© BBCChildren's bodies are maturing faster
Unhealthy lifestyles and unstable family environments may be contributing to a fall in the age that girls reach puberty, research suggests.
Psychologist Dr Aric Sigman found girls are reaching puberty 18 months earlier than their mothers, and almost two years earlier than their grandmothers.
He found girls currently start puberty at an average of 10.25 years of age.
His findings echo previous research suggesting 'precocious puberty' is a growing trend.
A study by scientists in Bristol in 2000 suggested one girl in six reaches puberty before the age of eight.
One theory is that puberty is triggered by the hormone leptin, which is produced by fat tissue.
Girls are getting progressively heavier with each generation, and as they carry higher levels of fat, they are also likely to have higher levels of leptin.Unhealthy lifestyle
Dr Sigman's study focused on three generations of family units, each with a daughter aged 16 to 20, a mother aged 40 to 50, and a grandmother aged 65 to 75.
He found the modern generation eats far more sweets and junk food than its predecessors, but less fresh fruit and vegetables.
Today's young women were also much more likely to travel by car than on foot.
They were also much more to lead a 'coach potato' lifestyle, taking little exercise.
The combination of a poor diet, and an sedentary lifestyle was fuelling obesity rates, said Dr Sigman.Stress factor
However, he also found evidence that a stressful home life raised the likelihood of an early puberty.
Dr Sigman told the BBC News website that this was possibly an ancient evolutionary response.
"If a girl senses her environment is unstable then it may be that an evolutionary mechanism kicks in to try to ensure that her genes are passed on sooner rather than later," he said.
There is some evidence to suggest that puberty arrives earlier in girls who live with a stepfather.
It is postulated that this might be due exposure to the stepfather's chemical scents, or pheromones, which are likely to have a more profound effect than those associated with a birth parent.
Dr Sigman said it had been assumed that puberty was purely a biological phenomenon influenced by genetics.
"We have not really thought about the possibility that lifestyle factors might influence something so primitive and profound as the arrival of puberty, but it might very well be that they have an impact."
He said early puberty was associated with a number of risks.Tough time
For instance, girls were vulnerable to emotional disturbances, such as depression, and behavioural problems, such as taking up drinking, smoking or drugs.
Young girls who suddenly started to mature sexually were also less able to control their impulses than those who hit puberty at a later stage, he said. This might in part explain why rates of teenage pregnancy have risen in recent times.
"They tend to work on the hedonistic principle that if it's enjoyable, then it must be right," he said.
Dr Sigman, whos reported was commissioned by Clearasil, said it was important for parents to be aware that puberty might arrive early for their daughters, and to try to talk to them in advance about the changes they would experience.
Professor Peter Clayton, a paediatric endocrinologist at the University of Manchester, told the BBC News website it was still unclear what factors might be driving a reduction in the age of onset of puberty.
He said: "Certainly in our clinical practice, we treat more girls with precocious puberty than we did 10 years ago, but this may be a reflection of awareness rather than a true shift in physical development."
Professor Ilpo Huhtaniemi, an expert in reproductive biology at Imperial College London, said: "There is still lots that we do not understand about this phenomenon.
"For instance, girls adopted from third world countries to well-off countries seem to have a very high incidence of precocious puberty."