Editing Adolescence (section)

==Biological development==
===Puberty in general===
{{Main|Puberty}}
[[File:Cubes teenager.jpg|thumb|left|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 10–11 for girls and 11–12 for boys.<ref name=Kail>{{cite book| last = Kail | first = RV |author2=Cavanaugh JC | title = Human Development: A Lifespan View | isbn = 978-0-495-60037-4 | publisher = [[Cengage Learning]] | year = 2010|access-date= September 11, 2014| page = 296 | edition = 5th|url=https://books.google.com/books?id=E-n5E7oyCgoC&pg=PA296}}</ref><ref name="PAmf.org">{{Cite news|title=Teenage Growth & Development: 11 to 14 Years|publisher=[[Palo Alto Medical Foundation]]|url=http://www.pamf.org/teen/parents/health/growth-11-14.html|quote=For girls, puberty begins around 10 or 11 years of age and ends around age 16. Boys enter puberty later than girls-usually around 12 years of age-and it lasts until around age 16 or 17.}}</ref> Every person's individual timetable for puberty is influenced primarily by [[heredity]], although environmental factors, such as diet and exercise, also exert some influences.<ref name=Tanner/><ref name=Kaplowitz01>{{Cite journal|doi=10.1542/peds.108.2.347 |vauthors=Kaplowitz PB, Slora EJ, Wasserman RC, Pedlow SE, Herman-Giddens ME |title=Earlier onset of puberty in girls: relation to increased body mass index and race |journal=Pediatrics |volume=108 |issue=2 |pages=347–53 |date=August 2001 |pmid=11483799 |url=http://pediatrics.aappublications.org/cgi/pmidlookup?view=long&pmid=11483799}}</ref> These factors can also contribute to [[precocious puberty|precocious]] and [[delayed puberty]].<ref name="Delayed puberty">{{cite web|first=Elizabeth|last=Cooney|title=Puberty gap: Obesity splits boys, girls. Adolescent males at top of the BMI chart may be delayed|work=[[NBC News]]|date=February 11, 2010|access-date=May 22, 2010|url=http://www.nbcnews.com/id/35332881|archive-date=January 11, 2016|archive-url=https://web.archive.org/web/20160111055843/http://www.nbcnews.com/id/35332881|url-status=dead}}</ref><ref name=Kaplowitz01/>

Some of the most significant parts of pubertal development involve distinctive physiological changes in individuals' height, weight, body composition, and [[circulatory system|circulatory]] and [[respiratory system|respiratory]] systems.<ref>Marshal, W. (1978). Puberty. In F. Falkner & J.Tanner (Eds.), ''Human growth'', Vol. 2. New York: Plenum.</ref> 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,<ref>{{cite journal |last1=Sisk |first1=Cheryl L |last2=Foster |first2=Douglas L |title=The neural basis of puberty and adolescence |journal=Nature Neuroscience |date=October 2004 |volume=7 |issue=10 |pages=1040–1047 |doi=10.1038/nn1326 |pmid=15452575 |s2cid=2932858 }}</ref> and an active role, referring to changes in hormones during adolescence that trigger behavioral and physical changes.<ref>{{cite book |last1=Coe |first1=Christopher L. |last2=Hayashi |first2=Kevin T. |last3=Levine |first3=Seymour |chapter=Hormones and Behavior at Puberty: Activation or Concatenation |pages=17–41 |chapter-url=https://books.google.com/books?id=w_J1oKCvCMEC&pg=PA17 |editor1-last=Gunnar |editor1-first=Megan R. |editor2-last=Collins |editor2-first=W. Andrew |title=Development During the Transition to Adolescence |date=1988 |publisher=Psychology Press |isbn=978-0-8058-0194-1 }}</ref>

