One in every six girls shows signs of puberty by the age of eight. So is it stress, pollution or obesity that's to blame?

Almost exactly 30 years ago, around 5,000 people in up-state Michigan were exposed to a chemical oestrogen that had been accidentally added to a feed given to cattle. A decade or so later, the girls whose mothers had been exposed to the hormone, were found to be going into puberty 12 months ahead of their classmates who had not consumed contaminated meat or milk.

Almost exactly 30 years ago, around 5,000 people in up-state Michigan were exposed to a chemical oestrogen that had been accidentally added to a feed given to cattle. A decade or so later, the girls whose mothers had been exposed to the hormone, were found to be going into puberty 12 months ahead of their classmates who had not consumed contaminated meat or milk.

According to scientists who have been investigating the case, the biological effect of oestrogens like this lurking in the environment may well be the reason why the age of puberty in industrialised countries of the West are going into puberty at an increasingly early age. But other researchers reject the notion, and say that better nutrition and obesity are to blame. Still more researchers suggest the declining age of sexual maturity is the result of having a cold and distant father, or a stepfather, or a depressed mother. A stressful home has also been blamed, and so too has lack of adolescent exercise, as well as child abuse, stress and media images.

Although the exact cause may be open to debate, there is now little doubt that the age of puberty for girls is coming down and that it has probably been tumbling for the last 150 years. One estimate is that there has been a drop of two to three months in each of the last 15 decades.

New research based on 17,000 girls shows that it is now not unusual for girls as young as seven to start developing breasts. The US study found that seven per cent of white girls and 27 per cent of black girls showed signed of development at seven. By the time the girls were eight, the proportions hitting early puberty had gone up to 15 per cent and 48 per cent. At 10, they had risen to 68 per cent and 95 per cent.

Earlier and similar research in the UK based on 630 girls also found that one in six had started to show signs of puberty by the time they were eight. Similar studies in Canada and New Zealand have also shown that the age for entering puberty is continuing to decline. But while the age change has been relatively easy to chart, the cause is proving far more elusive.

Puberty is that time of life when secondary sexual characteristics develop and when the sexual organs mature. What is beyond doubt is that the brain orchestrates this cascade of events, instructing the pituitary glad to secrete the hormones that drive the ovaries or testes. What is not known, is what causes the brain to start the process, and why in some girls it begins very early, while in other it can be much later.

Genes are involved, and girls often go into puberty around the time that their mother did, but they do not explain the declining age. Potential explanations range from the plausible and the likely, to the unexpected and the unlikely. The so-called stepfather syndrome has a number of backers and is based on the idea that unrelated adult males in the house give off pheromones which hasten the arrival of puberty and sexual maturing in the girls who are exposed to these chemicals.

Much of this research is based on animal studies, and detractors say its Achilles' heel is the very fact that it involves animals, usually rodents. They say that while rodents are pretty dumb, have a nocturnal lifestyle, live in burrows, and have poor eyesight, and may well rely on pheromones for sniffing out a mate, man has moved on to more sophisticated ways of mating.

Next up is the environmental oestrogen idea, the suggestion that oestrogen-like compound found in the environment in a multitude of modern products, from insecticides to plastic bags, may have an oestrogenic effect on the bodies of young girls.

A downside to this theory is that it does not take into account differences between industrialised countries, like Japan, where puberty still comes later that the UK. Nor does it allow for differences within countries, like the big difference in age of onset between different groups in the US, although supporters of the idea suggest genes may be involved.

What is now rapidly emerging as the dominant theory is that nutrition, bodyweight and lifestyle are the key triggers. "I think it is more to do with diet, lifestyle and energy metabolism. Puberty is not a function of chronological age, it is related to bodyweight, metabolism and a surplus of calories,'' says Dr Fran Ebling, a leading authority of the neurobiology of puberty at the University of Nottingham. "Reproduction is a costly process in terms of energy, with the costs of supporting the foetus and then lactation. Evolution has it that we don't embark on reproduction until we have appropriate energy resources. In the same way, some girls with anorexia do not have periods.''

He says that what has changed is that there are now more overweight children with more surplus calories. He suggests that one possible mechanism is through a hormone found in fat: "Fat produces a hormone called leptin which circulates in the blood and this is one of the signals that the pituitary gland reads so it knows how much fat is stored. If you have no leptin, you don't go through puberty."

Professor Paul Kaplowitz of Virginia Commonwealth University School of Medicine, who carried out the US study involving 17,000 girls agrees: "My own bias is that a major contributor to earlier puberty is the increasing prevalence of obesity over the past 25 years, especially in six- to 11-year-old girls."

The long-term medical effects, if any, of an early puberty are not known, although there have been suggestions of links with ovarian and breast cancer as well as an early menopause. Research also suggests that girls who go into puberty ahead of their contemporaries suffer more stress and worry more about their developing body.

With the declining age of puberty, parents are also having to face up to talking to their daughters about sexuality earlier. But according to psychologist Carol Jameson, that can be an advantage: "One bonus in to talking to a child early about puberty is that she is more likely to be open to a discussion at eight than she will be at 10, and far more open than a teenager.''

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