What's driving earlier puberty in girls?

Carolyn Moynihan
11 Aug 2010
Reproduced with Permission

Evidence that girls are reaching puberty as early as seven years of age is in the news this week following the publication of an article in the journal Paediatrics. It is a topic that has been debated for decades.

The new data shows that in the United States white girls are catching up with Black and Hispanic girls, who are still well ahead statistically when it comes to maturing early but among whom the trend has slowed right down.

More than 10 percent of white 7-year-old girls in the study, which was conducted in the mid-2000s, had reached a stage of breast development marking the start of puberty, compared to just 5 percent in a similar study conducted in the early 1990s.

Most lay people would be concerned about little girls' physical development getting ahead of their intellectual and emotional development. For one thing it could make them vulnerable to sexual interest by older males. They are also more likely to mix with older children and mimic older behaviour.

But the current study (which did not include menstruation) was driven by concern about increasing rates of breast cancer among women:

Early puberty in girls is a growing public health concern because studies have shown that girls who start puberty earlier are more likely to develop breast and uterine cancer later in life. The National Institutes of Health funded the study as part of a larger investigation into the environmental factors that contribute to breast cancer risk.

Experts are not certain about the factors driving the trend, which has been evident since the 1950s. Among the chief suspects are excess body fat which affects the level of hormones (estrogen and progesterone) that trigger puberty; environmental chemicals; and the environment in the womb resulting from maternal characteristics -- including first period before age 12, smoking during pregnancy, and being pregnant for the first time.

Some researchers have found links with television viewing, as Aric Sigman has noted:

Dr Sigman's report, which is based on his analysis of 35 scientific studies, claims that television viewing affects levels of melatonin, a hormone linked to when puberty occurs in girls. Melatonin levels increase in the evening, at the onset of darkness, but staring into a bright screen during this period hinders its production.

Another factor, cited by Leonard Sax in his book Girls On The Edge, is the absence of a girl's biological father. Sax writes:

Robert Matchock and Elizabeth Susman at Penn State University are convinced that pheromones are the mechanism whereby the presence of the biological father slows down the tempo of his daughter's sexual development. They believe that this phenomenon is hardwired in our species, as it is in many other mammals, in order to decrease the likelihood of a father having sex with his daughter. "Biological fathers send out inhibitory chemical signals to their daughters," says Matchock. "In the absence of these signals, girls tend to sexually mature earlier.

As for early exposure to the sex hormones oestrogen and progesterone, no-one seems to be mentioning the possible contribution of hormonal contraceptives. Could they, for example, help explain why a first pregnancy -- after coming off the pill etc -- is among the risk factors for starting puberty earlier?