Puberty refers to the years of rapid physical growth and sexual maturation that end childhood, producing a person of adult size, shape, and sexuality. The forces of puberty are unleashed by a cascade of hormones that produce external growth and internal changes, including heightened emotions and sexual desires.

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The process of puberty normally starts sometime between ages 8 and 14, and most physical changes are complete within four years—although some height, weight, and reproductive increases may occur for several more years. Over the past decades the age of puberty has declined, perhaps for both sexes, although the evidence is more solid for girls (Biro et al., 2013; Herman-Giddens, 2013).

For girls, the observable changes of puberty usually begin with nipple growth. Soon a few pubic hairs are visible, followed by a peak growth spurt, widening of the hips, menarche (the first menstrual period), full pubic-hair pattern, and breast maturation (Susman et al., 2010). The average age of menarche among normal-weight girls is about 12 years, 4 months (Biro et al., 2013).

For boys, the usual sequence is growth of the testes, initial pubic-hair growth, growth of the penis, spermarche (first ejaculation of seminal fluid), first facial hair, peak growth, deepening of the voice, and final pubic-hair and beard pattern (Biro et al., 2001; Herman-Giddens et al., 2012; Susman et al., 2010). The typical age of spermarche is just under 13 years. Age varies markedly within and between nations: These are averages in the United States.

HORMONES Puberty begins before those observable changes with an invisible event—a marked increase in hormones, which are body chemicals that regulate hunger, sleep, moods, stress, sexual desire, immunity, reproduction, and many other biological processes. The pituitary, deep within the brain, signals the hypothalamus, another brain structure, that puberty should begin. First to receive the signal are the adrenal glands, located above the kidneys at either side of the lower back.

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The pituitary also activates the gonads, or sex glands (ovaries in females; testes, or testicles, in males). One hormone in particular, GnRH (gonadotropin-releasing hormone), causes the gonads to enlarge and dramatically increase their production of sex hormones, chiefly estradiol in girls and testosterone in boys.

Estrogens (including estradiol) are female hormones and androgens (including testosterone) are male hormones, although both sexes have some of both. The ovaries produce high levels of estrogens and the testes produce dramatic increases in androgens. This “surge of hormones” affects bodies, brains, and behavior before any visible signs of puberty appear, “well before the teens” (Peper & Dahl, 2013, p. 134).

The activated gonads begin to release ova (at menarche) or sperm (at spermarche). Conception is possible, although peak fertility occurs four to six years later. Hormones also awaken interest in sex, as young teenagers fantasize—at first about people who are unlikely to reciprocate (celebrities, teachers) and then about their peers nearby.

Hormones may underlie differences in psychopathology. Compared to the other sex, adolescent males are almost twice as likely to develop schizophrenia and adolescent females more than twice as likely to develop major depression. Of course, hormones are never the sole cause of psychopathology (Tackett et al., 2014; Rudolph, 2014).

Remember that body, brain, and behavior always interact. Sexual thoughts themselves can cause physiological and neurological processes, not just result from them. Cortisol (another hormone) levels rise at puberty, and that makes adolescents quicker to become angry or upset (Goddings et al., 2012; Klein & Romeo, 2013).

Then emotions, in turn, increase hormones. For example, when other people react to emerging breasts or beards, that evokes thoughts and frustrations in the adolescent, which raises hormone levels, propels physiological development, and triggers more emotions. Thus the internal and external changes each affect the other.

BODY RHYTHMS The brain of every living creature responds to the environment with natural rhythms that rise and fall by the day and season. For example, in children, height increases more rapidly in summer and weight in winter. Many rhythms are circadian, which means they are on a daily cycle. That is why well-rested people tend to wake up at the same time on weekends as during the week, and they also get sleepy at the same time each night.

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In addition, some individuals (especially males) are more alert in the evening than in the morning. Other people are the opposite, filled with energy in the morning and then fading as the day wears on. Those differences are genetic.

In addition, at puberty, hormones cause a phase delay in the circadian sleep–wake cycles. That compels many teens to be wide awake and hungry at midnight but half asleep, with neither appetite nor energy, all morning. For teens who are already evening people, the phase delay increases the risk of problems: They are up when adults are asleep, but adults still make them get out of bed at dawn. Consequently, in many nations, sleep deprivation increases during adolescence (see Figure 9.1) (Roenneberg et al., 2012).

