Around a decade ago, the clinical psychologist Sheryl Gonzalez Ziegler created a course on puberty aimed at 12-year-old girls. “I thought, ‘Let’s get to them right before they become a teenager,’” she says.

It turned out to be a hit - and soon attracted a wider audience than she could possibly have imagined. “Very quickly parents started asking, ‘Can my 11-year-old come? Could my 10-year-old come? And then we eventually got to the point where it was like, ‘My nine-year-old needs to come.’”

Ziegler, who works in Denver, Colorado, recalls: “I wasn’t really prepared for that. And then I started hearing stories about endocrinologists reporting that more and more parents were bringing their kids in with these earlier puberty signs at 8 and 9 for girls and 9 and 10 for boys.”

Having researched the topic for her book The Crucial Years, she now believes that this is part of a global trend.

Puberty is sometimes presented as a cloud on each child’s horizon - and the data suggests that they are now weathering that storm earlier than ever. According to multiple studies, the average age of onset has been steadily decreasing for decades, with some signs of sexual development emerging as early as age 8.

Exactly why children’s development is changing, and the ways in which this may influence their capacity to learn, is highly nuanced - but unpicking those mechanisms may help parents and teachers to better serve children and adolescents in the future, no matter how quickly or slowly they mature.

What signals the onset of puberty?

Scientists studying children’s sexual development use multiple measures to mark the timing of puberty - each of which may tell a slightly different story.

For girls, one of the most reliable indicators is the age of their first period. In the early 19th century this was around 17-18 years; by the 1960s it had dropped to around 13, after which the rate of change appears to have slowed. By the early 2000s, the average lay at 11.9.

The first menstrual cycle comes towards the end of puberty, however, so scientists have turned to earlier markers, such as breast development. Analysing data from 30 global studies from 1977 to 2013, Camilla Eckert-Lind, at the University of Copenhagen in Denmark, and colleagues concluded that the average onset of puberty is occurring around three months earlier each decade. This meant that, by the 2010s, it was hitting the average girl at around nine and a half years.

The data for boys is patchier. “When I looked into this, I found it kind of frustrating that there were so few studies on boys’ puberty and the social consequences,” says Fartein Ask Torvik, a senior researcher at the Norwegian Institute of Public Health.

One of the few available studies comes from a team led by Kaspar Sørensen at the University of Copenhagen. In 2010, he examined two sets of data on testicular growth from 1991-1993 and 2006-2008. The researchers noted a three-month difference in the start of puberty between the two periods, which roughly aligns with the changes found among girls over the same time span.

However, such statistics can mask huge individual variation within the population. According to the NHS, “The average age for girls to start puberty is 11, while for boys the average age is 12. But it’s perfectly normal for puberty to begin at any point between the ages of 8 and 13 in girls and 9 and 14 in boys.”

The trend of children starting puberty earlier could be driven by many different factors. Some scientists suggest that unhealthy eating and reduced physical activity lie behind it. Fatty tissue releases leptin, a hormone that signals the presence of sufficient energy reserves to begin puberty, triggering the hypothalamus and pituitary gland to set off the complex physiological process of sexual development.

Compounding this, an unhealthy diet may reduce sensitivity to insulin, which could also lead the ovaries and adrenal glands to produce sex hormones. While longitudinal studies show a consistent link between obesity and early puberty onset in girls, the evidence for boys is far less clear cut.

So-called “endocrine-disrupting chemicals” in our environment may offer another explanation. The pesticides on our foods and the plastics in toys, food packaging and cosmetics can contain compounds such as bisphenol A (BPA), phthalates and PFAS that mimic the effects of human hormones, which could throw the body’s developmental processes off balance.

Another possible cause is stress, with some studies suggesting that childhood trauma may result in a cascade of physiological changes that kickstart puberty.

“It would be naive to propose that any one factor solely changes the timing of puberty,” conclude the authors of one academic review of the scientific evidence. “It is much more likely that many factors have small individual effects acting in concert with one another and that the combination of influences in each individual is what determines pubertal timing.”

Social consequences

How might the effects of this accelerated development be playing out in schools?

Puberty arrives with a host of neurological changes. “Some of the key areas of the brain that develop during adolescence are regions related to social cognition, emotional development and executive functioning,” explains Anne-Lise Goddings, an honorary consultant in paediatrics at University College London Hospitals NHS Foundation Trust and honorary senior clinical lecturer in the Department of Brain Sciences at Imperial College London.

The first two areas - related to social cognition and emotional development - are most clearly linked to puberty, Goddings explains, and the consequences can seem contradictory. Young people may seem more advanced in some ways (being better able to understand complex relationships in literature, for example) and less advanced in others (being very easily driven by peer pressure).

Exactly how the timing of puberty affects a child’s academic performance is still a matter of debate, with few studies providing a dispassionate look at the data. “It is an amazing research question that we should address in the future, but we don’t have the answer yet,” Torvik says.

‘In girls, early puberty is associated with increased risks of mental health symptoms’

The evidence so far suggests that the picture is nuanced. Torvik, for instance, has examined longitudinal data from the Twins Early Development Study, which recruited nearly 28,000 twins born between 1994 and 1996 in England and Wales. Many participants dropped out over the following years - a common enough phenomenon in any longitudinal study. At age 16, however, 13,477 of these individuals provided information about their GCSE grades.

Torvik and his colleagues used various measures of puberty timing, including information on menstruation and self-report questionnaires. Crucially, they also examined changes to their height, since a sudden growth spurt is a good indication that puberty has started in either sex. “It’s the measure that we believe the most,” Torvik says.