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 characteristic]]s (for example, a deeper voice and larger [[Adam's apple]] in boys, and development of [[breast]]s 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 [[Hormone|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 hormone]]s, tissue responsiveness or [[obesity]].<ref>{{Cite journal|vauthors=Nydick M, Bustos J, Dale JH, Rawson RW |title=Gynecomastia in adolescent boys |journal=JAMA |volume=178 |issue= 5|pages=449–54 |date=November 1961 |pmid=14480779|doi=10.1001/jama.1961.03040440001001 }}</ref>

[[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.<ref name="pamf.org">{{Cite news|title=Puberty – Changes for Males|publisher=pamf.org|access-date=February 20, 2009 |url=http://www.pamf.org/teen/health/puberty/physicalchanges.html| archive-url= https://web.archive.org/web/20090303201905/http://www.pamf.org/teen/health/puberty/physicalchanges.html| archive-date= March 3, 2009 | url-status= live}}</ref><ref name="www.ppwr">{{cite web|title=Getting The Facts: Puberty|publisher=ppwr|access-date=February 20, 2009|url=http://www.ppwr.on.ca/03_07.html|archive-url=https://web.archive.org/web/20090226191204/http://www.ppwr.on.ca/03_07.html|archive-date=February 26, 2009|url-status=dead}}</ref> 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.<ref name="pamf.org"/> 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.<ref name="pamf.org"/> 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.<ref name="www.ppwr"/><ref name="pbskids.org">{{Cite news|title=The No-Hair Scare|publisher=[[PBS]]|access-date=February 20, 2009|url=http://pbskids.org/itsmylife/body/puberty/article7.html|archive-url=https://web.archive.org/web/20090205115944/http://pbskids.org/itsmylife/body/puberty/article7.html|archive-date=February 5, 2009|url-status=dead}}</ref> Some men do not develop full facial hair for 10 years after puberty.<ref name="www.ppwr"/> Facial hair continues to get coarser, much darker, and thicker for another 2–4 years after puberty.<ref name="www.ppwr"/>

The major landmark of puberty for males is [[spermarche]], the first [[ejaculation]], which occurs, on average, at age 13.<ref name="Jorgensen & Keiding">(Jorgensen & Keiding 1991).{{full citation needed|date=October 2022}}</ref> For females, it is [[menarche]], the onset of menstruation, which occurs, on average, between ages 12 and 13.<ref name="Tanner">(Tanner, 1990).{{full citation needed|date=October 2022}}</ref><ref name="U.S. menarche">{{cite journal |vauthors=Anderson SE, Dallal GE, Must A |title=Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart |journal=Pediatrics |volume=111 |issue=4 Pt 1 |pages=844–50 |date=April 2003 |pmid=12671122 |doi=10.1542/peds.111.4.844}}</ref><ref name="Canadian menarche">{{cite journal |title=Age at menarche in Canada: results from the National Longitudinal Survey of Children & Youth|year= 2010 |pmid=21110899 |doi=10.1186/1471-2458-10-736 |pmc=3001737 |volume=10 |journal=BMC Public Health |page=736 |vauthors=Al-Sahab B, Ardern CI, Hamadeh MJ, Tamim H |issue=1 |doi-access= free }}</ref><ref name="UK menarche">{{cite journal |url=http://vstudentworld.yolasite.com/resources/final_yr/gynae_obs/Hamilton%20Fairley%20Obstetrics%20and%20Gynaecology%20Lecture%20Notes%202%20Ed.pdf |title=Obstetrics and Gynaecology |first=Diana |last=Hamilton-Fairley |edition=Second |publisher=Blackwell Publishing |journal= |access-date=December 9, 2011 |archive-date=October 9, 2018 |archive-url=https://web.archive.org/web/20181009065351/http://vstudentworld.yolasite.com/resources/final_yr/gynae_obs/Hamilton%20Fairley%20Obstetrics%20and%20Gynaecology%20Lecture%20Notes%202%20Ed.pdf |url-status=dead }}</ref> The age of menarche is influenced by heredity, but a girl's diet and lifestyle contribute as well.<ref name=Tanner/> Regardless of genes, a girl must have a certain proportion of body fat to attain menarche.<ref name=Tanner/> 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).<ref name=Tanner/><ref name=Kaplowitz01/> Girls who experience malnutrition or are in societies in which children are expected to perform physical labor also begin menstruating at later ages.<ref name=Tanner/>