To make it worse, “the blue spectrum light from TV, computer, and personal-device screens may have particularly strong effects on the human circadian system” (Peper & Dahl, 2013, p. 137). Watching late-night TV, working on the computer, or texting friends at 10 P.M. interferes with normal nighttime sleepiness. Sleeping late on weekends is a sign of deprivation, not compensation.

Sleep deprivation and irregular sleep schedules increase the risk of insomnia, nightmares, mood disorders (depression, conduct disorder, anxiety), and falling asleep while driving. In addition, sleepy people don’t learn as well as they do when rested. Many adults ignore these facts, as the following explains.

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That six-year range in onset of puberty (age 8 to 14) is troubling for parents and preteens who want to be ready but not premature. More precise prediction is possible.

GENES AND GENDER About two-thirds of the variation in age of puberty is genetic, evident not only in families but also in ethnic groups (Dvornyk & Waqar-ul-Haq, 2012; Biro et al., 2013). African Americans reach puberty, on average, about seven months earlier than European or Hispanic Americans; Chinese Americans average several months later.

The other major influence on age of puberty comes from the sex chromosomes. In height, the average girl is two years ahead of the average boy: The female height spurt occurs before menarche, whereas for boys the increase in height is relatively late, after spermarche. Thus, a sixth-grade boy with sexual fantasies about the taller girls in his class is neither perverted nor precocious; his hormones are simply ahead of his height.

FAT Body fat also affects age of puberty. Heavy girls reach menarche years earlier than malnourished ones do. Most girls weigh at least 100 pounds (45 kilograms) before menarche (Berkey et al., 2000). Although severe malnutrition always delays puberty, body fat may not be as necessary for boys. Indeed, obese boys are often delayed in puberty compared to boys who are neither slim nor overweight (Tackett et al., 2014).

Malnutrition causes many youths to reach puberty at age 15 or later in parts of Africa, whereas their genetic relatives in North America mature much earlier. Similarly, malnutrition is the main reason puberty began at about age 17 in sixteenth-century Europe.

Since then, puberty has occurred at younger and younger ages (an example of what is called the secular trend, the increases in human growth as nutrition improved). More food availability led to weight gain in childhood, and that has led to earlier puberty for girls and taller average height for both sexes (Floud et al., 2011; Fogel & Grotte, 2011).

One curious bit of evidence of the secular trend is that U.S. presidents have gotten taller. James Madison, the fourth president, was shortest at 5 feet, 4 inches; Barack Obama is 6 feet, 1 inch tall.

The secular trend seems to have stopped in developed nations, because adequate nutrition allows everyone to reach their genetic potential. Currently, fewer young men look down at their short fathers, or girls at their little mothers, unless their parents were born in Asia or Africa, where the secular trend is still evident.

There is one possible exception in developed nations to the statement “the secular trend has stopped.” Very early puberty, before age 8 (called precocious puberty), may be increasing, especially in girls, although it is still rare (perhaps 2 percent) (Sørensen et al., 2012). Sometimes precocious puberty is genetic (perhaps 20 percent), but the cause of the increase is largely unknown—perhaps childhood obesity or new chemicals in the environment.

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Some research finds that puberty is delayed, not accelerated, in boys who were exposed prenatally to phthalates and bisphenol A (K. Ferguson et al., 2014), or who experienced heavy doses of pesticides in boyhood (T. Lam et al., 2014). Phthalates may delay puberty in girls as well (Wolff et al., 2014).

Caution is needed here. No doubt, heavy doses of many chemicals and pesticides affect fish, frogs, insects, and birds, causing reproductive problems. However, as noted in Chapter 2, experts disagree about the impact on humans.

STRESS Stress hastens the hormonal onset of puberty, especially if a child’s parents are sick, drug-addicted, or divorced, or if the neighborhood is violent and impoverished. One study of sexually abused girls found that they began puberty seven months earlier, on average, than did a matched comparison group (Trickett et al., 2011). Particularly for girls who are genetically sensitive, puberty comes early if their family interaction causes high levels of cortisol but late if their family is supportive (Ellis et al., 2011; James et al., 2012).

