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While most are stunned by the news that a 9-year-old girl who had been raped gave birth in Mexico, doctors have known for years that the average age of puberty is declining.

A landmark 1997 study in the journal Pediatrics prompted a redefinition of early onset puberty, from 8 to 7 among Caucasian girls and 7 to 6 among African-American girls for early breast development.

While age of puberty is decreasing, the number of girls entering puberty early is increasing: about 16 percent of girls enter puberty by the age of 7, and about 30 percent by the age of 8, according to one study.

The main question is, Why?

A study published recently in the journal Nature Neuroscience seems to have fit another piece into the puzzle, when researchers were able to delay puberty in female rats by manipulating protein levels. They discovered a group of proteins that regulate the activity of a gene called the Kiss1 gene, which is required for puberty to occur.

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Study author Serjio Ojeda, a neuroscientist and professor in the Oregon Health & Science University School of Medicine, says that his team’s findings raise the possibility that environmental factors play a role in early onset puberty.

"It poses the question as to how are these epigenetic mechanisms responding to the environment?" he said. "These are difficult questions we haven't answered yet. Is it nutrition, man-made chemicals? That's the next step, how can we connect these environmental stimuli?"

"That's the crystal ball question," agrees Dr. William Crowley, Professor of Medicine at Harvard Medical School, the Chief of the Reproductive Endocrine Unit of the Department of Medicine at Massachusetts General Hospital, who has studied early onset puberty for 35 years but was not involved in Ojeda's study.

The research is starting to coalesce, he said.

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"The tipping point is very close," he said. "It's like when you're paddling down a great river and it gets very quiet just before the falls, and it looks pretty benign until you start to hear a dull roar. That's the kind of sense everybody has right now; you can kind of hear a dull roar."

But to obtain definitive answers, all of the genes that contribute to early onset puberty must be identified. Then, scientists may determine if the environment or other factors is modifying the genes.

So far, 14 genes that contribute to the disorder have been identified. But there could be dozens more: other complex trait disorders, such as Type II diabetes, have 70 different genes that contribute to the disease.

The process is slow, Crowley said, because you need large numbers of people -- 10,000 patients, say, and 10,000 controls. And the incidence of patients who have no puberty, whose genes are extremely helpful in solving the puzzle, is 1 in 50,000.

Ojeda's team took a different approach: They looked at various genes that might be candidates for affecting puberty and tested them in animals. But that approach is also time consuming because it involves such a great number of genes, and the ones that may provide the key are probably the ones no one has thought of, Crowley said.

Puberty starts when a hormone in the brain causes the pituitary gland to release more hormones. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) cause the ovaries to produce estrogen or testosterone, prompting physical changes.

In most cases of early onset puberty, the process simply starts early. Girls, African-Americans, and obese kids are more likely to experience early onset puberty. The condition is treatable with medicine.

Possible environmental factors include obesity, diet, and exposure to a variety of synthetic chemicals.