Puberty Upper body of a teenage boy. The structure has changed to resemble an adult form. Puberty is a period of several years in which rapid physical growth and psychological changes occur, culminating in sexual maturity.
The average age of onset of puberty is at 11 for girls and 12 for boys. Hormones play an organizational role, priming the body to behave in a certain way once puberty begins,  and an active role, referring to changes in hormones during adolescence that trigger behavioral and physical changes. It is the stage of life characterized by the appearance and development of secondary sex characteristics for example, a deeper voice and larger adam's apple in boys, and development of breasts and more curved and prominent hips in girls and a strong shift in hormonal balance towards an adult state.
This is triggered by the pituitary gland , which secretes a surge of hormonal agents into the blood stream, initiating a chain reaction to occur. The male and female gonads are subsequently activated, which puts them into a state of rapid growth and development; the triggered gonads now commence the mass production of the necessary chemicals.
The testes primarily release testosterone , and the ovaries predominantly dispense estrogen. The production of these hormones increases gradually until sexual maturation is met. Some boys may develop gynecomastia due to an imbalance of sex hormones , tissue responsiveness or obesity. The first facial hair to appear tends to grow at the corners of the upper lip, typically between 14 and 17 years of age.
This is followed by the appearance of hair on the upper part of the cheeks, and the area under the lower lip. Facial hair is often present in late adolescence, around ages 17 and 18, but may not appear until significantly later.
Early maturing boys are usually taller and stronger than their friends. Pubescent boys often tend to have a good body image, are more confident, secure, and more independent. However, early puberty is not always positive for boys; early sexual maturation in boys can be accompanied by increased aggressiveness due to the surge of hormones that affect them. Nearly half of all American high school girls' diets are to lose weight.
Girls attain reproductive maturity about four years after the first physical changes of puberty appear. Adolescence is marked in red at top right.
Growth spurt The adolescent growth spurt is a rapid increase in the individual's height and weight during puberty resulting from the simultaneous release of growth hormones, thyroid hormones , and androgens.
The weight gained during adolescence constitutes nearly half of one's adult body weight. The first places to grow are the extremities—the head, hands and feet—followed by the arms and legs, then the torso and shoulders. During puberty, bones become harder and more brittle. At the conclusion of puberty, the ends of the long bones close during the process called epiphysis.
There can be ethnic differences in these skeletal changes. For example, in the United States of America, bone density increases significantly more among black than white adolescents, which might account for decreased likelihood of black women developing osteoporosis and having fewer bone fractures there. This process is different for females and males. Before puberty, there are nearly no sex differences in fat and muscle distribution; during puberty, boys grow muscle much faster than girls, although both sexes experience rapid muscle development.
In contrast, though both sexes experience an increase in body fat, the increase is much more significant for girls. Frequently, the increase in fat for girls happens in their years just before puberty. The ratio between muscle and fat among post-pubertal boys is around three to one, while for girls it is about five to four.
This may help explain sex differences in athletic performance. These changes lead to increased strength and tolerance for exercise. Sex differences are apparent as males tend to develop "larger hearts and lungs, higher systolic blood pressure, a lower resting heart rate, a greater capacity for carrying oxygen to the blood, a greater power for neutralizing the chemical products of muscular exercise, higher blood hemoglobin and more red blood cells".
For example, girls tend to reduce their physical activity in preadolescence   and may receive inadequate nutrition from diets that often lack important nutrients, such as iron. Reproduction-related changes Primary sex characteristics are those directly related to the sex organs. In males, the first stages of puberty involve growth of the testes and scrotum, followed by growth of the penis.
The first ejaculation of seminal fluid generally occurs about one year after the beginning of accelerated penis growth, although this is often determined culturally rather than biologically, since for many boys first ejaculation occurs as a result of masturbation. Menarche , the beginning of menstruation, is a relatively late development which follows a long series of hormonal changes.
Changes in secondary sex characteristics include every change that is not directly related to sexual reproduction. In males, these changes involve appearance of pubic, facial, and body hair, deepening of the voice, roughening of the skin around the upper arms and thighs, and increased development of the sweat glands.
In females, secondary sex changes involve elevation of the breasts, widening of the hips, development of pubic and underarm hair, widening of the areolae, and elevation of the nipples.
Changes in the brain The human brain is not fully developed by the time a person reaches puberty. Between the ages of 10 and 25, the brain undergoes changes that have important implications for behavior see Cognitive development below.
However, the creases in the brain continue to become more complex until the late teens. The biggest changes in the folds of the brain during this time occur in the parts of the cortex that process cognitive and emotional information. However, this does not mean that the brain loses functionality; rather, it becomes more efficient due to increased myelination insulation of axons and the reduction of unused pathways.
The areas of the brain involved in more complex processes lose matter later in development. These include the lateral and prefrontal cortices, among other regions.
