The physical changes the body goes through in adolescence are the most rapid it has seen since infancy. The major physical changes, and some of the ways they affect behavior, are outlined here.


The most salient physical change during adolescence is puberty. Puberty marks the onset of the capability of reproduction. In the United States, the average age for girls to begin menstruating is 12.5 years, with European American girls typically starting a bit later than African American girls (Brooks-Gunn & Reiter, 1990). Most girls (95% of the population) reach puberty between the ages of 9 and 16. Boys lag behind girls by a few years; boys' average age for reaching sexual maturity is 14 years. Most boys (95% of the population) enter puberty between the ages of 10 and 19 (Brooks-Gunn & Reiter, 1990).

The ways in which puberty impacts adolescent behavior are related, in part, to the reactions of others. Once a child looks physically mature, he or she may be assumed to have greater mental and emotional maturity too, regardless of whether or not this is true. The timing of puberty—whether one matures early, late, or on time with respect to peers—has been shown to play an important role in whether pubertal changes are a positive or negative force in adolescents' lives. For boys, maturing early has some social benefits. Early-maturing boys are perceived as adult like and may be given more leadership roles. In addition, their increased strength and speed are valued by their peers in athletic arenas. Still, research has demonstrated psychological costs for early-maturing boys in terms of depression, anxiety, and feelings of hostility (Ge, Conger, & Elder, 2001).

For girls, maturing early seems to carry more substantial costs. These girls often report feeling awkward around their peers, tend to hang out with an older crowd, begin dating earlier than their peers, and may be pressured into sexual experiences before they are ready (Stattin & Magnusson, 1990). Boys who mature later than their peers might find that situation to be personally challenging, but research has not revealed any long-term negative effects. Maturing late may actually be advantageous for girls in terms of protecting them from negative body image issues associated with the naturally occurring weight gain of puberty.

Historically speaking, the average age for reaching puberty is decreasing. This secular trend may be due to better nutrition, sanitation, and control of infectious disease. Although the average age for girls to begin menstruating is unlikely to dip far below 12 years, a significant minority of girls may show one or more signs of puberty by age seven (Kaplowitz, 2004). Therefore, the effects of puberty may begin well before adolescence, often to the surprise of parents, teachers, peers, and the child herself.

Brain Development

Although the brain reaches 95% of its adult volume by age five or so, it continues to develop substantially in adolescence and even into young adulthood. Some changes that happen during this period make the brain more efficient: (a) unnecessary connections among brain cells are eliminated; (b) the formation of myelin sheaths around nerve fibers is completed, allowing messages to travel faster in the brain; (c) brain functions become more localized on either the right or the left hemisphere of the cerebral cortex; and (d) the corpus callosum (i.e., the cable of nerves connecting the brain's hemispheres) thickens. These changes are reflected in adolescents' greater memory and problem-solving abilities. But as the brain's efficiency increases, it loses the ability to easily take on new functions. The brain does not recover from trauma as easily, and if certain experiences have not happened before adolescence, the skills associated with them may be more difficult to acquire. For example, although most secondary language instruction happens during adolescence, our brains would actually be more receptive to learning this material earlier in life.

Changes in levels of some neurotransmitters (e.g., dopamine and serotonin) in the brain's limbic system have an impact on the way that adolescents respond to and interpret emotional events (Spear, 2000). Specifically, these changes make adolescents more emotional, more responsive to stress, and less responsive to rewards. This decline in responsiveness to rewards may result in feelings of boredom and a desire to seek out novelty and take risks.

One of the last parts of the brain to finish developing is the prefrontal cortex, or the area at the front of the brain. Dramatic changes in this area of the brain may account for striking changes in adolescents' behaviors (Spear, 2000). We use the prefrontal cortex for planning and anticipating the consequences of our actions. Because adolescents are still developing these neurological capabilities, they may have trouble thinking of the long-term impact of their risky behavior on their own. To compensate for the underdevelopment of the prefrontal cortex, the adolescent brain appears to rely heavily on another area of the brain, the amygdala, resulting in a tendency to react on instinct and behave impulsively (Spear, 2000).

Changes in Moods

Adolescents are noted for their rapid mood swings and general moodiness. Hormones are thought to be partially responsible for these emotional changes, especially among younger adolescents. Early in the pubertal process, when the hormonal system is being turned on, hormones fluctuate rapidly and result in fluctuating moods. In early adolescence, boys tend to become more irritable, aggressive, and impulsive, whereas girls tend to react with more depressed moods.

But beyond early adolescence, the direct link between hormones and moods is relatively weak. Stressful changes in adolescents' environments may be just as important as biological factors in influencing their moods. Using a method known as the experience sampling method, researchers outfitted adolescents with beepers in order to track the effect of contextual factors on moods. Adolescents carried the beepers with them everywhere they went for a week. The adolescents were beeped at random times. When they were beeped, adolescents recorded where they were, what they were doing, who they were with, and whether their mood was positive or negative. The researchers found that adolescents experienced emotional extremes (both positive and negative) more often than adults. However, changes in moods were linked to what the adolescent was doing rather than biological changes (Larson, Csikszentmihalyi, & Graef, 1980). Since adolescents change contexts and activities quite frequently, their moods change more often than do those of adults or children.

Sleep Requirements

Researchers have also suggested that adolescents' moodiness might be due to their failure to get enough sleep. In fact, most adolescents (and many adults) operate in a state of sleep deprivation (Maas, 2002)—a fact that is especially troubling given the importance of sleep in periods of rapid brain maturation (Dahl, 1999). Adolescents' preference for staying up late and then sleeping in are actually tied to biological changes. When allowed to determine their sleep schedules, most adolescents stay up until 1 A.M. and sleep until 10 A.M. However, school schedules often force adolescents to adhere to sleep schedules better suited to children or adults, leaving adolescents sleepy during school hours (Carskadon, Wolfson, Acebo, Tzischinsky, & Seifer, 1998). Girls tend to get less sleep than boys (likely due to the extra time girls use for grooming in the morning), and sleep loss has been associated with increases in depression and decreases in self-esteem across adolescence (Fredriksen, Rhodes, Reddy, & Way, 2004). As a group, adolescents are sleepiest between 8 A.M. and 9 A.M. and most alert after 3 P.M., posing obvious challenges to those trying to maintain their attention during class!