Adolescence

• 1. Introduction

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  Adolescence has no agreed-upon boundary. Scholars cannot settle on a precise definition, and some stretch it as early as age 10 and as late as 30. The World Health Organization fixes its own marker at ages 10 to 19, yet age, the source warns, is only a rough guide. This is the transitional stage between puberty and adulthood, usually mapped onto the teenage years. But its physical, psychological, and cultural expressions may begin earlier or run far later. A girl can reach reproductive maturity years before a boy finishes growing. A frontal lobe of the brain can keep reshaping itself into a person's 30s. So what actually changes during these years, and how much of the popular story about teenagers holds up against the evidence? The chapters ahead trace the body's surge, the brain's quiet rewiring, the mind's leap into abstraction, and the relationships that begin to crowd out the family.

• 2. The Body Surges

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  Puberty begins with a surge in hormone production triggered by the pituitary gland, which sends hormonal agents into the bloodstream and sets off a chain reaction. The gonads activate, and the testes release mostly testosterone while the ovaries dispense mostly estrogen. The average onset arrives at 10 to 11 for girls and 11 to 12 for boys, with heredity setting the timetable and diet and exercise nudging it.

  Spermarche, the first ejaculation, marks the major landmark for males and occurs on average at age 13. Menarche, the onset of menstruation, typically arrives around age 13 for females, give or take two or three years. A girl must reach a certain proportion of body fat to attain menarche at all. So girls with high-fat diets who are not physically active tend to menstruate earlier than ballet dancers or gymnasts whose activity reduces fat.

  Facial hair follows its own schedule in males. The first hairs appear at the corners of the upper lip, usually between 14 and 16, then spread into a moustache, up the cheeks, and down to a full beard. Some men do not grow full facial hair until 10 years after puberty.

  Growth itself arrives unevenly. The extremities come first, the head, hands, and feet, then the arms and legs, then the torso and shoulders, which is one reason an adolescent body can look out of proportion. The weight gained during adolescence makes up nearly half of a person's adult body weight. Girls usually reach full physical development around ages 15 to 17, while boys finish puberty around 16 to 17 and keep growing for about six years after the first visible changes.

• 3. Timing's Heavy Hand

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  Early-maturing boys are usually taller and stronger than their friends, which helps them catch the attention of potential partners and get picked first for sports. They often carry a good body image and act more confident, secure, and independent. Late-maturing boys can lose confidence by comparing themselves to peers who have already developed.

  Early puberty is not all advantage for boys. The hormonal surge can bring increased aggressiveness, and because they look older, these boys face pressure to act like adults even when their cognitive and social development lags behind their appearance. Studies show early-maturing boys are more likely to be sexually active and to take risks.

  Girls who mature early often grow more self-conscious, insecure, and dependent. They are more likely than their peers to develop eating disorders such as anorexia nervosa and bulimia nervosa. The source notes that nearly half of all American high school girls' diets are aimed at losing weight. Early-maturing girls may also field sexual advances from older boys before they are emotionally ready, date earlier, and face more unwanted pregnancies and earlier exposure to alcohol and drugs. Those with such experiences tend to do worse in school than their less-experienced peers.

• 4. The Rewiring Brain

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  The human brain reaches 90 percent of its adult size by age six, so it does not grow much during adolescence. The change is structural, not a matter of size. White matter increases in a roughly straight line, while grey matter traces an inverted-U pattern as synaptic pruning eliminates unnecessary neuronal connections.

  Pruning does not strip away function. The brain becomes more efficient through increased myelination, the insulation of axons, and the loss of unused pathways. The first regions pruned handle primary motor and sensory tasks. The areas tied to complex processes, including the lateral and prefrontal cortices, lose matter later.

  The prefrontal cortex carries some of the most developmentally significant changes, governing decision making, cognitive control, and other higher functions. As its myelination and pruning increase, connections to other brain regions strengthen, improving evaluation of risks and rewards and control over impulses. The dorsolateral region helps with impulse control and planning, the ventromedial with decision making, and the orbitofrontal with weighing rewards and risks.

  Three neurotransmitters drive much of this. Glutamate is excitatory, and because most pruned connections carry glutamate receptors, the early-adult brain ends up more inhibitory than excitatory. Dopamine, tied to pleasure and decision making, rises in the limbic system and feeds more strongly into the prefrontal cortex. Serotonin, which regulates mood, shifts in the limbic system to make adolescents more emotional and more responsive to rewards and stress. The source flags a common misconception worth retiring: that the brain matures at around age 25 and is immature before then.

• 5. Thinking About Thinking

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  Piaget described adolescence as the stage when thought turns abstract and egocentric thinking fades, letting a person reason from a wider perspective. By roughly ages 12 to 14, critical thinking and decision-making competence become comparable to those of adults. Improvements cluster in five areas: attention, memory, processing speed, organization, and metacognition.

  Processing speed sharpens between age five and middle adolescence, then levels off around age 14 or 15 and changes little afterward. Hypothetical and abstract thinking let adolescents contemplate possibilities outside what currently exists, plan ahead, and argue against a parent's or friend's assumptions. They grasp the higher-order logic of puns, proverbs, metaphors, and sarcasm, which children younger than nine often cannot comprehend at all.

