Adolescence

Two adolescents listening to music
Adolescents in Oslo

Adolescence (from Latin adolescere, meaning 'to grow up')[1] is a transitional stage of physical and psychological development that generally occurs during the period from puberty to legal adulthood (age of majority).[1][2][3] Adolescence is usually associated with the teenage years,[3][4][5][6] but its physical, psychological or cultural expressions may begin earlier and end later. For example, puberty now typically begins during preadolescence, particularly in females.[4][7][8][9][10] Physical growth (particularly in males) and cognitive development can extend into the early twenties. Thus, age provides only a rough marker of adolescence, and scholars have found it difficult to agree upon a precise definition of adolescence.[7][8][11][12]

A thorough understanding of adolescence in society depends on information from various perspectives, including psychology, biology, history, sociology, education, and anthropology. Within all of these perspectives, adolescence is viewed as a transitional period between childhood and adulthood, whose cultural purpose is the preparation of children for adult roles.[13] It is a period of multiple transitions involving education, training, employment, and unemployment, as well as transitions from one living circumstance to another.[14]

The end of adolescence and the beginning of adulthood varies by country. Furthermore, even within a single nation state or culture, there can be different ages at which an individual is considered mature enough for society to entrust them with certain privileges and responsibilities. Such privileges and responsibilities include driving a vehicle, having legal sexual relations, serving in the armed forces or on a jury, purchasing and drinking alcohol, purchase of tobacco products, voting, entering into contracts, finishing certain levels of education, marriage, and accountability for upholding the law. Adolescence is usually accompanied by an increased independence allowed by the parents or legal guardians, including less supervision as compared to preadolescence.

In studying adolescent development,[15] adolescence can be defined biologically, as the physical transition marked by the onset of puberty and the termination of physical growth; cognitively, as changes in the ability to think abstractly and multi-dimensionally; or socially, as a period of preparation for adult roles. Major pubertal and biological changes include changes to the sex organs, height, weight, and muscle mass, as well as major changes in brain structure and organization. Cognitive advances encompass both increment in knowledge and in the ability to think abstractly and to reason more effectively. The study of adolescent development often involves interdisciplinary collaborations. For example, researchers in neuroscience or bio-behavioral health might focus on pubertal changes in brain structure and its effects on cognition or social relations. Sociologists interested in adolescence might focus on the acquisition of social roles (e.g., worker or romantic partner) and how this varies across cultures or social conditions.[16] Developmental psychologists might focus on changes in relations with parents and peers as a function of school structure and pubertal status.[17] Some scientists have questioned the universality of adolescence as a developmental phase, arguing that traits often considered typical of adolescents are not in fact inherent to the teenage years.

Biological development

Puberty in general

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.[18][19] Every person's individual timetable for puberty is influenced primarily by heredity, although environmental factors, such as diet and exercise, also exert some influences.[20][21] These factors can also contribute to precocious and delayed puberty.[12][21]

Some of the most significant parts of pubertal development involve distinctive physiological changes in individuals' height, weight, body composition, and circulatory and respiratory systems.[22] These changes are largely influenced by hormonal activity. Hormones play an organizational role, priming the body to behave in a certain way once puberty begins,[23] and an active role, referring to changes in hormones during adolescence that trigger behavioral and physical changes.[24]

Puberty occurs through a long process and begins with a surge in hormone production, which in turn causes a number of 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. The male and female gonads are thereby activated, which puts them into a state of rapid growth and development; the triggered gonads now commence mass production of hormones. 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.[25]

Facial hair in males normally appears in a specific order during puberty: The first facial hair to appear tends to grow at the corners of the upper lip, typically between 14 and 17 years of age.[26][27] It then spreads to form a moustache over the entire upper lip. This is followed by the appearance of hair on the upper part of the cheeks, and the area under the lower lip.[26] The hair eventually spreads to the sides and lower border of the chin, and the rest of the lower face to form a full beard.[26] As with most human biological processes, this specific order may vary among some individuals. Facial hair is often present in late adolescence, around ages 17 and 18, but may not appear until significantly later.[27][28] Some men do not develop full facial hair for 10 years after puberty.[27] Facial hair continues to get coarser, darker and thicker for another 2–4 years after puberty.[27]

The major landmark of puberty for males is spermarche, the first ejaculation, which occurs, on average, at age 13.[29] For females, it is menarche, the onset of menstruation, which occurs, on average, between ages 12 and 13.[20][30][31][32] The age of menarche is influenced by heredity, but a girl's diet and lifestyle contribute as well.[20] Regardless of genes, a girl must have a certain proportion of body fat to attain menarche.[20] Consequently, girls who have a high-fat diet and who are not physically active begin menstruating earlier, on average, than girls whose diet contains less fat and whose activities involve fat reducing exercise (e.g. ballet and gymnastics).[20][21] Girls who experience malnutrition or are in societies in which children are expected to perform physical labor also begin menstruating at later ages.[20]

