9.1 Puberty

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 released by a cascade of hormones that produce external growth and internal changes, including heightened emotions and sexual desires.

This process normally starts between ages 8 and 14 and follows the sequence outlined in At About This Time. Most physical growth and maturation end about four years after the first signs appear, although some individuals add height, weight, and muscle until age 20 or so.

For girls, the observable changes of puberty usually begin with nipple growth. Soon a few pubic hairs are visible, then peak growth spurt, widening of the hips, the first menstrual period (menarche), full pubic-hair pattern, and breast maturation (Susman et al., 2010). The average age of menarche among normal-weight girls is about 12 years, 8 months (Rosenfield et al., 2009), although variation in timing is quite normal.

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, peak growth spurt, deepening of the voice, and final pubic-hair growth (Biro et al., 2001; Herman-Giddens et al., 2001; Susman et al., 2010). The typical age of spermarche is just under 13 years, close to the age for menarche.

Unseen Beginnings

Just described are the visible changes of puberty, but the entire process begins with an invisible event: a marked hormonal increase. Throughout adolescence, hormone levels correlate with physiological changes and self-reported developments (Shirtcliff et al., 2009).

Hormones are body chemicals that regulate hunger, sleep, moods, stress, sexual desire, immunity, reproduction, and many other bodily reactions, including puberty. The process begins deep within the brain when biochemical signals from the hypothalamus signal another brain structure, the pituitary.

The pituitary produces hormones that stimulate the adrenal glands, located above the kidneys, which produce more hormones. Many hormones that regulate puberty follow this route, known as the HPA (hypothalamus-pituitary-adrenal) axis (see Figure 9.1).

FIGURE 9.1 Biological Sequence of Puberty Puberty begins with a hormonal signal from the hypothalamus to the pituitary gland. The pituitary, in turn, signals the adrenal glands and the ovaries or testes to produce more of their hormones.

Puberty and Appearance Teenagers are often preoccupied with appearance as their bodies go through changes in puberty.

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The HPG (hypothalamus-pituitary-gonad) axis is another hormonal sequence. In adolescence, gonadotropin-releasing hormone (GnRH) is released by the hypothalamus, causing the pituitary to release gonadotropins (LH & FSH), which in turn activate the gonads. As a result, the gonads enlarge and increase their production of sex hormones, chiefly estradiol in girls and testosterone in boys.

These sex hormones affect the body’s shape and function, producing additional hormones that regulate stress and immunity (E. A. Young et al., 2008). Estrogens (including estradiol) are female hormones, and androgens (including testosterone) are male hormones, although the adrenal glands produce both hormones in both sexes.

A dramatic increase in estrogens or androgens at puberty produces mature ova or sperm, released in menarche or spermarche. This same hormonal rush awakens interest in sex and makes reproduction biologically possible, although peak fertility occurs four to six years later.

Hormonal increases affect psychopathology in sex-specific ways (Naninck et al., 2011; Steiner & Young, 2008). Psychological disorders in both sexes increase at adolescence, but males are twice as likely as females to become schizophrenic, whereas females are twice as likely to become depressed. This greater likelihood of psychopathology is also influenced by other psychological and environmental factors, such as stressful experiences.

Sexual Maturation

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

At the same time as maturation of the primary sex characteristics, secondary sex characteristics develop. Secondary sex characteristics are bodily features that do not directly affect fertility (hence they are secondary) but that visually signify masculinity or femininity. One secondary characteristic is shape. At puberty, males widen at the shoulders and grow about 13 centimetres taller than females, whereas girls develop breasts and a wider pelvis. Breasts and broad hips are often considered signs of womanhood, but neither is required for conception; thus, they are secondary, not primary, sex characteristics.

Age and Puberty

Parents often have a very practical concern: When will adolescence begin? Some fear precocious puberty (sexual development before age 8) or very late puberty (after age 16), but both are rare (Cesario & Hughes, 2007). Quite normal are increased hormones at any time from ages 8 to 14, with the precise age affected by genes, gender, body fat, and stress.

Genes and GenderAbout two-thirds of the variation in age of puberty is genetic, evident not only in families but also in ethnic groups (Ge et al., 2007; Susman et al., 2010; van den Berg & Boomsma, 2007). 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 average age of first menarche is 13 years in Australia and Russia; 13 years, 2 months in Norway; 12 years, 3 months in Greece; and 13 years, 3 months in Finland (Al-Sahab et al., 2010; Steingraber, 2007).

