7.2: Physical Development in Adolescence

Adolescence is a period that begins with puberty and ends with the transition to adulthood (approximately ages 10–20). Physical changes associated with puberty are triggered by hormones. Cognitive changes include improvements in complex and abstract thought, as well as development that happens at different rates in distinct parts of the brain and increases adolescents’ propensity for risky behavior because increases in sensation-seeking and reward motivation precede increases in cognitive control. Adolescents’ relationships with parents go through a period of redefinition in which adolescents become more autonomous, and aspects of parenting, such as distal monitoring and psychological control, become more salient. Peer relationships are important sources of support and companionship during adolescence yet can also promote problem behaviors. Same-sex peer groups evolve into mixed-sex peer groups, and adolescents’ romantic relationships tend to emerge from these groups. Identity formation occurs as adolescents explore and commit to different roles and ideological positions. Nationality, gender, ethnicity, socioeconomic status, religious background, sexual orientation, and genetic factors shape how adolescents behave and how others respond to them, and are sources of diversity in adolescence.

Adolescence Defined

Adolescence has evolved historically, with evidence indicating that this stage is lengthening as individuals start puberty earlier and transition to adulthood later than in the past. Puberty today begins, on average, at age 9-10 years for girls and 11–12 years for boys. This average age of onset has decreased gradually over time since the 19th century by 3–4 months per decade, which has been attributed to a range of factors including better nutrition, obesity, increased father absence, and other environmental factors (Steinberg, 2013). Completion of formal education, financial independence from parents, marriage, and parenthood have all been markers of the end of adolescence and beginning of adulthood, and all of these transitions happen, on average, later now than in the past. In fact, the prolonging of adolescence has prompted the introduction of a new developmental period called emerging adulthood that captures these developmental changes out of adolescence and into adulthood, occurring from approximately ages 18 to 29 (Arnett, 2000).

Adolescence begins with puberty. While the sequence of physical changes in puberty is predictable, the onset and pace of puberty vary widely. Several physical changes occur during puberty, such as adrenarche and gonadarche, the maturing of the adrenal glands and sex glands, respectively. Also during this time, primary and secondary sexual characteristics develop and mature. Primary sexual characteristics are organs specifically needed for reproduction, like the uterus and ovaries in females and testes in males. Secondary sexual characteristics are physical signs of sexual maturation that do not directly involve sex organs, such as development of breasts and hips in girls, and development of facial hair and a deepened voice in boys. Girls experience menarche, the beginning of menstrual periods, usually around 12–13 years old, and boys experience spermarche, the first ejaculation, around 13–14 years old.

During puberty, both sexes experience a rapid increase in height (i.e., growth spurt). For girls this begins between 8 and 13 years old, with adult height reached between 10 and 16 years old. Boys begin their growth spurt slightly later, usually between 10 and 16 years old, and reach their adult height between 13 and 17 years old. Both nature (i.e., genes) and nurture (e.g., nutrition, medications, and medical conditions) can influence height.

Because rates of physical development vary so widely among teenagers, puberty can be a source of pride or embarrassment. Early maturing boys tend to be stronger, taller, and more athletic than their later maturing peers. They are usually more popular, confident, and independent, but they are also at a greater risk for substance abuse and early sexual activity (Flannery, Rowe, & Gulley, 1993; Kaltiala-Heino, Rimpela, Rissanen, & Rantanen, 2001). Early maturing girls may be teased or overtly admired, which can cause them to feel self-conscious about their developing bodies. These girls are at a higher risk for depression, substance abuse, and eating disorders (Ge, Conger, & Elder, 2001; Graber, Lewinsohn, Seeley, & Brooks-Gunn, 1997; Striegel-Moore & Cachelin, 1999). Late blooming boys and girls (i.e., they develop more slowly than their peers) may feel self-conscious about their lack of physical development. Negative feelings are particularly a problem for late maturing boys, who are at a higher risk for depression and conflict with parents (Graber et al., 1997) and more likely to be bullied (Pollack & Shuster, 2000).

The adolescent brain also remains under development. Up until puberty, brain cells continue to bloom in the frontal region. Adolescents engage in increased risk-taking behaviors and emotional outbursts possibly because the frontal lobes of their brains are still developing. Recall that this area is responsible for judgment, impulse control, and planning, and it is still maturing into early adulthood (Casey, Tottenham, Liston, & Durston, 2005).

Puberty

Puberty is the period during which humans become sexually mature. Besides maturation of the primary sex organs (those involved directly in reproduction), secondary sex characteristics also emerge during puberty. Boys develop adult male traits (such as facial hair), and girls develop adult female traits (such as breasts). In addition, there is a period of rapid body growth during puberty, which results in sexual dimorphism in adult body size, composition, and shape.

