Puberty is one of the biggest, and often most awkward, changes that boys go through, and a new study shows it starts six months to two years earlier than previous data indicated. The findings raise concerns about the cause of the change and how boys could be affected by a sped-up timeline.

"Our question was, 'What's actually going on currently?' The data that was out there was old and of questionable validity," said Dr. Richard Wasserman, an author of the study and director of Pediatric Research in Office Settings at the American Academy of Pediatrics. "My interest [as a doctor] is, 'How do I know what's normal for boys?'"

In recent years, studies have drawn attention to early-onset puberty in girls in the U.S., including a 2010 paper that found more girls begin breast development at age 7 or 8 than a decade earlier. But the new study, published online in the journal Pediatrics on Saturday, is among the first to look at the issue in American boys.

"We had done the girl study," Wasserman said. "Now it was, 'Let's do a study with boys.'"

Some 200 health care providers in 41 states tracked signs of puberty among more than 4,000 boys. Puberty typically begins with the growth of the testicles and penis, followed by the appearance of body hair, muscle growth and the deepening or dropping of the voice.

On average, white and Hispanic boys began to show signs of puberty when they were just over 10 years old, while African-American boys tended to start puberty slightly earlier -- at just over 9 years old. These findings were compared to data from the 1970s that, to date, has served as the standard for health care professionals. The onset of puberty began six months to two years earlier than the previous averages.

"The question is, 'What's going on here?'" Wasserman said. "Is it nutrition -- either better nutrition or over-nutrition? Is there something in the environment?"

He and his co-authors wrote that they found the data "surprising," in part because the factors that have been linked to earlier physical development in girls, including obesity and endocrine disruptors, such as the industrial chemical BPA, are not known to be linked to earlier development in boys. (Endocrine-disrupting chemicals can mimic hormones in the body or block their effects, and studies have suggested that they can speed up or delay puberty, as well as contribute to sexual differentiation problems.)

Wasserman explained that this does not rule out a link between endocrine disruptors and early onset puberty, but simply that there are more questions than answers right now.

"There's not enough research to say," he said. "There's not enough research, period, in this area."

Preliminary studies have suggested that earlier puberty in girls is not only linked to social problems and poor self-esteem, but is also potentially linked to certain cancers. It is unclear whether earlier onset of puberty in boys has any long-term effects.

Dr. Lorena Siqueira, who is a director of adolescent medicine at Miami Children's Hospital and was not involved in the study, called the new findings "important" and agreed that they raise critical questions about whether environmental factors and obesity are somehow speeding up puberty in boys.

She cautioned, however, that there is very little room for comparison between the new study and previous data.

"Is there truly an earlier age of puberty? I'm not convinced of that," she said. "There is no baseline data. We're using this data that was published in the 1970s, but that was using white boys, in an institution, in England. Is that an appropriate comparison?"

Nonetheless, she and Wasserman stressed the importance of thorough physical examinations to track signs of puberty in boys, both so doctors can ensure proper development and so parents can help their children through any changes they're experiencing.

"It's important to know when changes are happening," Wasserman said, "so that parents can guide a child."