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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鈥檚 sexual development is affected by her family environment 鈥� 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 鈥� when periods start 鈥� 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 鈥渓ost childhood鈥� 鈥� 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.

鈥淢aturing early can have serious health repercussions for girls鈥�

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. 鈥淪mall changes in a girl鈥檚 development can have important downstream changes in fertility and social outcomes,鈥� Belsky says, 鈥渁nd large effects on opportunities for conception over her lifetime.鈥�

That stress can accelerate sexual development is now well established, but the idea that fathers play a central role in the maturation of their daughters is more recent. In 1999 and 2003, Bruce Ellis of the University of Arizona, Tucson, and his colleagues reported findings from studies in the US and New Zealand in which they followed 762 girls from age 5 to sexual maturity (Journal of Personality and Social Psychology, vol 77, p 387; Child Development, vol 74, p 801). The researchers used questionnaires and made several visits to the children鈥檚 homes, observing family relationships and taking special note of the warmth or harshness of interactions between girls and their fathers.

They found that daughters from homes in which the biological father was present tended to experience puberty and their first sexual encounter at a later age than those whose father was absent. The closer and more affectionate the father-daughter relationship, the later the child鈥檚 sexual development occurred. A supportive relationship between parents delayed puberty still further. In contrast, the biological father鈥檚 absence, or friction between parents, was associated with earlier puberty, sexual activity and pregnancy. Girls who had lived without their fathers from an early age were almost twice as likely to have completed puberty by the seventh grade (age 12 or 13) and were seven times more likely to experience pregnancy in adolescence. This effect was magnified by the presence of a stepfather: the more prolonged a girl鈥檚 exposure to a stepfather or mother鈥檚 boyfriend, the greater the chance of early puberty, although this was less pronounced in families where the mother and stepfather had a good, supportive relationship.

The study clearly shows that stressful family relationships and the absence of a girl鈥檚 father are each independently associated with earlier timing of puberty in daughters, both having a similar impact. Ellis suggests that girls 鈥渄etect and internally encode鈥� information about the quality of their relationship with their fathers, and that this calibrates the timing of their reproductive development and sexual behaviour in adolescence. In other words, like Belsky, he believes that girls have an evolved ability to adapt to their social environment, adopting different reproductive strategies depending on the circumstances in which they grow up.

If that is so, the question then arises of what the mechanism is that allows social and emotional experiences to influence physical maturation. One possibility is that it involves the stress hormone cortisol. The timing and speed of puberty is controlled by both the adrenal glands and the hypothalamic-pituitary-gonadal (HPG) axis. As these mature, they secrete increasing amounts of sex hormones, leading to a cascade of processes associated with sexual maturation. In a girl this leads to the development of breasts, the appearance of body hair, deposition of fat round the hips, widening of the pelvis and the beginning of periods. Ellis points out that stressful family situations and traumatic childhood experiences have been linked to elevated cortisol levels, and speculates that this could lead to a change in the timing or tempo of the maturation of the adrenal glands or HPG axis, which in turn could affect a child鈥檚 sexual development. Although little is known about the effects of cortisol on the development of the HPG axis, levels of the hormone do increase as puberty progresses.

The scent of a man

That might explain why family stress speeds up sexual maturation, but Ellis believes the father effect requires some additional explanation. He points out that in many species, including mice, pigs, goats and several primates, a female鈥檚 sexual development is inhibited by contact with members of her natal group, while exposure to unrelated adult males accelerates maturation. This effect is mediated by airborne chemicals known as pheromones, which play a role in various aspects of animal communication. Whether they also mediate human behaviour is hotly contested, but Ellis and others are convinced they are involved in the father effect.

