Sexy Science

It's well known that women have a much later peak of sexual activity than men, but what we don't usually realise is that it's one of the oldest ways for a woman to try to take control of her own fertility

MARIE STOPES, founder of the first free family planning clinic back in 1921, is one of six formidable women to be honoured in a set of stamps issued by the Royal Mail this week. Yet could she have possibly imagined the extent to which women can control their reproduction nowadays?

In Stopes's day, the issues were about spacing kids out and not producing too many of them. Nowadays, we have a gamut of relatively safe and convenient contraceptives, and a large proportion of women in the UK are free to pursue their careers, their travels, their dreams, with the reassurance that they can leave reproduction to some indeterminate time in the future.

Should that prove a difficult, or they fail to find Mr Right, then there's IVF, artificial donor insemination, or they can have their eggs frozen for future use.

But regardless of all that, something weird happens when women reach their thirties. It's like their psychology hijacks any plans they might have made. One friend said to me, "I can't believe how many women I know who accidentally got pregnant just after turning 30!" Accidentally on purpose, more like. Another friend, a bloke this time, found himself on the receiving end of thirtysomething babymaking anxiety. He had a brief but enjoyable no-strings-attached fling, or so he thought, but, lo and behold, a few weeks later the lady in question informed him that she was having his baby. Yes, he should have thought about using a condom.

In the 1950s the controversial scientist Alfred Kinsey reported that, while men's sexual peak occurred at around the age of 18 (based on the frequency of orgasms through any means available), women reached their peak in their early thirties. And, in a recent study of men and women's sexual habits across the world, David Schmitt of Bradley University, Illinois, US, has found a peak in women's sexual desire between the ages of 30 and 35. This is also the time when women are most likely to be unfaithful to long-term partners (for men the peak for all this is in the mid to late twenties).

So what is it with thirtysomething women that turns them, relatively speaking, into sex fiends?

Schmitt thinks it's about taking the opportunity to make babies before fertility takes a nosedive and the chances of genetic birth defects soar. Women's fertility reaches a peak in the early thirties but the percentage of fertile monthly cycles begins a steady decline after the age of 35. And, relative to women under 20, the probability of having a child with Down's syndrome, for instance, doubles between 25 and 30, triples between 30 and 35, but soars by a massive 800 per cent between 35 and 40.

Men can't be too complacent about these issues either. Male fertility gradually decreases from the age of 20 onwards, and one study found that for a 50-year-old man it takes on average 32 months to father a baby, while for a man under 20 it's just six months. And children of older dads are also more likely to have genetic disorders, although age doesn't have such a big effect as with women. Nevertheless, older men are particularly likely to father children with autism, epilepsy or schizophrenia.

Anyway, back to women. Of course plenty of women have healthy babies much later than the age of 35, but it seems women's behaviour has evolved to up the ante in the early thirties to ensure they don't miss the boat. This might take the form of more sex with their current partner or, for single women, an all-out bid to find "The One". Thirty-something women also seem more motivated to go for a bit on the side, and by having sex with multiple men a woman increases the chances that one of them will impregnate her.

Schmitt says that women's early-thirties sexual peak happens across cultures, and it's likely that these tendencies evolved way back in our hunter-gatherer ancestry. So however sophisticated our technology, at least some of our behaviour when it comes to the timing of making babies hasn't changed much over the last hundred millennia or so.