  Metacognition means thinking about thinking, monitoring one's own mental activity. It feeds self-control, studying, introspection, and self-consciousness. It can also produce two forms of adolescent egocentrism, the imaginary audience and the personal fable, which likely peak at age fifteen along with self-consciousness in general.

  Risk-taking draws heavy research because most adolescent injuries trace to risky behavior, from alcohol and drugs to reckless driving and unprotected sex. The evidence favors a particular conclusion: adolescents and adults think about risk in similar ways but hold different values and so reach different conclusions. Adolescents give more weight to rewards, especially social rewards. Some argue there are evolutionary benefits, since without a willingness to take risks teenagers would lack the motivation to leave their family of origin.

• 6. Building An Identity

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  G. Stanley Hall opened the formal study of adolescent psychology with his book Adolescence in 1904. The first president of the American Psychological Association, Hall defined adolescence as ages 14 to 24 and viewed it as a time of internal turmoil and upheaval, the sturm und drang, drawing on Darwin's evolutionary theory and Freud's psychodynamic theory. His assertions stood largely uncontested until the 1950s, when Erik Erikson and Anna Freud advanced their own theories.

  The search for identity begins in the adolescent years, when young people try on different appearances and behaviors and cycle through identities to find one that fits. Erikson argued that everyone has a self-concept but not everyone fully achieves identity, describing an identity crisis that must be resolved through exploration before commitment. James Marcia later built the standard method for testing progress, scoring answers across occupation, ideology, and interpersonal relationships into four categories: identity diffusion, foreclosure, moratorium, and achievement. Research since shows self-examination beginning early in adolescence, but identity achievement rarely occurring before age 18.

  Clothes and fashion carry real weight in this process, helping teens explore new identities, separate from parents, and bond with peers. Environment matters too. Studies by the American Psychological Association show that adolescents from less privileged upbringings have a harder time developing their identity.

  Self-esteem completes the picture, and the source corrects a popular belief: there is no empirical evidence for a significant drop in self-esteem across adolescence. Barometric self-esteem fluctuates rapidly and can cause distress, but baseline self-esteem stays highly stable. Girls tend to draw high self-esteem from supportive friendships, while boys derive it more from successfully influencing friends and from romantic competence.

• 7. Pulling Toward Peers

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  Communication with peers increases sharply during adolescence, and peer relationships grow more intense and more influential than at any earlier stage. This period brings a dramatic increase in time spent with peers and a decrease in adult supervision. Adolescents choose peer groups based on traits they share, and group norms and values fold into their own self-concept.

  Susceptibility to peer pressure increases during early adolescence, peaks around age 14, and declines after that. Peers can push academic motivation upward or push experimentation with drugs, drinking, vandalism, and stealing. Among Spanish teenagers, emotional rather than solution-based reactions to problems have been linked to physical aggression against peers.

  Adolescents organize socially into cliques on a small scale and crowds on a larger one. Cliques start as exclusive, single-sex groups and can act as a collective parent, telling members what to do. Toward late adolescence they merge into mixed-sex groups as teenagers begin pairing off, then break down further as socialization turns couple-oriented. Crowds gather around a shared interest and often feed stereotypes like jocks or nerds.

  Family roles shift at the same time. When children go through puberty, parent-child conflict often rises and the familial bond loosens, with arguments over curfew, clothing, and privacy. Yet on important life issues, most adolescents still share their parents' attitudes and values. A study by Adalbjarnardottir and Blondal in 2009 found that 14-year-olds who saw their parents as authoritative were more likely to finish secondary education by age 22.

• 8. Romance And Risk

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  By age 15-53 percent of adolescents have had a romantic relationship that lasted at least one month over the previous 18 months. A 2002 American study found that people aged 15 to 44 reported an average age of first sexual intercourse of 17.0 for males and 17.3 for females. In a 2008 study conducted by YouGov for Channel 4-20 percent of 14 to 17-year-olds surveyed said their first sexual experience came at 13 or under in the United Kingdom.

  Most sexual experience in this period happens within romantic relationships rather than casual hookups, and the distinction matters. Among young adolescents, heavy sexual activity is often tied to violence, depression, and poor relationship quality. For older adolescents, sexual activity within a romantic relationship correlated with lower levels of deviant behavior after controlling for genetic risks, unlike sex outside a relationship.

  Dating violence reaches into adolescent relationships. When surveyed, 12 to 25 percent of adolescents reported physical violence in a relationship, and a quarter to a third reported psychological aggression. In heterosexual couples there is no significant difference between male and female aggressors, unlike in adult relationships. Female adolescents from minority populations face increased risk of intimate partner violence.

  Sexual health risks climb as well. One in four sexually active teenagers will contract a sexually transmitted infection. Girls 15 to 19 have higher rates of gonorrhea than any other age group, and one-quarter of all new HIV cases occur in people under 21. According to a March survey by the Kaiser Family Foundation, 81 percent of parents want schools to discuss condoms and contraception, yet over a quarter of sex education instructors report being told by school boards and administrators not to teach about contraceptives at all.