The timing of puberty can have important psychological and social consequences. Early maturing boys are usually taller and stronger than their friends.<ref name=abbassi1998>{{Cite journal|author=Abbassi V |title=Growth and normal puberty |journal=Pediatrics |volume=102 |issue=2 Pt 3 |pages=507–11 |year=1998 |doi=10.1542/peds.102.S3.507 |pmid=9685454 |s2cid=24733669 }}</ref> 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.<ref name="Garn">{{cite journal |last1=Garn |first1=Stanley Marion |title=Physical growth and development |journal=American Journal of Physical Anthropology |date=June 1952 |volume=10 |issue=2 |pages=169–192 |doi=10.1002/ajpa.1330100215 |pmid=14952566 }}</ref> 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.<ref name=Garn/> 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 [[Cognition|cognitive]] and [[Social change|social development]] may lag behind their appearance.<ref name=Garn/> Studies have shown that early maturing boys are more likely to be sexually active and are more likely to participate in risky behaviors.<ref name=Susman>{{cite book |doi=10.1002/0471264385.wei0612 |chapter=Puberty, Sexuality, and Health |title=Handbook of Psychology |year=2003 |last1=Susman |first1=Elizabeth J. |last2=Dorn |first2=Lorah D. |last3=Schiefelbein |first3=Virginia L. |isbn=978-0-471-26438-5 |chapter-url=https://books.google.com/books?id=odIVt4_OGqcC&pg=PA295 |pages=295–324 |editor1-last=Weiner |editor1-first=Irving B. |editor2-last=Freedheim |editor2-first=Donald K. |editor3-last=Lerner |editor3-first=Richard M. |editor4-last=Easterbrooks |editor4-first=M. Ann |editor5-last=Mistry |editor5-first=Jayanthi }}</ref>

For girls, early maturation can sometimes lead to increased self-consciousness, a typical aspect in maturing females.<ref name="Pamf.org">{{Cite news|title=Teenage Growth & Development: 15 to 17 Years|publisher=[[Palo Alto Medical Foundation]]|access-date=February 20, 2009 |url=http://www.pamf.org/teen/parents/health/growth-15-17.html| archive-url= https://web.archive.org/web/20090226174419/http://www.pamf.org/teen/parents/health/growth-15-17.html| archive-date= February 26, 2009 | url-status= live}}</ref> Because of their bodies' developing in advance, pubescent girls can become more insecure and dependent.<ref name="Pamf.org"/> Consequently, girls that reach sexual maturation early are more likely than their peers to develop [[eating disorder]]s (such as [[anorexia nervosa]]). Nearly half of all American high school girls' diets are to lose weight.<ref name="Pamf.org"/> In addition, girls may have to deal with sexual advances from older boys before they are emotionally and mentally mature.<ref>(Peterson, 1987).{{full citation needed|date=October 2022}}</ref> In addition to having earlier sexual experiences and more unwanted pregnancies than late maturing girls, early maturing girls are more exposed to [[Alcoholic beverage|alcohol]] and [[drug abuse]].<ref>Caspi et al.1993{{full citation needed|date=October 2022}}</ref><ref>Lanza and Collins, 2002{{full citation needed|date=October 2022}}</ref> Those who have had such experiences tend to not perform as well in school as their "inexperienced" peers.<ref>{{cite book |last1=Stattin |first1=Håkan |title=Pubertal Maturation in Female Development |last2=Magnusson |first2=David |year=2018 |isbn=978-1-315-78901-9 |page=25 |doi=10.4324/9781315789019 |s2cid=143088283}}</ref>

