The secular trend in development is the fact that puberty is
starting earlier for boys and girls now than it did a hundred years
ago. The end of puberty (when girls get their period, for example)
has leveled off somewhat, but the beginning of puberty is still
getting earlier, especially for girls. It's common for girls today
to start developing breasts (which is often considered to be the
onset of puberty) much earlier than their ancestors did.
Nutrition may play a role in why this occurs. The onset of
puberty in Western nations such as the U.S., for example, is
typically years higher than it is in developing nations.
There are a number of reasons why early-onset puberty occurs,
including obesity, to the addition of hormones to meats and the
chemicals in hair and beauty products. Psychosocial causes, such as
the absence of a father or experiencing a traumatic event, have
also been held responsible.
Fat cells produce estrogen. Because estrogen kicks puberty into
gear, overweight and obese girls are more likely to undergo puberty
at younger ages than girls who aren't considered to be obese.
Even girls at normal weights are experiencing puberty earlier,
however. Researchers have put the blame on chemicals known as
endocrine disruptors. Examples of such disruptors include
bisphenol-A, which is found in plastics, as well as pesticides and
polychlorinated biphenyls. They've all been known to have an
estrogen-like effect on the body.