The average age of puberty is falling, according to a study of 20,654 healthy Chinese girls aged 3 to 20. On average the girls developed breast buds by 9 and pubic hair by 11, notably earlier than what used to be the norm. A 15-year Danish study similarly concludes that girls today experience initial breast development a year earlier than they did in the early 1990s. Both reports are in sync with a landmark 1997 study of 17,000 girls by the American Academy of Pediatrics, which found that Caucasian girls were developing breasts 6 to 12 months earlier than they did 40 years ago.
Better nutrition—leading to taller, heavier girls who mature younger—probably plays a role. Environmental exposure to hormone-mimicking chemicals may have an effect too. Pediatrician Barbara Cromer of Case Western Reserve University notes that many pesticides and plastics contain synthetic estrogens, and that cattle fattened with estrogen have up to five times as much of it in their tissue as do untreated cattle. “Early puberty could represent a ‘canary in the coal mine’ for excessive estrogen in our environment,” she says. If so, the next generation of young women are at greater risk of health problems. Elevated exposure to estrogen over a long period is linked with higher breast cancer rates in adulthood and earlier onset of risky sexual activity.