Both 12 The ancestors of these 12-year-old boys came from northern Europe and West Africa, respectively. Their genes have dictated some differences between them, including the timing of puberty.

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Research in the United States indicates that African-Americans reach puberty about seven months earlier than European- or Hispanic-Americans, whereas Chinese-Americans average several months later. In Canada, results from the National Longitudinal Survey of Children and Youth (NLSCY) indicated regional differences in the age of onset of menarche: Girls in British Columbia reached menarche at the youngest age (11 years, 5 months), while girls Ontario reached this stage at the oldest age (13 years, 9 months). The average age of menarche in Canada is 12.72 years (Al-Sahab et al., 2010).

Genes on the sex chromosomes have a marked effect. In height, the average girl is about two years ahead of the average boy. However, the female height spurt occurs before menarche, whereas for boys the increase in height is relatively late, occurring after spermarche (Hughes & Gore, 2007). Thus, when it comes to hormonal and sexual changes, girls are only a few months ahead of boys.

Body FatAnother major influence on the onset of puberty is body fat, at least in girls. Heavy girls reach menarche years earlier than malnourished ones do. Most girls must weigh at least 45 kilograms before they experience their first period (Berkey et al., 2000).

Worldwide, urban children are more often overfed and underexercised compared with rural children. That is probably why puberty starts earlier in the cities of India and China than it does in more remote villages, a year earlier in Warsaw than in rural Poland, and earlier in Athens than in other parts of Greece (Malina et al., 2004).

Body fat also explains why youth 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. Puberty has occurred at younger ages every century since then. This is one result of the secular trend: More food has allowed biological advances. Over most of the twentieth century, each generation experienced puberty a few weeks earlier and grew a centimetre or so taller than did the preceding one (Floud et al., 2011).

One hormone causes increased body fat and then triggers puberty: leptin, which stimulates the appetite. Leptin levels in the blood show a natural increase over childhood, peaking at puberty (Rutters et al., 2008). Curiously, leptin affects appetite in females more than it does in males (Geary & Lovejoy, 2008), and body fat is more closely connected to the onset of puberty in girls than in boys. In fact, the well-established finding that body fat precipitates puberty may not be true for boys in nations where malnutrition is rare: One study found that, unlike girls, U.S. boys who are heavy in childhood reach puberty later, not earlier, than others (J. M. Lee et al., 2010).

Too Early, Too Late

Few adolescents care about speculation regarding hormones or evolution. Only one aspect of pubertal timing matters to them: their friends’ schedules. No one wants to be first or last; every adolescent wants to hit puberty “on time.” Research finds that a wise hope since, for both sexes, early and late puberty increase the rate of almost every adolescent problem.

GirlsThink about the early-maturing girl. If she has visible breasts at age 10, boys tease her. She will need to fit her developing body into a school chair designed for smaller children; she might hide her breasts in bulky sweaters; she might also refuse to undress for gym. Early-maturing girls tend to have lower self-esteem, more depression, and poorer body image than do other girls (Compian et al., 2009). Sometimes early-maturing girls have older boyfriends, which gains them status—but also increases their risk of drug and alcohol use, eating disorders, relational bullying, and victimization of physical violence (from that same boyfriend) (DeRose et al., 2011; Schreck et al., 2007).

Delayed puberty in girls can be hereditary, but it can also be due to malnutrition, chromosomal abnormalities, genetic disorders, or illness. If puberty is delayed, girls may become distressed by the differences in their bodies compared to others.

Teen Development Although this Australian girl looks physically mature, in reality she is only 13 years old and lacks social maturity.

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BoysThere was a time when early-maturing boys became leaders in high school and successful men later in life (M. C. Jones, 1965; Taga et al., 2006). Since about 1960, however, the risks associated with early male maturation have outweighed the benefits.

For the past few decades, early-maturing boys have been more aggressive, law-breaking, and alcohol-abusing than later-maturing boys (Biehl et al., 2007; Lynne et al., 2007). As a result, they have more trouble with parents, schools, and the police. Speed of change adds to the problems. If puberty is both early and quick, boys are likely to become depressed (Mendle et al., 2010). In adolescence, male depression may appear as anger: The flailing, fuming 12-year-old boy may actually be more sad than mad. Boys who reach puberty late may also have problems, becoming more anxious, depressed, and afraid of sex than other boys (Lindfors et al., 2007).

Growing Bigger and Stronger

For every child, puberty begins a growth spurt—an uneven jump in the size of almost every body part. 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. Many pubescent children are temporarily big-footed, long-legged, and short-waisted.

