"You are a woman now." Many of us heard those words when we got our first period, a landmark event in a woman's life. It is a day we all remember, sometimes with embarrassment, but it's a subject most women have rarely been able to read about. In the recently released "My Little Red Book," 18-year-old Rachel Kauder Nalebuff broke the unspoken taboo by collecting dozens of essays on the topic from a diverse group of women, both famous and not. Erica Jong writes about the "Fear of Fourteen" and Cecily Von Ziegesar, author of the "Gossip Girl" series, describes an unfortunate sex-ed class on the subject. The collection is just one of a host of new books on the subject; preteens now have dozens of detailed guides to the physical and psychological changes that precede and accompany a girl's first menstrual period. But the mysteries of when and why girls begin puberty have gotten a lot of attention from the scientific community recently, too—research has expanded from the biology of menstruation to include some of the complex sociological and emotional factors involved in when and why girls begin maturing.

Scientists have been particularly puzzled by what appears to be a slight decline in the average age of puberty. The dip isn't much—from two months to a year, depending on the study. No one yet knows why, although there are a number of intriguing theories, nor even whether it's a problem. In the 1960s, says Dr. Paul Kaplowitz, an endocrinologist at Children's National Medical Center in Washington, D.C., girls typically developed breasts and pubic hair between ages 10 and ll. Now, he says, it has fallen to between 10 and 10½. Race also appears to play a role. For example, African-American girls tend to show the first signs of puberty—breast growth and pubic hair—a year earlier than white girls.

Some scientists say they suspect the change is related to the growing numbers of children who are overweight or obese. "That is my preferred theory," says Kaplowitz, author of "Early Puberty in Girls." "There is lots of evidence that where there is an increase in obesity, there's an increase in precocious puberty." The idea is that puberty begins once girls reach a certain weight or body-fat content, so if that point is reached earlier, puberty begins earlier. Fat is also a source of estrogen, so young girls with a lot of body fat would be exposed to more of the hormone.

But while a number of recent studies have found a strong association between early puberty and increased body-mass index (BMI), that link doesn't explain all cases. Kaplowitz notes that girls who are not overweight may still experience puberty earlier than average. In some cases, genetics may offer an explanation: "If a mom had her first period at age 10, 10½, there's a 50 percent chance that her daughter will mature early," Kaplowitz says.

Other scientists think fathers may play a role in some surprising ways. Bruce Ellis, a professor of family studies and human development at the University of Arizona, has been studying the effect of family structure on puberty. He has found that young girls who spent more time living in a home without their birth fathers were more likely to go through puberty early. One explanation might be chemical: animal studies have shown that exposure to the pheromones (chemical triggers) of biological fathers appears to slow down puberty in females. But other factors could be at work as well.

In a study published last year in the journal Developmental Psychology, Ellis and Jacqueline Tither of the University of Canterbury in New Zealand studied sisters in 90 intact families and 70 families where the parents had divorced. The most striking finding concerned sisters in divorced families where the father had engaged in high-risk behavior (defined as a history of violence, depression or imprisonment) and then left home. Younger sisters in those families tended to go through puberty nearly a year earlier than their older sisters, indicating that emotional stress at a certain period early in life may affect a girl's normal development.