Researchers struggle to define the span of time between child and adult .

 On the days when Roy Laver was summoned to be measured, he knew to  expect two things. “You were going to be cold,” he says. “And you were  going to be starkers — in the nude — with lots of adult people looking  at you.”Every few months from 1949 to 1971, dozens of youths,  Laver included, were marched to a specially outfitted (but poorly  heated) wing of the Harpenden branch of the National Children’s Home,  near London, UK. The home was a charity-run institution that took in  neglected youngsters. Laver, now 80, put up with periodic testing there  as part of a ground-breaking study to define adolescence. Each  battery of tests took about 3 hours. There were calipers to assess body  fat, photographs and X-rays to record growth, and a physical and dental  exam. The boys and girls were pleased to miss an afternoon of school,  says Laver, but they came to dread the calipers’ pinch on their scrawny  frames. “If you didn’t have a lot of fat, it bloody hurt,” he says. “It  took six or seven tries to find some.”The data from that study  are still used today to track a child’s progression through growth  spurts and puberty towards sexual maturity — a journey that the study’s  lead investigator, paedia-trician James Tanner, used to define a firm,  physical beginning and end for adolescence. But few researchers  feel that same certainty today. Adolescence is nestled between two  transitions — the end of childhood and the beginning of adulthood — with  malleable borders. Epidemiological data suggest a trend towards earlier  puberty in some countries, including the United States and China; at  the other end of the spectrum, societal changes and a growing awareness  of the timeline of brain development are pushing the accepted threshold  for adulthood well into the twenties.  

Adolescence: A collection

Scientists, clinicians and policy-makers now find themselves  wrestling with these shifting frontiers and asking anew: when does  childhood end, when does adulthood begin, and what exactly happens in  the middle? Without a good definition of what adolescence is,  it’s difficult to nail down what protections and responsibilities  adolescents should have. The debate is taking on more urgency as  researchers have realized that adolescence helps to set the trajectory  for adult life, and as judges and doctors try to fix boundaries on when a  person is competent to make adult decisions. “Clarity regarding our  conceptualization of adolescence is not just semantic nit-picking,” says  Jay Giedd, a neuroscientist at the University of California, San Diego.  “It has profound implications for clinical, educational and judicial  systems.” The lack of a clear definition could also be choking  off funding for research on adolescence, which tends to take a back seat  to research on early childhood. “One of the reasons adolescence has  suffered in terms of financial support and has generally been put at the  back of the queue is because the definition is quite difficult,” says  Anne-Lise Goddings, a developmental neuroscientist at University College  London. “It’s hard to give money to something when you can’t say what  it is.”Turning pointsGenerations of researchers have painted adolescence as a span of unremitting hazards on the road to adulthood.  In 1904, US psychologist G. Stanley Hall wrote an influential  two-volume opus on adolescence, which he concluded was between the ages  of 14 and 24. Hall, who focused his analysis largely on white boys,  promoted the idea that adolescence is a time of upheaval. He blamed the  mass media — in the form of cheap fictional pamphlets called ‘penny  dreadfuls’ — as well as ‘immoral’ activities such as drinking and  dancing for leading youths astray. That narrative of fear still  prevails, says Nancy Lesko, a sociologist of education at Columbia  University in New York City. Lesko worries that the widespread  perception of adolescents as irrational risk-takers could bias everyone  from policy-makers to researchers. “My guess is that there are lots of  changes going on in adolescence that we’re missing because adults,  including researchers, are still clinging to these dominant narratives,”  she says. Half a century after Hall, Tanner embarked on his  Harpenden growth study. It began as an effort to track the effects of  improved, post-war nutrition on child development, but would become the  most influential quantitative picture of adolescence. Tanner’s team  meticulously detailed changes in height, weight, fat, breast and  testicle sizes, the thickness of pubic hair, and a host of other  physiological characteristics. Tanner broke down his data into a  sequence that has been simplified into the ‘Tanner stages’. Stage I  describes the body before the onset of puberty. In Tanner’s study, this  occurred at about 11 years old, on average, in girls, with boys  following six months later. During Stages II, III and IV, the breasts  and testes enlarge. Menarche, a woman’s first menstruation, is a  relatively late event and typically begins at Stage IV. Stage V marks  puberty’s completion: for the adolescents in the children’s home, this  happened at about age 15. The value of the study was in defining the  progression of events associated with adolescence; Tanner himself noted  that the ages at which they occurred varied too much to be of wide use.There  are questions about how relevant Tanner’s data are to today’s children,  says Celia Roberts, a sociologist at Lancaster University, UK. Some of  the participants had arrived at the children’s home after enduring years  of neglect. During Laver’s time at the home, nutritious food was in  short supply: most meals leant heavily on potatoes, whereas meat, in the  form of Spam, was served about twice a week. Laver took to scraping the  cooking pots to scrounge a few extra bites, earning him the nickname  ‘fatty’ — although he says he would have been considered slender by  today’s standards. Despite its shortcomings, Tanner’s catalogue  of puberty has become entrenched. Researchers have suggested other  scales, some based on different definitions of maturity, others based on  hormone levels or voice changes. None has had Tanner’s staying power,  says Frank Biro, a paediatrician specializing in adolescent medicine at  Cincinnati Children’s Hospital in Ohio. Even if the growth curves are a  little off, the stages provide a useful point of reference, he says.  Biro keeps a signed copy of one of Tanner’s books on his shelf and still  uses the scale in his research. 

