The study echoes research on girls, which has now established a scientific consensus that they are showing breast development earlier than in the past.
The study did not try to determine what might be causing earlier puberty, although it mentioned changes in diet, less physical activity and other environmental factors as possibilities. Experts said that without further research, implications for boys are unclear.
Dolores J. Lamb, a molecular endocrinologist at Baylor College of Medicine and president of the American Society for Reproductive Medicine, who was not involved in the study said:
“This should perhaps set a standard going forward for being very attentive to puberty in boys and being mindful that they’re developing earlier. Whether the difference is as large as what they say on some papers 40 years ago is not clear.” However, she added, “this is going to be incredibly useful to pediatricians and urologists.”
The new study also found that African-American boys began puberty earlier than whites and Hispanics, a result that other studies have shown also applies to African-American girls. Researchers said that difference is most likely driven by the role of genes in puberty.
On average, black boys in the study showed signs of puberty, primarily identified as growth of the testicles, at a little older than 9, while white and Hispanic boys were a little older than 10.
Several experts said the study should not be seized upon as cause for alarm, but rather as a way to help parents and doctors gauge what to be aware of in boys’ development and whether to start conversations about social issues sooner.
For the study, researchers enlisted about 200 pediatricians in 41 states to record information on 4,131 healthy boys ages 6 to 16 during their well-child exams.