I seem to remember that although an improvement in diet and general health
has been the most important factor in decreasing the average age of
menarche, exposure to daylight length was also a factor.
Girls in the tropics (where day-length vs night-length does not vary a great
deal between winter and summer) entered menarche earlier than girls in the
latitudes further away from the equator. The further away from the equator,
the larger the disparaty between summer day/night and winter day/night and
meanarche is postponed. Something to do with the pineal gland.
I also deemly remember that girls born blind or blinded in early childhood
where the damage involves the retina and/or optic nerve, had a tendency to
enter menarche later – apparently the day/night signals to the pineal gland
come via the retina & optic nerve.
The effect from the pineal may only be small, since menarche will
eventually take place.
Has anyone any updated infromation on this?