Puberty is happening earlier now. Girls in the West, particularly those living in poverty, get breast buds a year or two earlier in the 21st Century than they did in the mid-20th Century, and on average the age of menarche—a girl’s first period—has fallen by about 3 ½ months per decade. Boys’ development is accelerating too. We see this trend tracking childhood obesity rates and other endocrine disruptors, and there has been considerable handwringing in the media about this divergence from nature.
The worry is justified in the sense that the sorts of things that currently cause it are worrying, as I wrote recently in Nature. Childhood obesity is worrying. Endocrine disruptors in the environment are worrying. Sexual abuse, which also leads to earlier menarche, is awful and must be fought. But as for the age of puberty itself, the story of the last half-century or so—that our twisted lifestyles warp our children into adults before their time—is only part of a much larger and more interesting story.
Throughout human history, puberty has hopped back and forth along the timeline of our lifespans. Boy’s voices tend to break around age 11 now, whereas in 1960 that age range centered around the 13 year mark. But keep scrolling back and you’ll find choirmaster’s records show chorister’s voices breaking at age 18 in the mid-1700s. Girls get their periods on average when they are 12 now, whereas in 1920, the average age in the United States and France was 14. Again, scroll farther back and you get up to age 16 in Northern Europe during the 1800s.
Plastics, fire retardants and childhood obesity were not really a thing between the mid-18th and mid-20th centuries, so what happened? During that stretch of time, lots of good things happened that brought down the age of puberty. Wherever medicine, sanitation and food improved, we saw a corresponding acceleration of development.
Urban sites in medieval England show that out of 994 adolescent skeletons aged 10-25 years, medieval girls tended to mark menarche around 15 years old unless they lived in London, in which case the age was 17 years.
What’s really interesting, though, is that this getting of periods when you’re sixteen is not the archetypal human state of nature either. It was kind of a blip that happened at the beginning of modern times—a retardation that followed a deterioration of living conditions even as the march of human progress achieved lots of impressive milestones.
Pediatric endocrinologist Anastasios Papadimitriou has done a fascinating review of the evolution of the age at menarche from prehistoric to modern times. Reports include Aristotle’s, which noted that ejaculation in boys and menarche in girls both occurred at 14. Then again, he also believed that men had more teeth than women. Sanskrit texts from 500BC to 500AD consistently report menarche to happen at age 12. Galen (2nd Century AD) wrote that menstruation began “with the completion of the 14th year”.
Tang-dynasty Chinese medicine records menarche at 14 years and puberty for men at 16 years, but this is confounded by the numerology that makes the number 7 a dominant force for women and 8 for men.
Once you get into prehistory, the real head-scratching begins. Peter Gluckman and Mark Hanson of New Zealand’s National Research Centre for Growth and Development tried to arrive at an estimate based on life expectancy, expected population growth rates and psychosocial development, as well as comparative biology. Chimpanzee development closely parallels humans with a 3-4 year delay for the extended childhood growth period in humans, and chimps progress through puberty from 6 to 9 years of age.
Gluckman and Hanson estimate that Paleolithic and Neolithic women reached menarche around age 10, with full reproductive maturity (when they are most likely to start bringing babies to term) around age 11-13. Only with settled agricultural societies and the poor health that came with them did this start to delay to the classical and medieval menarche ages.
If prehistoric women, then, were slightly more precocious than our current crop of artificially advanced girls, age of puberty must only be meaningful within the context of what causes it. Though long-term estrogen exposure is a risk factor in a bunch of things, the age of puberty is mostly important as a reminder to ask why. Our kids are growing up too quickly now, but bigger problems can be found by looking for the causes.