Girls have usually reached full physical development around ages 15–17,<ref name="MedlinePlus archive 001950 Puberty and adolescence"/><ref name="PAmf.org"/><ref name="PAMf.org">{{Cite news|title=Teenage Growth & Development: 15 to 17 Years|publisher=[[Palo Alto Medical Foundation]]|url=http://www.pamf.org/teen/parents/health/growth-15-17.html}}</ref> while boys usually complete puberty around ages 16–17.<ref name="PAmf.org"/><ref name="PAMf.org"/><ref name="Marshall17677">Marshall (1986), p. 176–7</ref> 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.<ref name="MedlinePlus archive 001950 Puberty and adolescence"/> In contrast, boys develop more slowly but continue to grow for about six years after the first visible pubertal changes.<ref name="Garn"/><ref name="Marshall17677"/>

[[File:Child development stages.svg|thumb|center|800px|Approximate outline of development periods in [[child development|childhood]] and early adulthood development. Adolescence is marked in red at top right.]]

===Growth spurt===
The physical development of girls during their teenage years can be broken down into three distinct stages. At the start, which generally coincides with the beginning of rapid growth, there is the development of breast buds and pubic hair. The peak period of physical growth occurs approximately one year later in concert with stage two of sexual maturity. Approximately 1 to 1.6 years after the onset of secondary sex characteristics, girls enter into the third stage which typically includes menarche. By this time, they will have finished their growth spurt and experience a notable broadening of the hips as well as an adult fat distribution. Additionally, breast development is complete and hair in both the pubic region and armpits (axillary hair) will be darker and more widespread. In comparison to girls, it can be tricky to define when exactly sexual development in boys begins.

For boys, puberty typically takes around 5 years to finish, as opposed to just {{frac|3|1|2}} years for girls (menarche). By this point in time, they have already experienced their growth spurt and there are evident changes in their body shape – wider hips and fat distribution is more adult-like. Breast development will also be completed by this stage. In boys, four stages in development can be correlated with the curve of general body growth at adolescence. The initial sign of sexual maturation in boys usually is the "fat spurt". The maturing boy gains weight and becomes almost chubby, with a somewhat feminine fat distribution. This probably occurs because estrogen production by the Leydig cells in the testes is stimulated before the more abundant Sertoli cells begin to produce significant amounts of testosterone. During this stage, boys may appear obese and somewhat awkward physically. Approximately 1 year after the scrotum begins to increase in size, stage II can be seen. During this time, there is a redistribution of subcutaneous fat and the start of pubic hair growth. Following 8 to 12 months of the peak velocity in height gain, stage III ensues. This period is marked by an obvious widenening of hips with a more adult-like fat distribution and full development of the breasts. All together, these three stages culminate in a complete growth spurt for most individuals. At this time, axillary hair appears and facial hair appears on the upper lip only. A spurt in muscle growth also occurs, along with a continued decrease in subcutaneous fat and an obviously harder and more angular body form. Pubic hair distribution appears more adult but has not yet spread to the medial area of the thighs. The penis and scrotum are near adult size. Stage IV for boys, which occurs anywhere from 15 to 24 months after stage III, is difficult to pinpoint. At this time, the spurt of growth in height ends. There is facial hair on the chin and the upper lip, adult distribution and color of pubic and axillary hair, and a further increase in muscular strength.<ref>{{Cite book |last=Proffit |first=William R. |url=https://www.worldcat.org/oclc/1089435881 |title=Contemporary orthodontics |date=2019 |others=Henry W., Jr. Fields, Brent E. Larson, David M. Sarver |isbn=978-0-323-54387-3 |edition=Sixth |location=Philadelphia, PA |oclc=1089435881}}</ref>

