In the study, lead author Marcia Herman-Giddens from the University of North Carolina's School of Public Health
and her colleagues show that boys are starting to sexually develop six months to two years earlier than medical textbooks say is standard.
This research has been a long time coming. Herman-Giddens first documented early puberty in girls in 1997, and several studies have since backed up those findings
One of the reasons it took so long to do a comprehensive study on early puberty in boys, Herman-Giddens said, is that the onset is more difficult to identify. For girls, breast development and the start of a menstrual cycle are obvious clues. For boys, the onset of puberty comes in the form of enlarged testes and the production of sperm.
Researchers responded: " 'Yikes, we don't want to ask about that!' " Herman-Giddens said with a laugh.
But ask they did -- 212 practitioners across the country examined more than 4,100 boys aged 6 to 16. The practitioners recorded information on the boys' genital size and pubic hair appearance.
Researchers assigned each boy's data to one of five stages -- Stage 1 being pre-puberty, Stage 2 being the onset of puberty and Stage 5 being adult maturity. They then compared the ages and puberty stages of all the boys. The rigorous study was designed to report on only physical changes, not hormonal.
The results were broken down by race: African-American boys start hitting Stage 2 first, at about 9 years old, while non-Hispanic white and Hispanic boys begin developing around 10 years old. "This should have an impact on the public health community," Herman-Giddens said.
But the researcher is concerned about using the numbers as a new standard for pediatricians. "That might be normal now," she said, "but that doesn't mean it's normal in the sense of what's healthy or what should be."
One of the reasons she's worried is that our environment may be playing a role in accelerating puberty.
"The changes are too fast," Herman-Giddes said. "Genetics take maybe hundreds, thousands of years. You have to look at something in the environment. That would include everything from (a lack of) exercise to junk food to TV to chemicals."
Dr. Megan Kelsey, an assistant professor of pediatric endocrinology with Children's Hospital Colorado
, said several studies have shown an association between childhood obesity and early puberty in girls.
Fat tissue has the ability to convert other hormones into estrogen, which experts believe may lead to early breast development. Fat also creates the hormone leptin, which is necessary for the onset of puberty, Kelsey said.