CHICAGO (AP) – When it comes to the birds and the bees, some parents may want to have that talk with their boys a little sooner than they expected.

Researchers have found signs of puberty in American boys up to 2 years earlier than previously reported – age 9 on average for blacks, 10 for whites and Hispanics. 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.