The adolescent growth spurt is a rapid increase in the individual's [[human height|height]] and weight during puberty resulting from the simultaneous release of growth hormones, [[thyroid hormone]]s, and [[androgens]].<ref name="Steinberg, L. 2008">{{Cite book |last=Santrock |first=John W. |title=Adolescence |publisher=McGraw-Hill |year=2013 |isbn=9780078035487 |edition=15th}}</ref>{{Rp|pages=55–56}} 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&nbsp;cm (4&nbsp;inches) per year for males and 9&nbsp;cm (3.5&nbsp;inches) per year for females.<ref name="Susman, E. 2004">{{Cite book |last1=Susman |first1=Elizabeth J. |title=Handbook of Adolescent Psychology |last2=Rogol |first2=Alan |publisher=John Wiley & Sons |year=2004 |editor-last=Lerner |editor-first=Richard M. |edition=2nd |location=Hoboken |pages=20 |chapter=Puberty and Psychological Development |editor-last2=Steinberg |editor-first2=Laurence}}</ref> 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.<ref name="Susman, E. 2004"/> Teenage and early adult males may continue to gain natural muscle growth even after puberty.<ref name=Garn/>

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.<ref name=Tanner1971>{{cite journal |last1=Tanner |first1=J. M. |title=Sequence, Tempo, and Individual Variation in the Growth and Development of Boys and Girls Aged Twelve to Sixteen |journal=Daedalus |date=1971 |volume=100 |issue=4 |pages=907–930 |jstor=20024040 }}</ref> 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, [[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.<ref>{{cite journal |author1=Gilsanz V. |author2=Roe T. |author3=Mora S. |author4=Costin G. |author5=Goodman W. | year = 1991 | title = Changes in vertebral bone density in Black girls and White girls during childhood and puberty | journal = New England Journal of Medicine | volume = 325 | issue = 23| pages = 1597–1600 | doi = 10.1056/NEJM199112053252302 | pmid = 1944449 | doi-access = free }}</ref>

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.<ref>{{cite journal |author1=Smoll F. |author2=Schutz R. | year = 1990 | title = Quantifying gender differences in physical performance: A developmental perspective | journal = Developmental Psychology | volume = 26 | issue = 3| pages = 360–369 | doi = 10.1037/0012-1649.26.3.360 }}</ref>

Pubertal development also affects [[circulatory system|circulatory]] and [[respiratory system|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".<ref>Peterson, A., & Taylor, B. (1980). The biological approach to adolescence: Biological change and psychological adaptation. In J. Adelson (Ed.), ''Handbook of adolescent psychology'', pp.129. New York: Wiley.</ref>

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<ref>{{cite journal | author = Goran M. | year = 1998 | title = Developmental changes in energy expenditure and physical activity in children: Evidence for a decline in physical activity in girls before puberty | journal = Pediatrics | volume = 101 | issue = 5| pages = 887–891 | display-authors = 1 | doi = 10.1542/peds.101.5.887 | pmid = 9565420 | last2 = Gower | first2 = BA | last3 = Nagy | first3 = TR | last4 = Johnson | first4 = RK }}</ref><ref>{{cite journal |author1=Savage M. |author2=Scott L. | year = 1998 | title = Physical activity and rural middle school adolescents | journal = Journal of Youth and Adolescence | volume = 27 | issue = 2| pages = 245–253 | doi = 10.1023/A:1021619930697 |s2cid=142764336 }}</ref> and may receive inadequate nutrition from diets that often lack important nutrients, such as iron.<ref>{{cite journal |author1=Johnson R. |author2=Johnson D. |author3=Wang M. |author4=Smiciklas-Wright H. |author5=Guthrie H. | year = 1994 | title = Characterizing nutrient intakes of adolescents by sociodemographic factors | journal = Journal of Adolescent Health | volume = 15 | issue = 2| pages = 149–154 | doi = 10.1016/1054-139X(94)90542-8 | pmid = 8018688 }}</ref> These environmental influences, in turn, affect female physical development.

