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, 1962.
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
Bone maturation
Vaginal cell cornification
Cervical mucous secretion
Proliferative endometrium
present on biopsy
Plasma estradiol measurement
Other correlates with he
start of puberty:
Skeletal age
Body water content
Critical lean body weight
Fat cells produce leptin
Age at onset of puberty and
rate of puberty are primarily
controlled by genetics.
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 per decade.
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
Smaller families
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)
Sedentary lifestyle
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:
Better nutrition
Chemicals such as DDE, PCBs,
Bisphenol A, and phthalates,
which mimic sex hormones.
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
Observation bias
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
Early data
Ancient Rome 12-14
Medieval Europe 12-14
Medieval Middle East 12-13
Nineteenth Century
Manchester 1840s
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 the data].
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 by
touch).
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 clinics
The subjects were all between
the ages of 3 and 12. No older
girls were included.
There is very little prior
data concerning the onset of
secondary sex characteristics.
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.]