Last year, Robert Matchock of Pennsylvania State University in Altoona and Elizabeth Susman at Penn State鈥檚 University Park campus published results from a study of almost 2000 US college students. Like Ellis, they found that the absence of a biological father was associated with earlier menarche, but they also found that the presence of half-brothers and stepbrothers had the same effect. They, too, say pheromones are the key. 鈥淏iological fathers send out inhibitory chemical signals to their daughters,鈥� says Matchock. 鈥淚n the absence of these signals, girls tend to sexually mature earlier.鈥� In addition, other pheromones produced by unrelated males serve to accelerate puberty. The study also found that girls living in towns and cities had earlier menarche than those from rural areas. 鈥淯rban living would increase the probability of women interacting with novel males and coming into contact with their chemical cues,鈥� Matchock observes. Unlike Belsky, Matchock does not attribute his findings to an evolved reproductive strategy that allows girls to adapt to different family environments. He thinks they indicate that humans, like other mammals, have a mechanism to prevent inbreeding.

Other researchers are more directly critical of the idea that girls adapt to different kinds of upbringing. Some question the notion that the environment we grow up in has ever been a reliable predictor of how things will be when we come to have children ourselves. Changes in family fortunes could bring new opportunities for children, or a society-wide phenomenon such as a war could completely turn around the optimal life-path for individuals. Other critics point out that if kids are responding to their early experiences, then those reared in the same environment 鈥� in the same family, for example 鈥� should reach puberty at similar ages and have similar sexual strategies later. Yet research has produced little evidence of this.

In answer to these charges, Belsky points out what every parent knows: that some children are more susceptible to parental influences than others. He suggests this could explain why kids growing up in the same household end up with differing approaches to sex and reproduction. This would also give parents a better chance of seeing at least some of their children through to reproductive success. If each is genetically predisposed to react to the environment to a different extent, parents can optimise their parenting effort by concentrating on the kids who need it most.

This line of thinking may also go some way to counter suggestions that the father effect is simply genetic, arising from the work of David Comings of the City of Hope medical centre in Duarte, California, for example. He has found that daughters of absent fathers are more likely to have a gene coding for increased androgen sensitivity, which in men has been linked to aggression, promiscuity and impulsive behaviour. In girls, androgen sensitivity is associated with early menarche and precocious sexual activity, so it is conceivable that a gene that predisposes a man to behave in a way likely to lead to the break-up of a relationship might just happen also to predispose a daughter born of that relationship to early sexual maturity. Perhaps this gene could also account for research published last year by David Perrett of the University of St Andrews and Lynda Boothroyd of Durham University, both in the UK, showing that daughters of absent fathers or warring parents have a more masculine appearance (Proceedings of the Royal Society B, vol 273, p 2375). Belsky suggests, however, that genetic predisposition and rearing environment could both play a part in the father effect 鈥� though to different degrees in different children. 鈥淵ou can think in terms of two different genotypes,鈥� he says. 鈥淥ne that is more or less 鈥榩rogrammable鈥�, based on experience, and one that is fixed.鈥�

What are parents to make of all this? The good news is that despite the much-debated fracturing of family structures in the west in recent decades, we may not actually be depriving our children of their childhood. Although divorce rates have risen dramatically in the past few decades, this has not been matched with a comparable drop in the age of puberty, according to William Cameron Chumlea of Wright State University in Dayton, Ohio. In a recent study of 2510 girls in the US, he found that their median age at menarche was 12.43 years, just 0.34 years younger than the 1973 figure, a difference that is not statistically significant (Pediatrics, vol 111, p 110). Even here there is disagreement, however. Marcia Herman-Giddens of the University of North Carolina, Chapel Hill, and her colleagues question Chumlea鈥檚 interpretation of the data and point to other studies suggesting that girls are indeed maturing earlier than several decades ago (Pediatrics, vol 113, p 911).

The truth is that it may be impossible to predict the effect of your own behaviour and relationships on your child鈥檚 sexual development. Belsky still believes there is a take-home message for parents, though. Girls who become sexually mature at a young age face particular risks, and parents can help them avoid these. 鈥淕irls are not getting into trouble by themselves,鈥� he says. 鈥淚f your daughters are maturing early, be especially alert to who they鈥檙e hanging out with and be especially vigilant when they attract the attention of older boys.鈥� It sounds like good old-fashioned parenting, but it may make more sense than ever in today鈥檚 family environment.