The student writes about menarche and the charts above,
I studied the onset of puberty in general, and focused in part on menarche.
In short, I found that the secular trend (here, secular means existing
or continuing through ages or centuries - in other words, how the ages
at first menstruation has changed through the centuries) was not, in fact,
a modern drop in age of onset, but rather due to a 19th century rise in
onset, probably due to nutritional factors. The trend was publicized by
Tanner and colleagues. Many of the charts I've included come from his research.
Part of the problem with Tanner's data is that he based the early estimates
(i.e. the age of onset in the 1860s) on small studies done on children
in less-than-ideal conditions - orphans, rural laborers, and the like.
If you look at the chart giving ages in various parts of the world, you'll
see that in New Guinea, the average age is much higher than elsewhere,
probably due to poor nutrition. You'll also see that everywhere else, the
average age is about the same, and that the populations the data is based
on are "well-off" or "middle class," etc.
Bibliography for the presentation, below, and charts, above:
Bullough, Vern. "Menarche and Teenage Pregnancy: A Misuse of Historical
Data." In Menarche, Sharon Golub, ed. Lexington: DC Heath and
Company, 1983. pp. 187-193.
Cray, Don, et al. "Teens Before Their Time." Time.
Oct. 30, 2000. p.66+
Ellis, Bruce J. and Judy Garber. "Psychosocial Antecedents of Variation
in Girls' Pubertal Timing: Maternal Depression, Stepfather Presence, and
Marital and Family Stress." Child Development. March-April
2000, p. 485(17).
Herman-Giddens, Marica, et al. "Secondary sexual characteristics
and menses in young girls seen in office practice." Pediatrics.
April 1997, p. 505(8).
Tanner, J.M. Foetus Into Man: Physical Growth from Conception to
Maturity. Cambridge: Harvard University Press, 1978.
Tanner, J.M. Growth at adolescence, with a general consideration
of the effects of hereditary and environmental factors upon growth and
maturation from birth to maturity. Oxford: Blackwell Scientific Publications,
For several years, researchers have been noticing a possible decline
in the age of girls at the onset of puberty. Is this really happening,
and if so, why, and what does it mean?
What causes the onset of puberty?
Androgens released during adrenarche may cause the secretion of pubertal
hormones (i.e. estrogen).
The rise in estrogen causes thelarche (a.k.a. breast development) -
the visible indicator or estrogen secretion.
Other possible indicators of estrogen secretion include:
Body fat distribution
Vaginal cell cornification
Cervical mucous secretion
Proliferative endometrium present on biopsy
Plasma estradiol measurement
Other correlates with he start of puberty:
Body water content
Critical lean body weight
Fat cells produce leptin
Age at onset of puberty and rate of puberty are primarily controlled
Precocious puberty: can be pathological in nature (caused by tumors
in central nervous system, encephalitis, or head trauma) or idiosyncratic.
Precocious puberty has been recorded since classical times. Girls who
go through precocious puberty usually reach menopause at a normal age.
In North America, girls are considered precocious if development begins
before age 8, though this data may have to be re-evaluated.
The Secular Trend
Tanner first described the secular trend in 1962. ["Secular"
means occurring through the centuries.]
According to Tanner, the average age of menarche dropped from about
17 to 12.8 during the period 1830-1962. The rate of decline was 4 months
Tanner has also noticed a decline in the age of initiation of the growth
spurt. The trend seems to have stopped, with the age of menarche leveling
off at 12.6.
Causes of the trend:
The most widely held belief is that the trend has occurred due to improved
nutrition. Children today are bigger and heavier than in the past. Improved
nutrition allows for normal growth. Lower classes and rural children have
also seen a drop in the age of onset of puberty.
Other causes may include:
Generally improved environmental circumstances
Genetic isolates - a.k.a. natural selection
Gradual change in world temperature
Drop in incidence of disease
Obesity (onset of menarche has a correlation with the body fat percentage)
Marcia Herman-Giddens and colleagues reported in 1997 that secondary
sex characteristics are appearing earlier than is currently documented.
The study was cross-sectional, using 255 doctors in 65 different practices.
Their findings include: Age at menarche has not dropped in the last 45
years, but age at the onset of secondary sex characteristics has.
African-American girls develop earlier than white girls by 1-1.5 years.
The mean age at onset of breast development is 8.87 years for African-American
girls and 10.51 years for white girls. The mean age at onset of axillary
(armpit) hair growth is 10.8 for A.A. girls and 11.8 for white girls. African-American
girls are taller and heavier than white girls of the same age.
Possible causes include:
Chemicals such as DDE, PCBs, Bisphenol A, and phthalates, which mimic
Hormones in meat and dairy products
Fat cells (obesity)
MTV [N.B. someone suggested that watching sexualized images on the media
caused children to develop earlier. It's pretty far-fetched]
Stress at home - broken homes, abuse
Problems with the Documentation of the Secular Trend
Tanner's data is suspect because he used a small study group in establishing
the original age of 17, and then used them to compare with groups elsewhere.
Tanner's establishment of the age range for normal development was based
on a group of 192 lower-class girls in a children's home, who may have
had low-quality care prior to the study.
History shows us another trend:
Historical Data on Age at Menarche
Ancient Rome 12-14
Medieval Europe 12-14
Medieval Middle East 12-13
working class women 15.7
upper class women 14.6
London 1855 (hospital patients) 15.5
Germany 1869 15.7
Scotland 1870 15.6-16.6
London 1880 (middle class) 15
U.S.A. late 19th century 12-14
Early 20th Century
USA 1905 14-15.7
So, we can see that there does appear to be a trend, but it is not as
great as Tanner suggested. Research shows that menarche has dropped from
14-15 years to 12-13 [N.B. this chart was one I made myself, copying the
data from one of the books I used. I'll need to get a proper citation for
If we look at another chart (Table 3), we can see that in modern populations,
people who are poorly nourished (New Guinea) start menstruating later.
The first chart shows a similar trend, with working class women developing
later than their upper-class counterparts.
It appears that the case is not that girls are currently developing
unusually early, but that 19th-century girls were unusually late, though
within normal parameters.
Herman-Giddens' data is also flawed:
Tanner stages of development are established by eye (breast tissue and
fat tissue are easily confused by eye, though they can be distinguished
Too many different doctors participated in the study
Only 9.6% of the subjects were African-American
Non-random selection - the participants were all clients in suburban
The subjects were all between the ages of 3 and 12. No older girls were
There is very little prior data concerning the onset of secondary sex
No data on other racial groups were included in the study.
Conclusion - what does the secular trend mean?
In every primate population where an artificial food source has been
introduced, the results have been:
Increase in body weight
Fall in age at menarche
Fall in the interbirth interval
Decline in infant mortality
In the majority of primates and domestic animals, sexual maturity is
achieved before epiphyseal closure (bones) and completion of adult dentition.
There appears to be a relative delay in dental emergence in mammals
that can be related to increasing body size and domestication.
Since these same patterns are being seen in humans, we can say that
humans have been domesticated.
[End of the student's paper.]