The timing of puberty can have important psychological and social consequences. Early maturing boys are usually taller and stronger than their friends.[33] They have the advantage in capturing the attention of potential partners and in being picked first for sports. Pubescent boys often tend to have a good body image, are more confident, secure, and more independent.[34] Late maturing boys can be less confident because of poor body image when comparing themselves to already developed friends and peers. 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.[34] Because they appear older than their peers, pubescent boys may face increased social pressure to conform to adult norms; society may view them as more emotionally advanced, despite the fact that their cognitive and social development may lag behind their appearance.[34] Studies have shown that early maturing boys are more likely to be sexually active and are more likely to participate in risky behaviors.[35]

For girls, early maturation can sometimes lead to increased self-consciousness, a typical aspect in maturing females.[36] Because of their bodies' developing in advance, pubescent girls can become more insecure and dependent.[36] Consequently, girls that reach sexual maturation early are more likely than their peers to develop eating disorders (such as anorexia nervosa). Nearly half of all American high school girls' diets are to lose weight.[36] In addition, girls may have to deal with sexual advances from older boys before they are emotionally and mentally mature.[37] In addition to having earlier sexual experiences and more unwanted pregnancies than late maturing girls, early maturing girls are more exposed to alcohol and drug abuse.[38] Those who have had such experiences tend to not perform as well in school as their "inexperienced" peers.[39]

Girls have usually reached full physical development around ages 15–17,[3][19][40] while boys usually complete puberty around ages 16–17.[19][40][41] Any increase in height beyond the post-pubertal age is uncommon. Girls attain reproductive maturity about four years after the first physical changes of puberty appear.[3] In contrast, boys develop more slowly but continue to grow for about six years after the first visible pubertal changes.[34][41]

Approximate outline of development periods in child and teenager development. 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.[42] Males experience their growth spurt about two years later, on average, than females. During their peak height velocity (the time of most rapid growth), adolescents grow at a growth rate nearly identical to that of a toddler—about 10.3 cm (4 inches) per year for males and 9 cm (3.5 inches) per year for females.[43] In addition to changes in height, adolescents also experience a significant increase in weight (Marshall, 1978). The weight gained during adolescence constitutes nearly half of one's adult body weight.[43] Teenage and early adult males may continue to gain natural muscle growth even after puberty.[34]

The accelerated growth in different body parts happens at different times, but for all adolescents it has a fairly regular sequence. The first places to grow are the extremities—the head, hands and feet—followed by the arms and legs, then the torso and shoulders.[44] This non-uniform growth is one reason why an adolescent body may seem out of proportion.

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.[45]

Another set of significant physical changes during puberty happen in bodily distribution of fat and muscle. 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.[46]

Pubertal development also affects circulatory and respiratory systems as an adolescents' heart and lungs increase in both size and capacity. 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".[47]

Despite some genetic sex differences, environmental factors play a large role in biological changes during adolescence. For example, girls tend to reduce their physical activity in preadolescence[48][49] and may receive inadequate nutrition from diets that often lack important nutrients, such as iron.[50] These environmental influences in turn affect female physical development.

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.[51] At the time that the penis develops, the seminal vesicles, the prostate, and the bulbourethral gland also enlarge and develop. 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.[44] Boys are generally fertile before they have an adult appearance.[42]

In females, changes in the primary sex characteristics involve growth of the uterus, vagina, and other aspects of the reproductive system. Menarche, the beginning of menstruation, is a relatively late development which follows a long series of hormonal changes.[52] Generally, a girl is not fully fertile until several years after menarche, as regular ovulation follows menarche by about two years.[53] Unlike males, therefore, females usually appear physically mature before they are capable of becoming pregnant.

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.[54] The changes in secondary sex characteristics that take place during puberty are often referred to in terms of five Tanner stages,[55] named after the British pediatrician who devised the categorization system.

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). The brain reaches 90% of its adult size by the time a person is six years of age.[56] Thus, the brain does not grow in size much during adolescence. 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.[56]

Over the course of adolescence, the amount of white matter in the brain increases linearly, while the amount of grey matter in the brain follows an inverted-U pattern.[57] Through a process called synaptic pruning, unnecessary neuronal connections in the brain are eliminated and the amount of grey matter is pared down. 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.[58]

The first areas of the brain to be pruned are those involving primary functions, such as motor and sensory areas. 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.[59] Some of the most developmentally significant changes in the brain occur in the prefrontal cortex, which is involved in decision making and cognitive control, as well as other higher cognitive functions. 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.[60] This leads to better evaluation of risks and rewards, as well as improved control over impulses. 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.[61] Because of this, by early adulthood the synaptic balance in the brain is more inhibitory than excitatory.

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.[62] The balance of excitatory to inhibitory neurotransmitters and increased dopamine activity in adolescence may have implications for adolescent risk-taking and vulnerability to boredom (see Cognitive development below).

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 .[63]