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CHICAGO (AP) - Researchers have found signs of puberty in American boys up to two years earlier than previously reported.

Other studies have suggested that girls, too, are entering puberty younger. Why is this happening? Theories range from higher levels of obesity and inactivity to chemicals in food and water, all of which might interfere with normal hormone production. But those are just theories, and they remain unproven.

Doctors say earlier puberty is not necessarily cause for concern. And some experts question whether the trend is even real.

Earlier research based on 20-year-old national data also suggested a trend toward early puberty in boys, but it was based on less rigorous information. The new study involved testes measurements in more than 4,000 boys. Enlargement of testes is generally the earliest sign of puberty in boys.

The study in online in Pediatrics to coincide with the American Academy of Pediatrics' national conference in New Orleans.