 In today’s adolescents, Tanner’s stages may be shifting because the  onset of puberty — Stage II — is starting earlier in many populations.  Tanner already saw signs of this in his studies. Although food was  scarce at the children’s home, the residents there were better fed than  many children living a century before. He thought that this improved  nutrition could account for a reduction in the average age at menarche  since the mid-nineteenth century1. That trend towards earlier puberty has continued, and is most notable in girls. A study2  of Danish children found that the age of onset of breast development  dropped by a full year between 1991 and 2006, to just younger than 10  years. In China, between 1985 and 2010, the age at which a girl gets her  first period also dropped by a year3.  Biro’s studies have shown differences between ethnic groups: since the  late 1990s, the age at which breast development begins has decreased  more among white girls in the United States than among African American  girls, who were already developing breasts at a younger age. The  evidence strongly suggests that in many parts of the world, this change  is at least partly due to the rise in overweight and obese children,  Biro says. Several mechanisms might link obesity to the time of onset of  puberty. One is that excess body fat increases oestrogen production,  which in turn stimulates the growth spurt and breast development  associated with puberty. (For boys, the trends are more complex: one  study4  of US boys found that overweight white and African American boys enter  puberty earlier than boys who are underweight, but obese boys tend to  enter puberty a little later.)Researchers are trying to decipher  what impact this earlier entry into puberty has on the normal  progression of adolescence. “You have some of these girls who at age  seven and eight are clearly into puberty,” says Biro. “But are they  adolescents yet?”In some ways, they might be. Studies have found  that earlier-maturing girls exhibit risk-taking behaviours that are  typically associated with adolescence5.  And in a survey of more than 700 magnetic resonance imaging scans from  275 people, Goddings and her colleagues found some changes in the brain  that correlated with the Tanner stage of their subjects. For example,  the growth of the amygdala, a brain region involved in processing  emotions, was influenced by chronological age and by the onset of  puberty6. Goddings  urges caution when interpreting the results: she does not yet know  whether the growth patterns trigger behavioural changes common to  adolescence or whether they are a response to such changes. A 9-year-old  girl, for example, whose breasts have begun to develop might be treated  differently by the people around her. This shift could prompt her to  behave differently from a prepubescent girl of the same age, with  corresponding changes in brain activation. “If a 9-year-old goes through  puberty that probably does change something about the brain,” says  Goddings. “And yet in many ways they are still a 9-year-old.” On the thresholdThe  definition of the end of adolescence — the emergence into adulthood —  is even more nebulous. Although Tanner equated this with the end of  puberty, many today use a different definition of adolescence that  extends well beyond that point. This could have implications for  everything from determining when a criminal can be tried as an adult to  when a youth becomes responsible enough to make their own medical  decisions. There is no physical measurement that can capture the  beginning of adulthood, say researchers. Often they set the end of  adolescence on the basis of social roles. “We don’t have an equivalent  physical definition of the end of adolescence,” says John Coleman, a  psychologist at the University of Oxford, UK. “It’s lacking a clear  definition because it combines social and physical developmental  factors.”Those social roles vary widely by culture and era,  opening up the definition to a range of interpretations. Tanner tethered  the end of puberty to the onset of adulthood, which coincided with  social forces in post-war Britain: Laver, for example, left the  children’s home at 16 to take a job as a clerk in a police station while  living with relatives.But in many societies today, the  conventional markers of adulthood are slipping to later in life. Young  people spend more years at school, live with their parents for longer,  and delay marriage and parenthood. Marriage, in particular, has  historically been a key marker for adulthood in many cultures, says  anthropologist Alice Schlegel at the University of Arizona in Tucson.  The average age of women at first marriage has risen by two years  globally over the past two decades, according to the United Nations. In  some countries, that increase is more dramatic: in Brazil the average  age has increased by 6 years to 27, and in several European countries  the age is creeping over 30 (see ‘Age at marriage’). 

