The study, widely considered the most reliable attempt to measure puberty in American boys, estimates that boys are showing signs of puberty six months to two years earlier than was reported in previous research, which historically taught that 11 ½ was the general age puberty began in boys. But experts cautioned that because previous studies were smaller or used different approaches, it is difficult to say how much earlier boys might be developing.

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, which was to be announced at the Academy of Pediatrics national conference on Saturday and published online in the journal Pediatrics, 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.