The tendency for girls to mature earlier (and boys too) is known as "secular
trend" in growth and development research.  Secular trend is readily
observable in many growth variables -- wherever DIETS have improved so that
nutrition is no longer limiting on human growth, and wherever HEALTH has
improved by any means (immunizations, antibiotics, insect-vector
eradication, draining of swamps, improved sanitation, clean water supplies,
sewage treatment, etc. etc. etc., even better emotional health, such as the
end of a civil war).  When inadequate diet, disease, and emotional stress
are removed as limiting factors on human growth, children are able to reach
their genetic potential for growth.  This manifests itself as:

a positive secular trend for height at each age -- the 10 year olds of today
are taller than the 10 year olds of their parent's generation

a positive secular trend for height at adulthood -- the adults of today are
taller than the adults of their parent's generation, who in turn are taller
than the adults of the grandparental generation, if the improvement in
diet/disease status has been going on for some time

a negative secular trend for age at menarche (first menstruation) -- the
girls of today go through puberty and become fertile at an earlier age than
their mothers did, who in turn did so earlier than their grandmothers did

Remember that evolution through natural selection operates to maximize the
number of offspring.  Human females have very limited reproductive capacity
compared to any other animal -- 9 months for gestation, at least 3 years of
lactation for a good chance of surviving (prior to formula and modern
medicine), and only about 30 years (15-45 years of age) for reproducing in.
Thus, you were lucky if you were able to have 6-8 children.

Anything that can increase the potential for childbearing, such as earlier
menarche, shorter birth spacing without sacrificing health of the child,
etc., will be strongly selected for.  The human body "monitors" itself, and
gears up for pregnancy and lactation at the earliest possible point that the
body can handle it in terms of maturity and fat stores.

In places where girls are malnourished and have to cope with lots of
diseases, they may not go through puberty and begin to menstruate and become
fertile until they are 18 or 19 or 20 years old or even older, as their
bodies are not yet capable of carrying a pregnancy to term and breastfeeding
for several years.  Where conditions are much better, girls may go through
puberty as early as 9 or 10 years of age, as their bodies are already
capable of sustaining pregnancy and lactation, even if their brains/emotions
are not.  In the United States, we seem to have hit the lower minimum of age
at menarche, with an average of 12.6 years, holding steady for about 25
years now.  Some girls start much earlier, some much later, but an AVERAGE
of 12.6 years seems to be about the lower biological limit of genetic
potential for beginning our reproductive careers.

Similarly, an AVERAGE height of about 5'9" for males and 5'6" for females
seems to be the upper limit for adult height in terms of human genetic
potential.  Remember, I'm talking here about population averages.
Everywhere around the world, wherever people who have been traditionally
thought to be "genetically short" have had access to better diets in
childhood and better health in childhood, their heights approach this
average.  This is most clearly seen in the US in populations from southeast
Asia, such as the Vietnamese, and from Latin America, such as the Mayans.
When children of these populations are born in the US to conditions of good
nutrition and health, they grow very much like American children, and very
unlike children in their home communities.  Thus, the shortness of
Vietnamese and Mayan people in their homelands is due to inadequate diet,
lots of diseases, and emotional turmoil.  You can see this easily if you go
sit at a big mall and watch several generations of families who are out
shopping.  In Texas, it is most clearly seen in the Hispanic families
recently immigrated from Mexico -- the grandmother is 4'10", the mother is
5'2" and the 12 year old daughter is 5'5".  We call these "stairstep
generations."

It often takes several generations for the maximum genetic potential to be
reached, because the size of the mother affects the size of the baby.  After
several generations of living under good conditions, populations around the
world have the same average height as Americans, 5'9" for males and 5'6" for
females, give or take a 1/2".  Asian populations will often still be up to
1/2" shorter, and African populations up to 1/2" taller than the US
averages.  And remember that there will of course be LOTS of individual
variation due to different genetic potential within each family and
population.

The only known population-level exception to the foregoing are the so-called
"Pygmies" of the central African rainforests such as the Mbuti and the Baka,
who seem to have a genetic variation that impedes the production of
insulin-like growth factor number one, resulting in them being several
inches shorter than US averages even under the best of environmental conditions.

End of lecture for today.

Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology and Nutrition
Texas A&M University