"In higher-income families, father absence predicted earlier puberty, but it did not in lower-income, father-absent (households)," said study leader Julianna Deardorff.

"Girls in upper-income households without a father were at least twice as likely to experience early onset of puberty, as demonstrated by breast development," she said. The researchers defined higher income as $50,000 or more a year.

Early maturation in girls is linked with emotional and substance use problems and earlier sexual activity. These girls also face a higher risk for breast cancer and other reproductive cancers later in life.

Previous research has linked absent-father households and earlier puberty, but this study adds more information, said Deardorff, an assistant professor of public health at the University of California, Berkeley.

"We were looking at very early signs," such as breast development and the growth of pubic hair, she said. Other researchers have focused on the start of menstruation without looking at factors such as income and ethnicity, according to background information in the study.

Girls are reaching puberty earlier in the United States, where the average age of menstruation is about 12 years, Deardorff said. Recent research has found some girls starting to develop breasts as early as age 7 or 8.

For their study, published Sept. 17 in the Journal of Adolescent Health, Deardorff and her colleagues followed 444 girls, aged 6 to 8 at the start, and their mothers. They gathered extensive data on factors such as weight, height, stage of breast and pubic hair development, father's presence and income. Eighty percent of the girls said their fathers did not live with them.

After two years of follow-up, the researchers saw earlier breast development in higher-income girls in absentee-dad homes across the board, but noted earlier pubic hair growth only in black girls from richer families. Having another male, such as a stepfather, in the home didn't change the findings.

They also found, as other researchers have, that higher body-mass index (BMI), a calculation based on height and weight, was also linked with earlier puberty.

The authors said they can only speculate on the reasons behind the connections. Exposure to more artificial light from TVs or computers is one theory. Other possibilities include weak maternal bonding, assuming a single mother is working long hours, or contact with chemicals that may have estrogenic effects — perhaps hair straighteners in the case of black preteens.

"It's possible girls in those homes are exposed to different environmental exposures, for example, toxins," Deardorff said. They may be exposed more to cosmetics and other personal care products, for instance, and some experts have expressed concerns about what they see as hormone-disrupting chemicals in those products.

Anthony Bogaert, a professor of community health sciences at Brock University in St. Catharines in Canada reported a link between absent fathers and early puberty in boys and girls in in 2005 in the Journal of Adolescence.

Looking at a national sample, he found that those who had had an absent father at 14 had likely had an early age of puberty. For his study, he defined puberty for girls as the onset of menstruation and for boys, voice change.

"It is difficult to know why this relationship occurs — for example, stress, genes — so more work needs to be conducted on the exact mechanism underlying it," he said. This new study is an "interesting" addition to the literature on the topic, he said.

How can parents in father-absent homes compensate, if at all?

Until researchers determine the cause of the connection, Bogaert said it's premature to offer suggestions.

Deardorff suggested focusing on other links, such as the higher BMI and its association with earlier puberty.

"Probably one of the primary targets is going to be body weight and physical activity (to maintain a healthy weight)," she said.