===Reproduction-related changes===
Primary sex characteristics are those directly related to the [[sex organ]]s. In males, the first stages of puberty involve growth of the testes and scrotum, followed by growth of the penis.<ref name=Tanner1971/> At the time that the penis develops, the [[seminal vesicle]]s, 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 the first ejaculation occurs as a result of masturbation.<ref name=Tanner1971/> Boys are generally fertile before they have an adult appearance.<ref name="Steinberg, L. 2008"/>{{Rp|page=54}}

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.<ref>{{cite journal |author1 = Dorn L.D. | author2 = Nottelmann E.D. | author3 = Sussman E.J. | author4 = Inoff-Germain G. | author5 = Chrousos G.P. | last6 = Cutler | year = 1999 | title = Variability in hormone concentrations and self-reported menstrual histories in young adolescents: Menarche as an integral part of a developmental process | journal = Journal of Youth and Adolescence | volume = 28 | issue = 3| pages = 283–304 | doi = 10.1023/A:1021680726753 | s2cid = 142626631 }}</ref> Generally, a girl is not fully fertile until several years after menarche, as regular ovulation follows menarche by about two years.<ref>Hafetz, E. (1976). Parameters of sexual maturity in man. In E. Hafetz (Ed.), ''Perspectives in human reproduction'', Vol. 3: ''Sexual maturity: Physiological and clinical parameters.'' Ann Arbor, MI: Ann Arbor Science Publishers.</ref> Unlike males, therefore, females usually appear physically mature before they are capable of becoming pregnant.

Changes in [[secondary sex characteristic]]s 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 [[Areola|areolae]], and elevation of the [[Nipple|nipples.]]<ref name="Steinberg, L. 2008" />{{Rp|pages=57–58}} The changes in secondary sex characteristics that take place during puberty are often referred to in terms of five [[Tanner scale|Tanner stages]],<ref>{{cite web |url=http://healthvermont.gov/family/toolkit/tools%5CJ-1%20CARD%20Tanner%20Stages.pdf |title=Tanner stages |access-date=September 15, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20101205151003/http://healthvermont.gov/family/toolkit/tools/J-1%20CARD%20Tanner%20Stages.pdf |archive-date=December 5, 2010 }}</ref> named after the British pediatrician who devised the categorization system.

===Changes in the brain===
The human brain is not finished developing by the time a person reaches puberty, or even finishes it. The frontal lobe of the brain has been known to shape itself well into one's 30s.<ref>{{cite journal | doi=10.1073/pnas.1105108108 | title=Extraordinary neoteny of synaptic spines in the human prefrontal cortex | journal=Proceedings of the National Academy of Sciences | date=August 9, 2011 | volume=108 | issue=32 | pages=13281–13286 | last1=Petanjek | first1=Zdravko | last2=Judaš | first2=Miloš | last3=Šimić | first3=Goran | last4=Rašin | first4=Mladen Roko | last5=Uylings | first5=Harry B. M. | last6=Rakic | first6=Pasko | last7=Kostović | first7=Ivica | pmid=21788513 | pmc=3156171 | bibcode=2011PNAS..10813281P | doi-access=free }}</ref> Neuroscientists often cannot agree precisely on when this developmental period ends or if there is an exact age for the end of brain development.<ref>{{cite journal | pmid=28009272 | doi=10.1016/j.neuron.2016.10.059 | title=Searching for Signatures of Brain Maturity: What Are We Searching For? | journal=Neuron | date=December 21, 2016 | volume=92 | issue=6 | pages=1164–1167 | last1=Somerville | first1=Leah H. | s2cid=207217254 | doi-access=free }}</ref> Below the age of about roughly 30, the human brain has been implicated in human behavior and [[Immaturity|social immaturity.]] However, there has been no empirical study indicating a causal relationship with the development of the prefrontal cortex in adolescence and into early adulthood with any irrational behaviors.<ref>{{cite journal | pmid=19699416 | pmc=2892678 | doi=10.1016/j.jadohealth.2009.05.016 | title=Adolescent Maturity and the Brain: The Promise and Pitfalls of Neuroscience Research in Adolescent Health Policy | journal=The Journal of Adolescent Health | date=September 2009 | volume=45 | issue=3 | pages=216–221 | last1=Johnson | first1=Sara B. | last2=Blum | first2=Robert W. | last3=Giedd | first3=Jay N. }}</ref> The brain reaches 90% of its adult size by six years of age.<ref name="Casey, B. J. 2008">{{cite journal |author1=Casey B. J. |author2=Getz S. |author3=Galvan A. | year = 2008 | title = The adolescent brain | journal = Developmental Review | volume = 28 | issue = 1| pages = 62–77 | doi = 10.1016/j.dr.2007.08.003 | pmid = 18688292 | pmc = 2500212 }}</ref> Thus, the brain does not grow in size much during adolescence.

