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.
Image: Tween boy, via Shutterstock