The link between stress and puberty is one explanation for the fact that children born in one country and adopted in another tend to experience early puberty, especially if their first few years of life were in an institution or a chaotic home. An alternate explanation is that their age at adoption was underestimated, so puberty appears to occur early but actually is at the expected time (Hayes, 2013).

Why would cortisol trigger puberty? An answer comes from evolutionary theory. Thousands of years ago, if harsh conditions threatened survival of the species, adolescents needed to reproduce early and often to increase the chance that at least some children would reach adulthood. By contrast, in peaceful times, puberty could occur later, allowing children to postpone maturity and instead enjoy extra years of childhood nurturance from parents and grandparents.

Of course, this evolutionary rationale no longer applies. Today, early sexual activity and teen parenthood are more likely to harm communities than help the species. However, the genome has been shaped over millennia; if there is a puberty-starting allele that responds to social conditions, it still responds as it did thousands of years ago.

TOO EARLY, TOO LATE For most adolescents, these links between puberty, stress, and hormones are irrelevant. The only timing that matters is their friends’ schedules. No one wants to be too early or too late.

Think about the early-maturing girl. If she has visible breasts at age 10, the boys her age tease her; they are unnerved by the sexual creature in their midst. She must fit into a school chair designed for smaller children; she might hide her breasts in large T-shirts and bulky sweaters. Early-maturing girls tend to have lower self-esteem, more depression, and poorer body image than do other girls (Galvao et al., 2014; Compian et al., 2009).

Some early-maturing girls have older boyfriends, who are attracted to their womanly shape and girlish innocence. Having an older boyfriend bestows status, but it also increases problems (including drug and alcohol abuse) that arise from older teens (Mrug et al., 2014).

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The effects of early maturation on boys depend on context. In the United States, early-maturing boys who were born around 1930 often became leaders in high school and high wage earners as adults (Jones, 1965; Taga et al., 2006). Since about 1960, however, the risks associated with early male maturation have outweighed the benefits.

In the twenty-first century, early-maturing boys are more aggressive, law-breaking, and drug-abusing than the average boy (Biehl et al., 2007; Lynne et al., 2007; Mendle et al., 2012). This is not surprising: A boy who is experiencing rapid increases in testosterone, and whose body looks more like a man than a child, is likely to cause trouble with parents, peers, schools, and the police.

Late puberty may also be difficult, especially for boys (Benoit et al., 2013). Slow-developing boys tend to be more anxious, depressed, and afraid of sex. Girls are less attracted to them, coaches less often want them on their teams, peers bully or tease them.

Puberty causes two sets of changes. One set is marked by the growth spurt—a sudden, uneven jump in the size of almost every body part, turning children into adults. The other set is sexual; turning girls and boys into men and women who could become parents.

THE GROWTH SPURT Growth proceeds from the extremities to the core (the opposite of the earlier proximodistal growth). Thus, fingers and toes lengthen before hands and feet, hands and feet before arms and legs, arms and legs before the torso. This growth is not always symmetrical: One foot, one breast, or even one ear may grow later than the other.

Because the torso is the last body part to grow, many pubescent children are temporarily big-footed, long-legged, and short-waisted. If young teenagers complain that their jeans don’t fit, they are probably correct—even if those same jeans fit when their parents paid for them a month earlier. (Advance warning about rapid body growth occurs when parents first have to buy their children’s shoes in the adult section.)

As the growth spurt begins, children eat more and gain weight. Exactly when, where, and how much weight they gain depends on heredity, hormones, diet, exercise, and gender. By age 17, the average girl has twice the percentage of body fat as her male classmate, whose increased weight is mostly muscle.

A height spurt follows the weight spurt; then a year or two later a muscle spurt occurs. Thus, the pudginess and clumsiness of early puberty are usually gone by late adolescence. During these years, all the muscles increase in power.

Inner organs grow as well. Lungs triple in weight, allowing adolescents to breathe more deeply and slowly. The heart doubles in size as the heartbeat slows, decreasing the pulse rate while increasing blood pressure. Consequently, endurance improves: Some teenagers can run for miles or dance for hours. Fortunately, red blood cells increase in both sexes, dramatically more in boys, aiding oxygen transport during intense exercise.

