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.

The process of puberty normally starts sometime between ages 8 and 14. Most physical growth and maturation ends about four years after the first signs appear, although some individuals (especially boys) add height, weight, and muscle until age 20 or so. Over the past decades the age of puberty has decreased, 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, the first menstrual period (menarche), a 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) although variation in timing is quite normal.

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For boys, the usual sequence is growth of the testes, initial pubic-hair growth, growth of the penis, first ejaculation of seminal fluid (spermarche), appearance of facial hair, a peak growth spurt, deepening of the voice, and final pubic-hair growth (Biro et al., 2001; Herman-Giddens et al., 2012; Susman et al., 2010). The typical age of spermarche is just under 13 years, almost a year later than menarche. Averages as well as age vary markedly, as soon discussed: The averages above are for well-nourished adolescents in the United States.

Just described are the visible changes of puberty, but the entire process begins with an invisible event—a marked increase in hormones. Hormones are body chemicals that regulate hunger, sleep, moods, stress, sexual desire, immunity, reproduction, and many other bodily functions and processes, including puberty. Throughout adolescence, hormone levels correlate with physiological changes and self-reported developments (Shirtcliff et al., 2009).

You learned in Chapter 8 that the production of many hormones is regulated deep within the brain, where biochemical signals from the hypothalamus signal another brain structure, the pituitary, to go into action. The pituitary produces hormones that stimulate the adrenal glands, located above the kidneys at either side of the lower back. The adrenal glands produce more hormones.

Many hormones that regulate puberty follow this route, known as the HPA (hypothalamus-pituitary-adrenal) axis (see Figure 14.1). Abnormalities of the HPA axis in adolescence are associated with eating disorders, anxiety, and depression. These conditions and many other types of psychopathology, appearing for the first time or worsening at puberty, are connected to hormones and genes (Dahl & Gunnar, 2009).

Indeed, abnormalities of the HPA axis probably cause the sudden increases in clinical depression among young adolescent girls (Guerry & Hastings, 2011). Further, HPA disruptions are one result of childhood sexual abuse (discussed soon) (Trickett et al., 2011).

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Late in childhood, the pituitary activates not only the adrenal glands but also the gonads, or sex glands (ovaries in females; testes, or testicles, in males), following another sequence called the HPG (hypothalamus-pituitary-gonad) axis. 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. These hormones affect the body’s shape and function and produce additional hormones that regulate stress and immunity (Young et al., 2008).

Estrogens (including estradiol) are female hormones and androgens (including testosterone) are male hormones, although both sexes have some of both. The biochemical messages from the HPG axis activate the ovaries to produce high levels of estrogens and the testes to 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 produce mature sperm or ova, released in menarche or spermarche. Conception is possible, although fertility peaks several years later.

Hormonal increases and differences may also underlie sex 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 depression. Of course, hormones are not the sole cause of psychopathology in anyone (Tackett et al., 2014; Rudolph, 2014).

One psychological effect of hormones at puberty has been proven: Hormones awaken interest in sex. The first sexual objects are usually unattainable—a film star, a teacher—but by mid-adolescence, fantasies typically settle on another teenager.

Emotional surges and lustful impulses may begin with hormones, but remember that body, brain, and behavior always interact. Sexual thoughts themselves can cause physiological and neurological processes, not just result from them.

Cortisol levels rise at puberty, and that makes adolescents quicker to become angry or upset (Goddings et al., 2012; Klein & Romeo, 2013). Then those emotions, in turn, increase levels of various other hormones. Bodies, brains, and behavior each affect the other two.

For example, when other people react to emerging breasts or beards, that evokes thoughts and frustrations in the adolescent, which then raise hormone levels, propel physiological development, and trigger emotions, which then affect people’s reactions. Thus the internal and external changes of puberty each impact the other.

The brain of every living creature responds to the environment with natural rhythms that rise and fall by the hours, days, and seasons. For example, body weight and height are affected by time of year: Children’s growth rate increases for height in summer and for weight in winter. Some biorhythms are on a day–night cycle that occurs approximately every 24 hours, called the circadian rhythm. (Circadian means “about a day.”) Puberty affects both seasonal and daily biorhythms.

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The hypothalamus and the pituitary regulate the hormones that affect patterns of stress, appetite, sleep, and so on. At puberty, these hormones cause a phase delay in the circadian sleep–wake cycles. The delay is in the body’s reaction to daylight and dark.

