Girls who come from higher-income households and don't live with their dads are more likely to enter puberty earlier than average, a new study suggests.

The finding surprised researchers, who had never before considered income levels when testing for factors that could contribute to early puberty.

"We thought it'd be an under-resourced thing, where it'd be the lower-income girls" who start puberty early, said study researcher Julianna Deardorff, assistant professor of maternal and child health at University of California, Berkeley. "But it was the total opposite."

The study is based on two years of data on 444 6- to 8-year-old girls, 80 of whom did not live with their biological fathers. Researchers monitored signs of puberty by looking for early signs of breast development and growth of pubic hair.

Girls from households with an income higher than $50,000 were more likely to start developing breasts within that two-year period than girls from households with lower incomes. And within that group of girls from high-income households, African American girls were more likely to have early growth of pubic hair than other girls.

Most girls begin puberty between the ages of 10 and 14, according to the National Institutes of Health.

Previous studies linking father absence with early puberty were retrospective, with scientists asking women and older teens to remember when they got their first period. This ongoing study is different because it tracks the development of younger girls as they enter puberty, and it takes into account body mass index (BMI), ethnicity and income, Deardorff said.

It's impossible to pinpoint a "normal" age of entering puberty, but any girls who started developing breasts or pubic hair during the time of the study were considered to be early, Deardorff said.

Doctors are concerned about early puberty because it has been associated with increased risk of breast and ovarian cancers later in life.

Increasing evidence is showing that many girls now enter puberty earlier than they did even just a few decades ago. A study published August in the journal Pediatrics found that 10 percent of 7-year-old Caucasian girls showed the beginnings of breast development, double the number that a 1997 study found.

"If you have early puberty compiled with other contextual stresses, you're at high risk for issues in the long term," Deardorff said. "There's a higher risk of sexual initiation, substance use, anxiety and depression."

Other studies have shown that early puberty is associated with being overweight, being extremely physically active and having a mom who also started puberty early.

But many other explanations have been put forth, too, she said.

A girl who doesn't live with her father may have a more unstable family environment, the researchers said, or could be exposed to more men in her home, who could emit pheromones, that could contribute to earlier puberty.

It could be that higher-income girls are more often exposed to the artificial light of computer and TV screens, which studies show accelerates puberty in animals, Deardorff said. Or, a mother who must work to support to the family could inadvertently lead to weak maternal bonding, which may play a role in earlier puberty, she said.

Beauty products commonly used by African American girls, such as hair straighteners, could have estrogenic effects.

But none of these explanations have been proven, Deardorff said. Now that the girls in her study are between the ages of 10 and 12, she hopes to learn if environmental factors are involved in the onset of menstruation.

"We have an opportunity to see what else is accounting for [the early development]," she said. "We can find out, 'Is it really the dad not being there that's the cause?'"

The study was published Sept. 17 in the Journal of Adolescent Health.