One organ system, the lymphoid system (including tonsils and adenoids), decreases in size, so teenagers are less susceptible to respiratory ailments. As a result, mild asthma often switches off at puberty—half as many teenagers as children are asthmatic (MMWR, June 8, 2012), and teenagers have fewer colds and allergies than younger children.

When the larynx grows, the voice lowers, especially noticeable in boys. Another organ system, the skin, becomes oilier, sweatier, and more prone to acne. Hair also changes, becoming coarser and darker, with new hair under arms, on faces, and over sex organs. Specifics depend on genes as well as on hormones.

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Often teenagers cut, style, or grow their hair in ways their parents do not like, as a sign of independence. To become more attractive, many adolescents spend considerable time, money, and thought on their visible hair—growing, gelling, shaving, curling, straightening, highlighting, brushing, combing, styling, dyeing, wetting, and/or drying. In many ways, hair is far more than a growth characteristic; it is a display of sexuality.

SEXUAL CHARACTERISTICS The body characteristics that are directly involved in conception and pregnancy are called primary sex characteristics. During puberty, every primary sex organ (the ovaries, uterus, penis, and testes) increases dramatically in size and matures in function. By the end of the process, reproduction is possible.

At the same time, development occurs in secondary sex characteristics. They do not directly affect reproduction (hence they are secondary) but signify gender.

One secondary characteristic is shape. Young boys and girls have similar shapes, but at puberty males widen at the shoulders and grow about 5 inches taller than females, while girls widen at the hips and develop breasts. Those curves are considered signs of womanhood, but neither breasts nor wide hips are required for conception; thus, they are secondary, not primary, sex characteristics.

Secondary sex characteristics are important psychologically, if not biologically. Breasts are an obvious example. Many adolescent girls buy “minimizer,” “maximizer,” “training,” or “shaping” bras, hoping that their breasts will conform to their idealized body image.

During the same years, many overweight boys are horrified to notice a swelling around their nipples—a temporary result of the erratic hormones of early puberty. If a boy’s breast growth is very disturbing, tamoxifen or plastic surgery can reduce the swelling, although many doctors prefer to let time deal with the problem (Morcos & Kizy, 2012).

All the changes of puberty depend on adequate nourishment, yet many adolescents do not eat well. They often skip breakfast, binge at midnight, guzzle down unhealthy energy drinks, and munch on salty, processed snacks. Some eat so poorly that their health is damaged lifelong.

NUTRIENTS MISSING Family dinners correlate with healthy adolescent eating and well-being. However, this is tricky: If parents insist on family dinners, clashing directly with the teenager’s wish for independence, the benefits of such meals may be undercut by a sullen, angry diner (Meier & Musick, 2014).

Most adolescents consume enough calories, but in 2013 only 16 percent of high school seniors ate the recommended three or more servings of vegetables a day (MMWR, June 13, 2014). Deficiencies of iron, calcium, zinc, and other minerals are common.

Because menstruation depletes iron, anemia is more likely among adolescent girls than among people of any other age or sex. Boys may also be iron-deficient if they engage in physical labor or intensive sports: Muscles need iron for growth and strength.

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The cutoff for iron-deficiency anemia is higher for boys than girls because boys require more iron to be healthy (Morón & Viteri, 2009). Yet in developed as well as developing nations, adolescents tend to spurn iron-rich foods (beans, egg yolks, and lean meat) in favor of iron-poor chips, sweets, and fries.

Similarly, although the daily recommended intake of calcium for teenagers is 1,300 milligrams, the average U.S. teen consumes less than 500 milligrams a day. About half of adult bone mass is acquired from ages 10 to 20, which means many contemporary teenagers will eventually develop osteoporosis (fragile bones), a major cause of disability, injury, and death in late adulthood.

One reason for calcium deficiency is that milk drinking has declined. In 1961, most North American children drank at least 24 ounces (about three-fourths of a liter) of milk each day, providing most of their daily calcium requirement. Fifty years later, only 12.5 percent of high school students drank that much milk and 19 percent (more girls than boys) drank no milk at all (MMWR, June 13, 2014).