For most people, daylight awakens the brain. That’s why people experiencing jet lag are urged to take an early morning walk. The phase delay at puberty makes many teens wide awake and hungry at midnight but half asleep, with no appetite or energy, all morning.

In addition to circadian changes in adolescent bodies, some individuals (especially males) are naturally more alert in the evening than in the morning, a genetic trait called eveningness. Exacerbated by pubertal phase delay, eveningness puts adolescents at risk for antisocial activities because they are awake when adults are asleep. Another result, if school is scheduled for adults, not teenagers, is that students are sleep deprived (Carskadon, 2011). In many nations, sleep deprivation increases during adolescence (see Figure 14.2) (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 weekend mornings is a sign of deprivation, not compensation.

Sleep deprivation and irregular sleep schedules lead to several specific dangers, including insomnia, nightmares, mood disorders (depression, conduct disorder, anxiety), and falling asleep while driving. In addition, sleepy people do not learn as well as they might when rested. Many adults ignore these facts, as the following Opposing Perspectives explains.

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Normally, pubertal hormones begin to increase between ages 8 and 14, and visible signs of puberty appear a year later. That six-year range is too great for many parents, teachers, and children, who want to know when a given child will begin puberty. Fortunately, if a child’s genes, gender, body fat, and stress level are known, some prediction is possible (see Visualizing Development, p. 451).

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 about seven months earlier than European or Hispanic Americans, while Chinese Americans average several months later. Ethnic differences are apparent on other continents as well. For instance, northern European girls reach menarche at 13 years, 4 months, on average; southern European girls do so at 12 years, 5 months (Alsaker & Flammer, 2006).

The sex chromosomes have a marked effect. In height, the average girl is about 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.

When it comes to hormonal and sexual changes themselves, though, girls may be less than a year ahead of boys. 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.

Another major influence on the onset of puberty is body fat. Heavy girls reach menarche years earlier than malnourished ones do. Most girls must weigh at least 100 pounds (45 kilograms) before they experience their first period (Berkey et al., 2000). Although malnutrition always delays puberty, body fat may not be as necessary for well-fed boys. Indeed, in developed nations obese boys are more often delayed in puberty, unlike girls (Crocker et al., 2014).

Body fat also explains why youths reach puberty at age 15 or later in some parts of Africa, although their genetic relatives in North America mature much earlier. Similarly, malnutrition may explain why 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 trend of changes in human growth as nutrition improved). Increased food availability has led to weight gain in childhood, and that has led to earlier puberty for girls and taller average height for both sexes. Because of the secular trend, for centuries every generation has reached puberty before the previous one (Floud et al., 2011; Fogel & Grotte, 2011).

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One curious bit of evidence of the secular trend is that U.S. presidents have been taller in recent decades than they were earlier (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 now nutrition allows everyone to attain their genetic potential. Currently, young men no longer usually look down at their short fathers, or girls at their little mothers, unless the parents were born in Asia or Africa, where the secular trend continues.

There is an exception in developed nations to the statement “the secular trend has stopped.” Puberty that begins before age 8, called precocious puberty, seems more common now, although still rare (perhaps 2 percent). The increase may be caused by childhood obesity or by exposure to new chemicals and may be more common in girls than boys.

Some research finds, however, that puberty is delayed, not accelerated, in boys who were exposed to phthalates and bisphenol A when they were in the womb (Ferguson et al., 2014) or who experience heavy doses of pesticides in boyhood (Lam et al., 2014). Another study found that phthalates delay puberty in girls of normal weight, but the authors note that other research has found earlier puberty (Wolff et al., 2014).

Caution, not panic, 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 4, experts disagree about the impact on humans of specific environmental toxins, as well as about what dose is safe.

Many researchers focus on sex hormones as factors that accelerate puberty. Of course, estrogens and androgens affect body shape, reproduction, height, and weight, but these sex hormones may also work directly to trigger the onset of puberty, even before visible changes occur.

Many scientists suspect that precocious or delayed puberty is caused by hormones in the food supply, especially in milk. Cattle are fed steroids to increase bulk and milk production, and hundreds of chemicals and hormones are used to produce most of the meat and milk that children consume. All these might affect appetite, body fat, and sex hormones, with effects suspected at puberty (Clayton et al., 2014; Wiley, 2011; Synovitz & Chopak-Foss, 2013).

Leptin, a hormone naturally produced by the human body, definitely affects the onset of puberty. Leptin affects appetite and energy; without it, puberty does not occur. However, high levels of leptin correlate with obesity. A girl with this problem may experience early puberty that ends relatively soon, stopping growth. Thus the heaviest third-grade girl may become the tallest fifth grader and then the shortest high school graduate.

