Adolescence is a time of dramatic physiological changes and social-role development. In Western societies it is the transition between childhood and adulthood that typi­cally spans the period between ages 12 and 20. Most of the major physical changes of adolescence take place during the first few years of this period. However, important and often profound changes in behavior and role expectations occur throughout this phase of life. By cross-cultural standards, adolescence in our society is rather extended.

In many cultures (and in Western society in preindustrial times), adult roles are assumed at a much earlier age. Rather than under­going a protracted period of child-adult status, the child is often initiated into adulthood upon reaching puberty.

Puberty (from the Latin pubescere, “to be covered with hair") is a term frequently used to describe the period of rapid physical changes in early adolescence. The mechanisms that trigger the chain of devel­opments are not fully understood. However, we do know that the brain coordinates the physical changes that occur during puberty and that the hypothalamus plays an especially important role in this pro­cess (Westwood, 2007). In general, when a child is between 8 and 14 years old, the hypothalamus increases secretions that cause the pitu­itary gland to release larger amounts of hormones known as gonado­tropins into the bloodstream (Westwood, 2007). These hormones stimulate activity in the gonads, and they are chemically identical in boys and girls. However, in males they cause the testes to increase testosterone production, whereas in females they act on the ovaries to produce elevated estrogen levels. From the age of 9 or 10 years, the levels of these gonadal steroid hormones begin to increase as the child approaches puberty (Bancroft, 2003). Girls typically enter puberty at age 10 or 11, whereas boys experience puberty a little later, at an average age of 12 years (Westwood, 2007). In the United States, 15% of girls begin puberty by age 7 (Newman, 2011). Research has demonstrated that overweight girls tend to enter puberty at an earlier age than the norm for girls (Diaz et al., 2008).

In response to higher levels of male and female hormones, external signs of characteristic male and female sexual maturation begin to appear. The resulting developments—breasts; deepened voice; and facial, body, and pubic hair—are called secondary sex characteristics. Growth of pubic hair in both sexes and breast budding (slight protuberance under the nip­ple) in girls are usually the earliest signs of puberty. A growth spurt also follows, stimulated by an increase in sex hormones, growth hormone, and a third substance called insulin-like growth factor 1 (Caufriez, 1997). A deficiency in growth hormone levels is associated with short stature in youth, a condition that can be remedied by the administration of growth hormone during puberty (Collett-Solberg, 2011; Root et al., 2011). The growth spurt even­tually terminates, again under the influence of sex hormones, which send signals to close the ends of the long bones. External genitals also undergo enlargement; the penis and testes increase in size in the male, and the labia become enlarged in the female (I Figure 12.1).

The only event of puberty that is clearly different in boys and girls is growth. Because estrogen is a much better facilitator of growth hormone secretion by the pituitary gland than is testosterone, as soon as a girl starts to show pubertal development, she starts to grow more quickly. Even though the magnitude of the pubertal growth spurt is roughly equal in both sexes, it begins about 2 years earlier in girls (Westwood, 2007). This is why the average 12-year-old girl is considerably taller than her male counterpart.

Under the influence of hormone stimulation, the internal organs of both sexes undergo further development during puberty. In girls the vaginal walls become thicker, and the uterus becomes larger and more muscular. Vaginal pH changes from alkaline to acidic as vaginal and cervical secretions increase in response to the changing hormone status. Eventually, menstruation begins; the first menstrual period is called menarche (discussed in Chapter 3). Initial menstrual periods can be irregular and can occur with­out ovulation. Some adolescent girls experience irregular menstrual cycles for several years before their periods become regular and predictable. Consequently, methods of birth control based on the menstrual cycle can be particularly unreliable for females in this age group. Most girls begin menstruating around the age of 12 or 13, but there is widespread variation in the age at menarche (Chumlea et al., 2003).

Sexuality During Childhood and adolescence

Acne

Beard

Voice change

Underarm hair, chest hair, muscle development

Pubic hair

Enlargement of penis, scrotum, testes

Ejaculation

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The median age at menarche for all girls in the United States and other developed nations, 12.43 years, has remained stable for the last 50 years (Segal & Stohs, 2007). Only 10% of U. S. girls are menstruating by age 11.1 years, but by age 13.75 years, 90% are menstruating (Chumlea et al., 2003). There are, however, significant differences in the ages at menarche for different racial and ethnic groups in the United States, as described in the following Sexuality and Diversity discussion.