Marcia Herman-Giddens first realized something was changing in young girls in the late 1980s, while she was serving as the director for the child abuse team at Duke University Medical Center. During evaluations of girls who had been abused, Dr. Herman-Giddens noticed that many of them had started developing breasts at ages as young as 6 or 7. "That did not seem right," said Dr. Herman-Giddens. A decade later, she published a study of more than 17,000 girls who underwent physical examinations at pediatricians' offices across the country. The numbers revealed that, on average, girls in the mid-1990s had started to develop breasts " typically the first sign of puberty " around age 10, more than a year earlier than previously recorded. The decline was even more striking in Black girls, who had begun developing breasts, on average, at age 9. Studies in the decades since have confirmed, in dozens of countries, that the age of puberty in girls has dropped by about three months per decade since the 1970s. A similar pattern, though less extreme, has been observed in boys. No one knows what risk factor " or more likely, what combination of factors " is driving the age decline or why there are stark race- and sex-based differences. Obesity seems to be playing a role. Researchers are also investigating ... chemicals found in certain plastics and stress. The girls with the earliest breast development in [a] 2009 study ... had the highest urine levels of phthalates, substances used to make plastics more durable.