Bigger and Stronger During puberty, teenage boys first gain weight, then grow taller, and then develop muscle mass; young adolescents, such as Susan’s nephew, David Chuang, from Reno, Nevada, emerge as promising athletes.

SAMUEL CHUANG

Sequence: Weight, Height, and MusclesAs the bones lengthen and harden (visible on X-rays) children eat more and gain weight. Exactly when, where, and how much weight they gain depends on heredity, hormones, diet, exercise, and gender. For instance, at age 17, the average girl has twice the percentage of body fat as her male classmate, whose increased weight is mostly muscle (Roche & Sun, 2003).

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.

Lungs triple in weight; consequently, adolescents breathe more deeply and slowly. The heart doubles in size as the heart beat slows, decreasing the pulse rate while increasing blood pressure (Malina et al., 2004). Red blood cells increase in both sexes, but dramatically more so in boys, which aids oxygen transport during intense exercise. Endurance improves: Some teenagers can run for long distances or dance for hours.

Both weight and height increase before muscles and internal organs: Athletic training and weight lifting should be tailored to an adolescent’s size the previous year to protect immature muscles and organs. Sports injuries are the most common school accidents. Injuries increase at puberty, partly because the height spurt precedes increases in bone mass, making young adolescents particularly vulnerable to fractures (Mathison & Agrawal, 2010).

Only one organ system, the lymphoid system (which includes the tonsils and adenoids), decreases in size, so teenagers are less susceptible to respiratory ailments. Consequently, mild asthma often disappears at puberty (Busse & Lemanske, 2005), and teenagers have fewer colds than younger children do. This is aided by growth of the larynx, which gives deeper voices to both sexes, dramatically noticeable in boys.

Skin and HairBecause of the increased hormones, the fatty acid composition of perspiration changes into more “adult” body odour. Secretion of oils from the skin also increases, which results in a greater susceptibility to acne.

Hair also changes. During puberty, hair on the head and limbs becomes coarser and darker. New hair grows under arms, on faces, and over sex organs. For males, visible facial and chest hair is sometimes considered a sign of manliness, although hairiness in either sex depends on genes as well as on hormones.

To become more attractive, many teenagers spend considerable time, money, and thought on their head hair—growing, gelling, shaving, curling, straightening, highlighting, brushing, combing, styling, dyeing, wetting, drying, and so forth. If parents dislike the styling choices their teens make, hair can become a point of contention, and for the teens, a sign of independence.

Body Rhythms

I Covered That Teachers everywhere complain that students are falling asleep in class. Maybe schedules, not disinterest or lack of motivation, are to blame.

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The brain of every living creature responds to the environment with natural rhythms that rise and fall by the hours, days, and seasons. Some biorhythms are on a day-night cycle of biological activity that occurs approximately every 24 hours; this cycle is called the circadian rhythm (circadian means “about a day”).

The hypothalamus and the pituitary regulate the hormones that affect biorhythms of stress, appetite, sleep, and so on. Hormones of the HPA axis at puberty cause a phase delay in sleep–wake cycles, making many teens wide awake and hungry at midnight but half asleep with little appetite or energy all morning. By contrast, many older adults are naturally alert in the morning and sleepy at night because of their circadian rhythms.

Biology (circadian rhythms) and culture (socializing with friends and technology) work in opposite directions, making teenagers increasingly sleep-deprived with each year of high school (Carskadon, 2011). Not only does insufficient sleep decrease learning and well-being, but so does an uneven sleep schedule (more sleep on weekends, erratic bedtimes) (Fuligni & Hardway, 2006; Holm et al., 2009). Parents who yell at their wide-awake teenagers to turn off the bedroom light at midnight often must also drag those same children out of bed at 6 a.m. to get ready for school. Many high schools begin before 8 a.m., even though the evidence indicates that a later start time improves adolescent learning (Kirby et al., 2011).

Research indicates that 25 percent of Canadians are sleep-deprived, while 60 to 70 percent of Canadian students report being very sleepy during their morning classes (Douglas Mental Health University Institute, 2012). Sleepy teenagers are more likely doze in school, fall asleep while driving, develop eating and mood disorders (depression, conduct disorder, anxiety), have poor relationships with their parents, and abuse substances (partly to wake up or sleep). These problems risk jeopardizing their future health, increasing the likelihood of future obesity, diabetes, and heart disease. (Mueller et al., 2011; Patrick & Schulenberg, 2011; Roenneberg et al., 2012).