When does puberty occur? The timing of puberty depends in part on gender, with puberty typically occurring earlier in girls than boys. Besides gender, the timing of puberty is influenced by genetic and environmental factors. Although there is considerable individual variation in the age of onset, duration, and tempo of the physical changes of puberty, the sequence of these changes is relatively consistent among individuals.

Hormonal Control of Puberty

In both boys and girls, the changes of puberty are triggered by the hypothalamus in the brain. For unknown reasons, the hypothalamus starts releasing pulses of gonadotropin-releasing hormone (GnRH). This hormone travels directly to the anterior pituitary gland and stimulates it to secrete hormones that target the gonads (testes in males and ovaries in females). The main pituitary hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH stimulates the testes to produce sperm and follicles in the ovaries to mature and secrete estrogen. LH stimulates the testes to secrete testosterone and the ovaries to secrete estrogen. Testosterone and estrogen (primarily estradiol, a form of estrogen), in turn, stimulate the development of primary and secondary sex characteristics and contribute to the spurt in physical growth. It should be noted that both sexes have both hormones. It is the balance of the hormones that determines which characteristics develop, more estrogen/estradiol leading to the development of female and more testosterone leading to male. Both hormones play important roles for both sexes.

Puberty in Males

In the United States, puberty in boys generally begins between the ages of 11 and 12 years and is usually over by the age of 18. During puberty, the testes and scrotum start to increase in size first, followed by the penis. At the same time that the penis is growing, the seminal vesicles, prostate, and bulbourethral glands are also growing and developing. Secondary sex characteristics, such as pubic hair, also develop. Additional physical changes that occur in boys during puberty include the appearance of facial and body hair and deepening of the voice as the vocal cords increase in size. The first ejaculation generally occurs by the age of 13 years. Even this early in puberty, the semen may contain some sperm. Although full fertility may not be gained for another year or two, boys are generally fertile before they have completed their adolescent growth and achieved an adult appearance.

Puberty in Females

Puberty in females typically begins a couple of years earlier than puberty in males. In the United States, girls begin puberty between the ages of nine and ten. Visible, external changes begin first, including the growth and development of the breasts and pubic hair. About two years after breast development begins, the internal reproductive organs — including the uterus and vagina — start to grow and develop.

One of the most significant changes in females during puberty is menarche, which is the first menstrual period. It marks the beginning of menstruation. In the United States, menarche occurs at an average age of 12.4 years. However, there is considerable variation in this age, with menarche at any age between eight and 16 considered normal. It generally takes a few years after menarche occurs before ovulation starts occurring regularly and a girl becomes fertile, although occasionally, ovulation starts earlier. Unlike males, most females attain their adult size and appearance before they can reproduce.

Adolescent Growth Spurt

The period of rapid growth in body size that occurs during puberty is called the adolescent growth spurt (AGS). Both height and weight increase at a rate that is faster than at any time since early childhood. There are also significant changes in body composition and body proportions. The adolescent growth spurt is controlled by hormones, including growth, thyroid, and sex hormones.

Growth in Height

The average boy and girl do not differ significantly from each other in growth rate before the AGS begins. However, by the time they have attained their final adult height, the average female is about 13 cm (5.1 in.) shorter than the average male. One reason is that the AGS occurs earlier in girls than in boys, so girls experience a shorter period of childhood growth, making them shorter, on average, when they begin the AGS in height. Another reason is that the peak height velocity (maximum rate of growth in height) is lower for the average girl than it is for the average boy.

In boys, the AGS in height usually starts by about the age of 11 years. The peak height velocity in boys occurs at about age 13.5 when growth in height is about 10.3 cm (4 in.) per year on average. Growth in height in boys ceases by about age 18 (or a bit later) when the ends of the long bones finally ossify at the epiphyses, so additional growth in height is no longer possible.

In girls, the AGS in height usually starts by the age of roughly 9.5 years. The peak height velocity in girls occurs at about age 11.5 years when growth in height is about 9 cm (3.5 in.) per year on average. Growth in height in girls is completed by about 16 years (if not earlier) when the closure of the epiphyses prevents any additional growth in height.

The accelerated rate of growth during the AGS happens at different times for various parts of the body, but it occurs in the same predictable sequence for both sexes. Generally, the extremities — including the head, hands, and feet — experience rapid growth first, followed by the arms and legs, and then by the trunk and shoulders. This non-uniform growth may make the adolescent body seem awkward and disproportionate until growth is completed.

Growth in Weight

Growth in weight shows a similar spurt during adolescence as growth in height. Growth in weight occurs partly because of the growth in height, but growth in muscle, bone, and (for girls especially) body fat also contributes to the growth in weight. In boys, the AGS in weight lags behind the AGS in height by about three months, whereas in girls the lag time is about six months.

SOURCES

Human Biology (Wakim & Grewal)
Human Development Life Span (Overstreet)

REFERENCES

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