 

And whereas Tanner could  only speculate about changes in the brains of his young subjects,  studies by today’s neuroscientists have bolstered the idea that  adolescence does not stop in the teenage years. The prefrontal cortex,  considered the seat of executive functioning and responsible for the  ability to plan ahead and to resist impulses, typically does not come  fully online until the early twenties — or later7.  “The brain doesn’t suddenly become adult at age 18,” says Sarah-Jayne  Blakemore, a neuroscientist at University College London.But if  not then, when? The hotchpotch of definitions in research articles,  social policies and laws around the globe reveals a wide range of  opinion about the end of adolescence (see ‘Sliding scales’). The World  Health Organization set its boundaries at ages 10 and 19, but Susan  Sawyer, chair of adolescent health at the University of Melbourne in  Australia, and her colleagues have argued8  that this upper boundary should be raised to 24. In 2017, New Zealand  revised its regulations regarding children in protective care: rather  than sending them out on their own at the age of 18, the government  continues to provide support into their twenties. The change came in  response to reports that the adolescents were not coping well with  independence at younger ages.As helpful as it would be to have a  fixed, biological end point to adolescence, neuroscientists are unlikely  to derive one, says Blakemore. Ultimately, the end point of adolescence  is a social construct, she says, with large differences between  cultures. And there is so much variation in brain function and structure  from person to person that it may not be possible to nail down a  suitable biological end point. “There is no such thing as an average  adolescent,” Blakemore says.That can confound research by making  it difficult to compare and interpret results across studies. And it  complicates social policies by yielding a patchwork of inconsistent  guidelines. But Beatriz Luna, who studies neurocognitive development at  the University of Pittsburgh in Pennsylvania, sees the differing  definitions as a reminder of the complicated and gradual changes that  occur in the brain as it specializes for adult modes of action — a  process that continues into adulthood. “I don’t believe the brain has an  abrupt change that will determine the end of adolescence,” she says.At  the National Children’s Home in the 1950s, however, there was no  discussion of when adulthood began, says Laver. At 16, Laver simply left  to take a job, later attending a technical college before embarking on a  career in the Royal Air Force. During Tanner’s study, Laver says he was  told that no one would see the pictures that had been taken of him,  starkers and shivering, at the National Children’s Home. But twice, some  of the older students stumbled across them in a nursing magazine. Black  bars obscured much of the faces in the photos, but Laver easily  recognized some of his classmates.Many physicians are unaware of  the full history of the Tanner scale, says Roberts, who has written  about the growth study. “When clinicians have heard me talk, they are  quite shocked,” she says. “We just wouldn’t do that to kids anymore.” But  Laver looks back on his days in Harpenden with affection, and with  respect for the charity’s efforts. The standards of child care were  different back then, he says. And participation in the study was  voluntary — sort of. “We were told to put your hand up if you would like  to have half a day off school,” Laver says. “And up went young Roy’s  hand.”  

Nature 554, 429-431 (2018) 

doi: 10.1038/d41586-018-02169-w

READ MORE:https://www.nature.com/articles/d41586-018-02169-w