The results were surprising. While an early first menstrual cycle was associated with poorer academic results, analyses using the other indicators suggested that accelerated physical development was beneficial to academic attainment. “Early puberty was associated with better school performance and it was more so for boys than for girls,” says Torvik.

He suspects that this reflects the complexities of puberty, which involves many different processes occurring together. “An early first period could perhaps indicate a specific [form of] maturity,” he says. “But by including multiple puberty indicators, we may have a better picture of the overall maturity.”

To understand why earlier puberty may sometimes be an advantage, we need to consider the ways that an adolescent’s personality changes over puberty.

Alithe van den Akker, of the University of Amsterdam, and colleagues, for instance, have examined data from the Texas Twin Project, which includes measures of hormones like DHEA, testosterone and progesterone, as well as participants’ self-reported perceptions of whether they had started to grow body hair or develop a deeper voice. Crucially, the project also included psychological questionnaires on various personality traits.

As you might expect, many adolescents showed a reduction in “agreeableness” as puberty kicked in. More importantly, however, the participants also experienced a dip in conscientiousness after the onset of puberty, followed by a rise again as they reached full maturity.

This is significant since conscientiousness is associated with organisation and self-discipline - both of which are essential for academic success. Early developers, Torvik speculates, have already passed through the trough and have come out the other side before they take their exams. “If you want to do well in school, you would like that drop [in conscientiousness] to have finished, and to see the increase again before you get your grades,” he says.

Impact on mental health

As intriguing as this hypothesis may be, Torvik is cautious about making any predictions about the broader trends in puberty onset and the academic consequences. Without more data, we can’t assess the full impact on children’s overall development, which is likely to be complex.

Goddings is similarly cautious. “Where puberty has been associated with development of social cognition and emotional development, these enhanced skills may mean that young people can get more engaged with education and find subjects and teachers that inspire them,” she says.

Rather than demonstrating an advantage to early puberty, however, she wonders whether Torvik’s study is instead showing the disadvantages of late puberty. “Pubertal timing and tempo are interlinked with many aspects of health and wellbeing,” she says. “It may be that these young people who have not yet reached the end of puberty by 16 have had other health or psychological challenges that have delayed or slowed their pubertal development.”

We should also be conscious of the effects of puberty on mental health. “In girls, there is relatively strong data that early puberty is associated with increased risks of mental health symptoms including anxiety, depression and self-harm,” says Goddings. “For boys the data are more mixed, with there appearing to be associations with poorer mental health with going through puberty particularly early or particularly late compared to same-aged peers.”

The consequences of early puberty could depend on the child’s environment and the amount of social support they receive. One study of 310 African American boys, for instance, found that earlier puberty was associated with delinquent behaviours, such as bullying and assault, but also with healthy social competence, such as the capacity to work well in a group. Which side came to the forefront depended on things like the behaviour of the boys’ peers and their parents’ disciplinary style. Early sexual development, therefore, may simply heighten susceptibility to existing risk factors.

‘The earlier developing you are, the more society treats you as older. People expect more from you’

However, in many cases children may just be responding to adults’ expectations of their behaviour, which can be biased by the signs of physical development. A child who goes through puberty early may be treated as if they are older than they are, without the experience to navigate those interactions.

“The earlier developing you are, the more society treats you as older,” says Ziegler. “People expect more from you, including your parents. They might expect an earlier developing child to watch their younger siblings or help around the house more.” This could be a source of considerable stress, particularly if the child hasn’t developed the emotional skills to deal with greater responsibility.

Alternatively, parents or teachers might see precocious development as a precursor to behavioural problems. A few years ago, a team of psychologists led by Rona Carter at the University Michigan showed elementary school teachers drawings of 10-year-old girls at various stages of pubertal development, and then asked them to predict each girl’s academic and social functioning, using items from the US Office of Social Security Administration Teacher Questionnaire, which is often used to assess children’s abilities and behaviour. They were asked to predict how likely the children would be to understand complex content, learn new material, pay attention when spoken to, make and maintain friendships or express their anger appropriately.

Carter and her colleagues found that the teachers were far more likely to expect the early developers to have both academic and behavioural problems. Those beliefs could easily become self-fulfilling prophecies, the researchers suggest. Various studies, after all, have shown that teachers’ expectancies can shape how they treat their pupils - making them more likely to give negative feedback, for instance, or less likely to provide them with challenging material - which can, in turn, influence their chances of success.

Raising awareness

Without much stronger evidence for the educational consequences, it is hard to make any clear recommendations for the ways schools might adapt to the earlier onset of puberty, though Ziegler has some ideas.

Puberty is known to interfere with our sleep cycles, for instance, so that adolescents stay awake later in the evening and wake later in the morning. This has led to campaigns for secondary schools to begin lessons later in the day, when adolescents are more alert and receptive. If more and more children begin puberty before the age of 11, then primary schools might need to make a similar move.

For now, Ziegler would simply like to see much more awareness of the trend. Many parents and teachers, she says, complain that eight- or nine-year-olds are acting like teenagers without realising that they are already going through some of the biological changes of puberty. Recognising that fact may make adults more understanding of the mood swings, lethargy and increased self-consciousness.

Ziegler has already found a lot of interest in her work from educators in the US. “I have been really getting a lot of requests from schools to come in and do their professional development days,” she says.

“Teachers are saying that they are seeing [early puberty] a lot but don’t know what to do with it. And it’s been very rewarding to get that type of feedback.”

Teachers might be acknowledging that there has been a shift. But for schools to get the clear answers they need on how to respond, it seems we need to wait for further research.

David Robson is the author of The Laws of Connection: 13 social strategies that will transform your life (Canongate)

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