Sexy Science - Dr Mairi McLeod

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THE sexual strategies we use come down at least in part to the family situation we grow up in. Last week BBC news announced that, in Scotland, a girl growing up in a deprived environment was five times more likely to get pregnant during her teenage years than a girl experiencing childhood in an affluent area.

Girls in poorer areas are also much more likely to grow up without their father in the household, and the results of a newly published study by Bruce Ellis, of the University of Arizona, and his colleague, Jacqueline Tither, suggest a way that these two issues could be linked.

It's fairly well established that father absence is linked with early puberty in daughters, which, in turn ,increases the chances of teenage pregnancy, but Ellis and Tither wanted to find out if the father absence causes this acceleration, or if some confounding factor such as a genetic link is responsible.

They interviewed pairs of sisters who differed in age from 160 families, some of which were intact and some where the parents had divorced. Questions focused on family composition and the relationship the girls had with their father. They looked at families where, for instance, one daughter might be five and the other 12 when a divorce occurred. The younger daughters had longer exposure to father absence and consistently reached puberty at a younger age than their older siblings, suggesting a causal link.

What matters, though, is not just that the father is around, but that he's having a positive involvement in his daughter's life. The most dramatic effect on sexual development was seen in sisters who had early experience of socially deviant behaviour by their father, followed by his absence, and Ellis thinks this accelerated puberty is an evolved response.

"The idea is that children adjust their development to match the environments in which they live," he said. "In the world in which humans evolved, dangerous or unstable home environments meant a shorter lifespan, and going into puberty earlier in this context increased chances of surviving, reproducing and passing on your genes."

And, in deprived areas, there are certainly more hazards than in affluent ones. Mortality rates are higher in poorer areas as is infertility because of pollutants, accidents and the ill health people suffer, so, if a girl wants to be reasonably sure of having at least a couple of children, it might be a good idea to get started a bit earlier. But could having kids early be linked to family disruption in these areas?

According to evolutionary psychologist Daniel Nettle, of Newcastle University, father absence is on the increase, especially in poorer families. For children born in the 1950s, more than 90 per cent of fathers were co-resident even in the most deprived areas, he says. But for children born in 2000 in England and Wales, while 96 per cent of dads were co-resident when the child was a year old, where the father had a professional or managerial job, in lower status families, only 70 per cent of dads were living in the home at their child's first birthday.

It seems that poorer, riskier environments affect men by making them tend to go for a lower investment strategy but with more partners, so they are less likely to stick around to help look after their children. But for women risky conditions can have the effect of making them get started on reproduction earlier.

Given the fact that teenage pregnancy can have bad outcomes for the baby, what is to be done about it? Well, these young women are responding to the conditions around them, and who's to say they're wrong? And given that family disruption often plays a large part in teenage pregnancy, it looks as if a little chat to young girls about their sexual habits isn't going to make much difference. The underlying problem in a lot of cases is deprivation and inequality.

"It's the physical danger, health support, and type of jobs that are available and the opportunities they have," says Nettle. "I think if you sorted those out, the socioeconomic differences in both father absence and teenage pregnancy would go away."