Fifteen years ago, researchers began to notice that girls were hitting puberty earlier than usual. Now they've discovered that boys are showing their first signs of sexual maturation an average of up to two years before their counterparts from a few decades ago

The Pediatric Research in Office Settings (PROS) study, which was based on pediatric visits by 4,131 generally healthy U.S. boys between the ages of 6 and 16, found the average age for puberty's onset was 10.14 years among non-Hispanic whites, 10.4 for Hispanics, 9.14 for African Americans. Previously, the average onset of puberty in boys had been pegged at about 11.

In 1997, the PROS published a finding which suggested girls were hitting puberty about a year earlier than what had been cited in most clinical textbooks—often as early as age 7.

In the case of accelerated puberty in girls, there is a theory that it is being triggered by more estrogen in food, such as soy-based products.

The researchers have no such theory as to why boys are reaching sexual maturation sooner these days. Nor do they classify the trend as a positive or a negative one.