During adolescence, myelination and synaptic pruning in the prefrontal cortex increases, improving the efficiency of information processing, and neural connections between the prefrontal cortex and other regions of the brain are strengthened. Specifically, developments in the dorsolateral prefrontal cortex are important for controlling impulses and planning ahead, while development in the ventromedial prefrontal cortex is important for decision making.
Changes in the orbitofrontal cortex are important for evaluating rewards and risks. Three neurotransmitters that play important roles in adolescent brain development are glutamate , dopamine and serotonin. Glutamate is an excitatory neurotransmitter.
During the synaptic pruning that occurs during adolescence, most of the neural connections that are pruned contain receptors for glutamate or other excitatory neurotransmitters.
Dopamine is associated with pleasure and attuning to the environment during decision-making. During adolescence, dopamine levels in the limbic system increase and input of dopamine to the prefrontal cortex increases. Serotonin is a neuromodulator involved in regulation of mood and behavior. Development in the limbic system plays an important role in determining rewards and punishments and processing emotional experience and social information. Changes in the levels of the neurotransmitters dopamine and serotonin in the limbic system make adolescents more emotional and more responsive to rewards and stress.
The corresponding increase in emotional variability also can increase adolescents' vulnerability. The effect of serotonin is not limited to the limbic system: Several serotonin receptors have their gene expression change dramatically during adolescence, particularly in the human frontal and prefrontal cortex.
This allows the individual to think and reason in a wider perspective. The age at which particular changes take place varies between individuals, but the changes discussed below begin at puberty or shortly after that and some skills continue to develop as the adolescent ages. The dual systems model proposes a maturational imbalance between development of the socioemotional system and cognitive control systems in the brain that contribute to impulsivity and other behaviors characteristic of adolescence.
One is the constructivist view of cognitive development. Based on the work of Piaget , it takes a quantitative, state-theory approach, hypothesizing that adolescents' cognitive improvement is relatively sudden and drastic. The second is the information-processing perspective , which derives from the study of artificial intelligence and attempts to explain cognitive development in terms of the growth of specific components of the thinking process.
Improvements in cognitive ability By the time individuals have reached age 15 or so, their basic thinking abilities are comparable to those of adults.
These improvements occur in five areas during adolescence: Improvements are seen in selective attention , the process by which one focuses on one stimulus while tuning out another. Divided attention , the ability to pay attention to two or more stimuli at the same time, also improves.
Improvements are seen in both working memory and long-term memory. Adolescents think more quickly than children. Processing speed improves sharply between age five and middle adolescence; it then begins to level off at age 15 and does not appear to change between late adolescence and adulthood. Adolescents are more aware of their thought processes and can use mnemonic devices and other strategies to think more efficiently. One manifestation of the adolescent's increased facility with thinking about possibilities is the improvement of skill in deductive reasoning , which leads to the development of hypothetical thinking.
This provides the ability to plan ahead, see the future consequences of an action and to provide alternative explanations of events. It also makes adolescents more skilled debaters, as they can reason against a friend's or parent's assumptions. Adolescents also develop a more sophisticated understanding of probability.
The appearance of more systematic, abstract thinking is another notable aspect of cognitive development during adolescence. For example, adolescents find it easier than children to comprehend the sorts of higher-order abstract logic inherent in puns, proverbs, metaphors, and analogies.
Their increased facility permits them to appreciate the ways in which language can be used to convey multiple messages, such as satire, metaphor, and sarcasm. Children younger than age nine often cannot comprehend sarcasm at all. Metacognition A third gain in cognitive ability involves thinking about thinking itself, a process referred to as metacognition. It often involves monitoring one's own cognitive activity during the thinking process.
Adolescents' improvements in knowledge of their own thinking patterns lead to better self-control and more effective studying. It is also relevant in social cognition, resulting in increased introspection , self-consciousness , and intellectualization in the sense of thought about one's own thoughts, rather than the Freudian definition as a defense mechanism.
Adolescents are much better able than children to understand that people do not have complete control over their mental activity. Being able to introspect may lead to two forms of adolescent egocentrism, which results in two distinct problems in thinking: These likely peak at age fifteen, along with self-consciousness in general.
Through experience outside the family circle, they learn that rules they were taught as absolute are in fact relativistic. They begin to differentiate between rules instituted out of common sense—not touching a hot stove—and those that are based on culturally-relative standards codes of etiquette, not dating until a certain age , a delineation that younger children do not make.
This can lead to a period of questioning authority in all domains. Thus, it is during the adolescence-adulthood transition that individuals acquire the type of wisdom that is associated with age.
Wisdom is not the same as intelligence: Risk-taking Because most injuries sustained by adolescents are related to risky behavior car crashes , alcohol, unprotected sex , a great deal of research has been done on the cognitive and emotional processes underlying adolescent risk-taking.