A new study finds that girls are more likely today than in the past to start developing breasts by age 7 or 8.

The research is just the latest in a flood of reports over the last decade that have led to concern and heated debate about whether girls are reaching puberty earlier, and why it might be happening.

Increased rates of obesity are thought to play a major role, because body fat can produce sex hormones. Some researchers also suspect that environmental chemicals that mimic the effects of estrogen may be speeding up the clock on puberty, but that idea is unproved.

The issue is of concern for both medical and psychosocial reasons. Studies suggest that earlier puberty, as measured by the age at first menstruation, can slightly increase the risk of breast cancer, probably because it results in longer lifetime exposure to the hormones estrogen and progesterone, which can feed some tumors.

Although the new study did not look at menstrual age, breast growth is also a sign of hormone exposure, and some researchers fear that early development might also mean an increased cancer risk.

Socially and emotionally, life can be difficult for a girl who has a child’s mind in a woman’s body and is not ready to deal with sexual advances from men and boys, or cope with her own hormone-spiked emotions and sexual impulses.

“Our analysis shows clearly that the white participants entered puberty earlier than we anticipated,” said Dr. Frank M. Biro, the first author of the study and the director of adolescent medicine at Cincinnati Children’s Hospital Medical Center.

Overweight girls were more likely to have more breast development, the study showed. But Dr. Biro said he did not think weight was the whole story. He said it was possible that environmental chemicals were also playing a role, and added that he and his colleagues were now studying the girls’ hormone levels and lab tests measuring their exposures to various chemicals.

“It’s certainly throwing up a warning flag,” Dr. Biro said. “I think we need to think about the stuff we’re exposing our bodies to and the bodies of our kids. This is a wake-up call, and I think we need to pay attention to it.”

Dr. Catherine Gordon, a pediatric endocrinologist and specialist in adolescent medicine at Children’s Hospital Boston, said that so far, most evidence showed that neither breast development nor menstrual age had changed for white girls of normal weight.

The new study included 1,239 girls ages 6 to 8 who were recruited from schools and examined at one of three sites: the Mount Sinai School of Medicine in Manhattan, Cincinnati Children’s Hospital or Kaiser Permanente Northern California/University of California, San Francisco. The group was roughly 30 percent each white, black and Hispanic, and about 5 percent Asian.

At 7 years, 10.4 percent of white, 23.4 percent of black and 14.9 percent of Hispanic girls had enough breast development to be considered at the onset of puberty.

At age 8, the figures were 18.3 percent in whites, 42.9 percent in blacks and 30.9 percent in Hispanics. The percentages for blacks and whites were even higher than those found by a 1997 study that was one of the first to suggest that puberty was occurring earlier in girls.

The new study is being published on Monday in the journal Pediatrics. It was paid for by government grants and conducted at hospitals that are part of the Breast Cancer and the Environment Research Centers, a group formed in 2003 after breast cancer advocates petitioned Congress to set aside money to study possible links between environmental exposures and breast cancer.

If there is an ideal age when girls should reach puberty, no one knows what it is, said Dr. Marcia E. Herman-Giddens, a researcher at the University of North Carolina, Chapel Hill. A girl needs a certain amount of body fat to start menstruating, and girls who are malnourished or ill may have delayed puberty.

In developed countries, the age of puberty dropped from the 19th to 20th centuries, as nutrition improved and infectious diseases were brought under better control, and it was seen as a sign of progress. But if the drop continues, at what point does it become pathological?

The debate over this issue started with a study published in 1997 by a research team led by Dr. Herman-Giddens. In the study, pediatricians around the country rated sexual maturation in 17,077 girls ages 3 to 12. The study found that breasts or pubic hair, or both, were far more common in 7- and 8-year-olds than medical textbooks had been reporting.

The researchers were also surprised to find that black girls developed significantly earlier than whites. But they cautioned that there had been few rigorous studies of puberty, so it was not clear whether their research was detecting a new trend or just discovering that the medical books were wrong.

The study led to a bit of a furor. Some endocrinologists doubted the findings and warned that if doctors and parents started blithely assuming that puberty at 7 or 8 was the new normal, they would overlook serious problems like endocrine diseases or tumors. But others warned that if the new findings were rejected, families would be frightened needlessly and fortunes wasted on batteries of tests for perfectly normal 7- and 8-year-old girls with budding breasts.

Dozens of studies have been published in the years since. Arguments continue, but many doctors accept the idea that heavier girls often develop earlier. And subsequent studies have also found that black and Hispanic girls mature earlier than whites, even when weight is taken into account. No one knows why. Though breasts may be sprouting earlier, the average age of first menstruation, between 12 and 13, has not really changed.

Dr. Vaneeta Bamba, director of the Diagnostic and Research Growth Center at the Children’s Hospital of Philadelphia, said that the 1997 study had “somewhat reshaped” endocrinologists’ thinking about the onset of puberty, but that most would still urge a thorough medical evaluation for any girl under 8 who was showing significant breast development or other signs of puberty. She said she doubted that the new study would change medical practice.

One objection to the 1997 study was that the pediatricians may have mistaken fat deposits for breast tissue in some girls, or differed in other ways in assessing the stage of breast development.

In the new study, the researchers went to great lengths to train examiners and make sure all were on the same page when it came to checking girls’ breasts and rating their stage of development.