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What was striking about Herman-Giddens' report was the onset of secondary sexual characteristics: breast buds and pubic hair. Significant numbers of white girls--some 15%--were showing outward signs of incipient sexual maturity by age 8, and about 5% as early as 7. For African Americans, the statistics were even more startling. Fifteen percent were developing breasts or pubic hair by age 7, and almost half by age 8.
The Pediatrics report answered many questions, but much about the subject remains a mystery. The study couldn't accurately gauge, for example, how much the average age of onset of breast development (as opposed to menstruation) has dropped or over what period. That's because a key piece of research that helped set the standard age at 11 was a small study in the 1960s of white girls raised in English orphanages. But Dr. John Dallas, a pediatric endocrinologist with the University of Texas Medical Branch in Galveston, points out that the British girls may have been poorly nourished--a factor known to delay puberty. African- American girls were studied even less rigorously. "For all we know," says Dallas, "African-American girls could have been earlier developers for a long time."
The Pediatrics study is also limited because it does not include enough Asian- American or Hispanic girls to draw conclusions about these groups. Herman-Giddens agrees: "We in the public health and medical community really need to get data on American girls of all racial and ethnic groups." They also need to get data on boys, who haven't been studied in any systematic way. Herman-Giddens is pursuing the question now but says it isn't easy. "With girls," she says, "you can see breasts budding. With boys, the equivalent sign is an increase in size of the testes. It's very subtle. Even a physician may not be aware of it if they are not looking carefully."
Finally, somebody needs to look at what's going on in other countries. Is this a peculiarly American phenomenon? Or are girls developing breasts and reaching puberty at younger and younger ages all over the world?
The uncertainties swirling around the phenomenon make it difficult for scientists to nail down a cause, but that hasn't stopped them from coming up with a long list of potential candidates. The theory that has the broadest support among scientists holds that early puberty is somehow tied up with a much more familiar phenomenon: weight gain. America is in the midst of an epidemic of overweight and obese kids; between the late '70s and the early '90s, the percentage of children ages 6 to 11 who were overweight nearly doubled, from 6.5% to 11.4%, according to the National Center for Health Statistics.
Dr. Paul Kaplowitz, a pediatric endocrinologist with the Virginia Commonwealth University School of Medicine in Richmond, Va., explains, "We've known for a long time that very overweight girls tend to mature earlier, and very thin girls, such as anorexics, tend to mature later than normal. We think mildly overweight girls may be maturing early as well." Kaplowitz emphasizes that the correlation is merely statistical; not every girl with a little extra baby fat will develop breasts early.
Exactly why obesity and early development should be linked is not well understood. But Kaplowitz suspects early breast development may be encouraged by a protein called leptin. "We know that fat cells produce leptin," he says. "And leptin is necessary for the progression of puberty." Another clue, according to Duke's Freemark, is that overweight girls have more insulin circulating in their blood. Says Freemark: "Those higher levels of insulin appear to stimulate the production of sex hormones from the ovary and the adrenal gland."
While the consensus favors a fat connection, other explanations haven't been ruled out. One is chemical pollution in the food chain--specifically, DDE, a breakdown product of the pesticide DDT, and PCBs, once used as flame retardants in electrical equipment. Both chemicals are plausible suspects because they mimic hormones that play a key role in the development of the reproductive system. Beyond that, says Dr. Walter Rogan, an epidemiologist at the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., both chemicals are ubiquitous in the environment, and they persist in the body for years after exposure.