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.<ref>{{cite journal|last1=Giedd|first1=Jay, N|title=Structural Magnetic Resonance Imaging of the Adolescent Brain|journal=Annals of the New York Academy of Sciences|year=2004|volume=1021|issue=1|pages=77–85|doi=10.1196/annals.1308.009|pmid=15251877|bibcode=2004NYASA1021...77G|s2cid=20947602}}</ref> 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 [[myelin]]ation (insulation of axons) and the reduction of unused pathways.<ref>{{cite journal |author1=Giedd J. N. |author2=Blumenthal J. |author3=Jeffries N. O. |author4=Castellanos F. X. |author5=Liu H. |author6=Zijdenbos A. |author7=Rapoport J. L. | year = 1999 | title = Brain development during childhood and adolescence: a longitudinal MRI study | journal = Nature Neuroscience | volume = 2 | issue = 10| pages = 861–863 | doi = 10.1038/13158 | pmid = 10491603 |s2cid=204989935 }}</ref>

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 cortex|prefrontal]] cortices, among other regions.<ref>{{cite journal |author1=Gogtay N. |author2=Giedd J. N. |author3=Lusk L. |author4=Hayashi K. M. |author5=Greenstein D. |author6=Vaituzis A. C. |author7=Thompson P. M. | year = 2004 | title = Dynamic mapping of human cortical development during childhood through early adulthood | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 21| pages = 8174–8179 | doi = 10.1073/pnas.0402680101 | pmid = 15148381 | pmc = 419576 | bibcode = 2004PNAS..101.8174G |doi-access=free }}</ref> 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.<ref>{{cite journal |author1=Segalowitz S. J. |author2=Davies P. L. | year = 2004 | title = Charting the maturation of the frontal lobe: An electrophysiological strategy | journal = Brain and Cognition | volume = 55 | issue = 1| pages = 116–133 | doi = 10.1016/S0278-2626(03)00283-5 | pmid = 15134847 |s2cid=16152759 }}</ref> 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 [[neurotransmitter]]s that play important roles in adolescent brain development are [[Glutamic acid|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.<ref>Weinberger, D.R., Elvevåg, B., Giedd, J.N. (2005). The Adolescent Brain: A Work in Progress. ''The National Campaign to Prevent Teen Pregnancy''.</ref> 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.<ref>{{cite journal | author = Spear L. P. | year = 2000 | title = The adolescent brain and age-related behavioral manifestations | url = http://faculty.weber.edu/eamsel/Classes/Child%203000/Adolescent%20Risk%20taking/Lectures/3-4%20Biological/Spear%20LV%20%20(2000).pdf| journal = Neuroscience and Biobehavioral Reviews | volume = 24 | issue = 4| pages = 417–463 | doi = 10.1016/S0149-7634(00)00014-2 | pmid = 10817843 | s2cid = 14686245 }}</ref> 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|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.<ref name="pmid24721318">{{cite journal | doi = 10.1016/j.euroneuro.2014.02.009 |author=Shoval, G. |author2=Bar-Shira O. |author3=Zalsman G. |author4=John J. Mann |author5=Chechik G. | pmid = 24721318 | year = 2014 | title = Transitions in the transcriptome of the serotonergic and dopaminergic systems in the human brain during adolescence | journal = European Neuropsychopharmacology | volume=24 | issue=7 | pages=1123–32|s2cid=14534307 }}</ref>