do. Thoughtless impulses and poor decisions are almost always to blame.

Any decision, from whether to eat a peach to where to go to college, requires balancing risk and reward, caution and attraction. Experiences, memories, emotions, and the prefrontal cortex help all of us choose to avoid some actions and perform others. Since the reward parts of adolescents’ brains (the parts that respond to excitement and pleasure) are stronger than the inhibition parts (the parts that urge caution) (Luna et al., 2013), many adolescents act in ways that seem foolhardy to adults.

Benefits of Adolescent Brain Development

It is easy to be critical of adolescent behavior and blame it on hormones, peers, culture, or brains. Yet remember from Chapter 1 that difference is not always deficit, and that gains as well as losses are part of every stage of life. There are benefits as well as hazards in every development, including in the brain.

With increased myelination and slower inhibition, reactions become lightning fast. Such speed is valuable. For instance, adolescent athletes are potential superstars, quick and fearless as they steal a base, tackle a fullback, or sprint when their lungs feel about to burst. Ideally, coaches have the wisdom to channel such bravery.

Furthermore, as the brain’s reward areas activate positive neurotransmitters, teenagers become happier. A new love, a first job, a college acceptance, or even an A on a term paper can produce a rush of joy, to be remembered and cherished lifelong.

There are constructive good evolutionary reasons for adolescents to question tradition. As social and ecological circumstances change, someone needs to ask whether having lots of children, or eating bacon every breakfast, or burning fossil fuels, is still a good practice. If every tradition were accepted uncritically, social practices would ossify and societies would die.

Yes, Not No Diving into cold water with your friends is thrilling if you are a teenage boy and a girl is watching. Adult prohibition increases the joy.

DENNIS MACDONALD/PHOTOEDIT

Further, it is beneficial that teenagers take risks and learn new things, because “the fundamental task of adolescence—to achieve adult levels of social competence—requires a great deal of learning about the social complexities of human social interactions” (Peper & Dahl, 2013, p. 135). That is exactly what teenagers’ brains enable adolescents to do.

Synaptic growth enhances moral development as well. Adolescents question their elders and forge their own standards. Values embraced during adolescence are more likely to endure than those acquired later, after brain connections are firmly established. This is an asset if adolescent values are less self-centered than those of children or are more culturally attuned than those of older generations.

The fact that the prefrontal cortex is still developing “confers benefits as well as risks. It helps explain the creativity of adolescence and early adulthood, before the brain becomes set in its ways” (Monastersky, 2007, p. A17). The emotional intensity of adolescents “intertwines with the highest levels of human endeavor: passion for ideas and ideals, passion for beauty, passion to create music and art” (Dahl, 2004, p. 21). As a practical application, since adolescents are learning lessons about life, those who care about the next generation need to help make sure those lessons are good ones.