Normally, body fat produces leptin (one reason puberty is delayed is if a person overexercises or undereats), and starvation increases leptin levels so that the hungry person will seek food. Once weight is gained, higher levels of leptin decrease the appetite (Elias & Purohit, 2013).

Most of the research on leptin has been done with mice; they become fat or thin depending on the levels of this hormone, but the consequences of high or low leptin, especially artificially added, are more complicated for humans than for mice.

In fact, all the research on the effects on humans of hormones and other chemicals, whether natural or artificial, is complex. The precise impact of all the substances in the air, water, or diet on the human sexual-reproductive system is unknown. It seems that the female system is especially sensitive not only to leptin but also to other factors in the environment. Leptin is one factor in puberty onset, but many other hormones, chemicals, genes, and psychosocial forces are involved (Elias, 2012).

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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 is stressful but late if their family is supportive (Ellis et al., 2011; James et al., 2012).

This is one explanation for the fact that many internationally adopted children 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 occurs at the expected time (Hayes, 2013).

Most developmentalists agree that age of menarche is influenced by genes and experience, as differential susceptibility would predict. Minor stresses—first day of a new school, summer camp away from home, a fight with a best friend—also trigger menarche in a girl whose body is ready.

Twenty years ago many scientists were skeptical about a direct link between stress and puberty. Perhaps early puberty caused stress rather than the other way around? Now, however, the link seems clear. (See A View from Science.)

For most adolescents, these links between puberty, genes, fat, stress, and hormones are irrelevant. Only one aspect of timing matters: 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 her developing body into a school chair designed for smaller children; she might hide her breasts in large T-shirts and bulky sweaters; she might refuse to undress for gym. 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).

Sometimes early-maturing girls have older boyfriends, who are attracted to their womanly shape and girlish innocence. Having an older boyfriend gives these girls status, but it also increases their risk of problems (including drug and alcohol abuse) that arise from an older peer group (Mrug et al., 2014). Early-maturing girls enter abusive relationships more often than other girls do. Is that because they are lonely and their social judgment is immature?

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Research over the past 100 years has always found that early female maturation is more often harmful than helpful, but cohort seems crucial for males. Early-maturing boys who were born around 1930 often became leaders in high school and earned more money 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, lawbreaking, and alcohol-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.

Early puberty is particularly stressful if it happens suddenly: The boys most likely to become depressed are those for whom puberty was both early and quick (Mendle et al., 2010). In adolescence, depression is often masked as anger. That fuming, flailing 12-year-old boy may be more sad than mad.

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. If a boy still looks childish, unlike the other boys and girls, he is likely to react in ways that are not healthy for him.

Every adolescent wants to hit puberty “on time.” They often overestimate or underestimate their maturation, or become depressed, if they are not average (Conley & Rudolph, 2009; Shirtcliff et al., 2009; Benoit et al., 2013). These effects are not caused by biology alone; family contexts and especially peer pressure can make early and late puberty worse (Mendle et al., 2012; Benoit et al., 2013).

Puberty that is late by world norms, at age 14 or so, is not troubling if one’s friends are late as well. Well-nourished Africans tend to experience puberty a few months earlier and Asians a few months later than Europeans, but they all function well if their peers are on the same schedule (Al-Sahab et al., 2010). This is true within those continents, as well as within nations such as the United States and Canada that are home to many teenagers with roots elsewhere.

For adolescents of all ethnic backgrounds, peer approval is more important than adult approval or historic understanding. The effects of early puberty vary not only by sex but also by ethnicity and culture. For instance, one study found that, in contrast to European Americans, early-maturing African American girls were not depressed, but early-maturing African American boys were (Hamlat et al., 2014a, 2014b).

European research finds that Swedish early-maturing girls were likely to encounter problems with boys and early drug abuse, but similar Slovak girls were not (Skoog & Stattin, 2014). Finally, among Mexican American boys, early maturers were likely to experience trouble with the police and with other boys if they lived in neighborhoods with relatively few Mexican Americans but not if they lived in ethnic enclaves (White et al., 2013).

None of these trends is true for all children, of course, as ethnicity is only one influence on development. However, all three studies confirm that contextual factors interact with biological ones, and both have significant implications for individuals. Always, relationships with peers and parents make off-time puberty better or worse (Benoit et al., 2013).

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