Fast-food establishments, serving tasty but not healthy food, cluster around high schools, with extra seating that encourages teenagers to hang out. This is especially true for high schools with large Hispanic populations, who are most at risk for obesity (Taber et al., 2011). Forty percent of Hispanic girls in U.S. high schools describe themselves as overweight, as do 27 percent of Hispanic boys (MMWR, June 13, 2014).

BODY DISSATISFACTION One reason for poor nutrition among teenagers is anxiety about body image—that is, a person’s idea of how his or her body looks. Two-thirds of U.S. high school girls are trying to lose weight, even though only one-fourth are actually overweight or obese (MMWR, June 8, 2012).

Body image dissatisfaction occurs in adolescents of both sexes and every ethnic group, although each adolescent is troubled by particular characteristics that spring from their ancestral genes. Film stars and media models make this worse.

Many adolescents obsess about being too short or too tall, too wide in the hips or too narrow in the face, too hairy or not hairy enough, with fingers too long or legs too fat, and so on. Self-acceptance is difficult at every age, particularly when body changes are new. Indeed, adolescents may become depressed because of some characteristic they wish they had or did not have (Kuzucu et al., 2014).

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For a few, dissatisfaction with body image can be dangerous, even deadly. Eating disorders are rare in childhood but increase dramatically at puberty, accompanied by distorted body image, food obsession, and depression (Le Grange & Lock, 2011). Many teenagers, mostly girls, eat erratically or ingest drugs (especially diet pills) to lose weight; others, mostly boys, take steroids or creatine (Calzo et al., 2015) to increase muscle mass. Both may switch from obsessive dieting, to overeating, to overexercising, and back again.

Rates of obesity are falling in childhood but increasing in adolescence and adulthood in almost every nation, including the United States. In 2001, more than 15 percent of high school students were obese in only three states (Kentucky, Mississippi, and Tennessee). In 2013, 22 states had rates that high (MMWR, June 13, 2014).

EATING DISORDERS Obesity is an eating disorder at every age, and it is discussed in several other chapters of this book. Here we focus on three other eating disorders that appear in adolescence.

Anorexia nervosa is characterized by voluntary starvation. It is the extreme of what many teenage girls think about their weight: They have a distorted body image, perceiving themselves as being too heavy. Those suffering from anorexia become very underweight, risking death by organ failure. Staying thin becomes an obsession.

Although anorexia existed earlier, it was not identified until about 1950, when some high-achieving, upper-class young women became so emaciated that they died. Soon anorexia was noticed among teenagers and young adults of every income, nation, and ethnicity.

The rate of anorexia spikes at puberty and again in emerging adulthood. Scientists disagree about the causes—genes, family influences, or culture—but in any case, sufferers and their families need life-saving help (Dring, 2015; Dodge & Simic, 2015).

About three times as common as anorexia is bulimia nervosa (also called the binge–purge syndrome). A person overeats compulsively, devouring thousands of calories within an hour or two, and then purges through vomiting or laxatives, risking many health problems.

Binging and purging are common among adolescents. For instance, a 2013 survey found that within the previous month 6.6 percent of U.S. high school girls and 2.2 percent of boys vomited or took laxatives to lose weight, with marked variation by state—from 3.6 percent in Nebraska to 9 percent in Arizona (MMWR, June 13, 2014).

The third adolescent eating disorder is newly recognized in DSM-5: binge eating disorder. Some adolescents periodically and compulsively overeat, quickly consuming large amounts of ice cream, cake, or some other kind of food until their stomachs hurt. In binge eating disorder, overeating is done secretly, at least weekly for months. The sufferer does not purge (this is not bulimia) but feels out of control, distressed, and depressed.

All adolescents are vulnerable to unhealthy eating, because autonomy, anxiety, and body image can lead to disorders. Teenagers try new diets, go without food for 24 hours (as did 19 percent of U.S. high school girls in one typical month), or take diet drugs (6.6 percent) (MMWR, June 13, 2014). Parents are slow to recognize eating disorders, and they often delay in getting help (Thomson et al., 2014).

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All eating disorders—obesity, anorexia, bulimia, and bingeing—have many causes, including cultural images, anxiety, depression, puberty, hormones, genes, childhood experiences, and current family pressures. Especially in adolescence, family function (not structure) is crucial in treatment (Tetzlaff & Hilbert, 2014; Dodge & Simic, 2015).