Why are girls growing up so fast?
- Why are girls growing up so fast?
- 14 February 2007
- Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.
- Mairi Macleod
Parenthood has never been easy, but these days it seems more onerous than ever. Parents worry about whether their children are getting enough exercise, a healthy diet, the right kind of stimulating play. They fret about safety, the corrupting influence of computer games and how to give their little darlings an intellectual head start. As if all that were not enough, they now have something else to beat themselves up about: the speed at which children mature may partly depend on how their parents conduct their own relationships. More and more evidence suggests that a girl's sexual development is affected by her family environment - and fathers play a crucial role.
There is no doubt that girls are growing up more quickly than they used to. The average age at menarche - when periods start - has plummeted over the past 150 years in western societies from around 17 years old down to 12 or 13. Boys are also maturing earlier, although far less is known about their accelerated development because their progress through puberty is more difficult to measure. Maturing early is not simply a vague matter of "lost childhood" - it can have serious health repercussions. The younger a girl is when she reaches puberty, the higher the likelihood that she will experience depression and breast cancer, indulge in substance abuse or risky sexual activity, or suffer teenage pregnancy and dissatisfaction with her body image. Early-maturing boys may face their own problems, but with so few studies into their development these are as yet unknown.“Maturing early can have serious health repercussions for girls”
While there is general agreement that the huge improvement in nutrition and health in developed countries underlies this accelerated development, it is becoming clear that this cannot be the whole story. Why, for example, do girls reach puberty at widely different ages in countries with similarly high standards of nutrition and healthcare. More puzzling still, why do girls who grow up without their biological father tend to mature earlier?
Back in 1991, psychologist Jay Belsky of Birkbeck College, University of London, and his colleagues came up with what they called the psychosocial acceleration theory. This suggests that girls who experience a lot of family stress will mature faster. They reasoned that what a girl experiences in her early years acts as a prediction of the likely availability of resources later in life. If she grows up in a socially harsh environment, and so cannot expect much support in later life, she will be better off if she adopts an accelerated reproductive strategy, including earlier onset of puberty and menarche, early first pregnancy, and short-term relationships with less parenting investment in each of her children. A girl growing up in a more stable, nurturing environment, on the other hand, will be better served by a high-investment reproductive strategy, including later puberty and onset of sexual activity, and more stable pair-bonding.
Several studies examining the relationship between family background and reproductive strategy have since revealed that family stress can accelerate the onset of menarche by around four to six months. That may not sound like much, but a slightly earlier puberty can translate into a drastic reduction in the age at which a girl can become pregnant. It takes a while after menarche to gain full fertility because girls do not begin ovulating straight away, but a study of Finnish girls more than two decades ago found that ovulation happened far more quickly if they matured earlier. The time from menarche until 50 per cent of cycles became ovulatory was just one year if menarche occurred before age 12, compared with 4.5 years if she was 13 or more. "Small changes in a girl's development can have important downstream changes in fertility and social outcomes," Belsky says, "and large effects on opportunities for conception over her lifetime."
The rest of the article is at New Sceintist