HORMONES have a lot to answer for. They give us spots, make us cry for no apparent reason and control our internal world. They also have a big influence on how attractive we are to the opposite sex. The latest of these mischievous molecules to come under the spotlight are the oestrogens - the feminine hormones.
This week we learned that women with high circulating oestrogen levels have especially attractive faces, according to research by psychologist Miriam Law Smith and her colleagues at the St Andrews University.
"Oestrogen is known to inhibit bone growth, unlike testosterone which has the opposite effect," says Ms Law Smith, "so females retain more childlike facial features."
This gives women their relatively large eyes and smaller jaws and brows compared to men. Women with high levels of oestrogen have especially feminine faces. But it's not only women's faces that are influenced by oestrogen. Another study has shown that breast size and waist-to-hip ratio in women are associated with oestrogen levels: those with a lot of the hormone sloshing around in their bloodstreams also tend to have narrow waists and large breasts - that is, the kind of hour-glass figure that men in Western societies at least, allegedly prefer.
So it seems that oestrogen is linked with features that men will find attractive when looking for a mate and there are good biological reasons for this: high oestrogen confers not only femininity in women, but also high fertility.
"It would be beneficial from an evolutionary perspective for men to choose feminine women as mates," says Ms Law Smith. "Men who picked more feminine females would have ended up fathering more babies and passing on more of their genes, so this preference would have become instinctual.
"Women with high levels of oestrogen are able to conceive more quickly."
Research has also shown that oestrogen levels are associated with the quality of a woman's eggs and the lining of the womb. Low levels of oestrogen make the cervical mucus difficult for sperm to penetrate thereby reducing the chances of conception.
Oestrogen does a lot of its work on a female's face and body shape during puberty when she becomes more "womanly" in appearance, but the hormone carries on having an effect on looks later in life. Last year, Craig Roberts of Liverpool University and co-researchers demonstrated that facial attractiveness in women increases around the time of ovulation, when they are most likely to conceive. This is also the time of the month when oestrogen levels are highest. While short-term fluctuations in circulating hormones are clearly not able to affect the shape of a woman's face or the size of her eyes, Ms Law Smith suspects that oestrogen could have an influence here.
"Oestrogen has soft tissue building properties, so that even short-term increases in its level could enhance skin texture, for example."
This could explain why women using contraceptive pills or HRT sometimes report an improvement in their skin quality.
So, oestrogen has positive long-term and short-term effects on a woman's attractiveness. This begs the question - why don't all women have high levels of the hormone? There must be a cost. No-one knows the answer to this for sure, but some researchers suspect that oestrogen has an immunosuppressant quality, so that only the healthiest women can afford to have high levels of the hormone.
If this is the case, then, intriguingly, oestrogen is an indicator of biological quality in women just as high testosterone levels indicate quality in men. Testosterone is known to be a damaging chemical and only very healthy men can tolerate high levels. Ian Penton-Voak of Bristol University and his colleagues demonstrated that men with high levels of testosterone in their saliva were judged to have more masculine faces with bigger jaws and heavier brows. It follows then that such masculinity is likely to be an indicator of health and high genetic quality, and we might expect women to be particularly attracted to such men.
In some circumstances this is, indeed, the case. However, testosterone can confer more than just a masculine looking face.
Male courtroom lawyers and firefighters tend to have high testosterone levels. "There seems to be a link between high testosterone and dominance in men," says Dr Penton-Voak. "There are also huge amounts of data linking high testosterone men with marital discord and divorce, but we are not clear about the direction of causality here."
While masculine looking men are likely to have good genes that can be passed on to their children, the less macho types may be more reliable husbands and fathers. So it's not surprising that scientists have discovered that women prefer masculine faces when they are looking for short-term relationships, but go for slightly feminine looking men's faces when they are in the market for long-term commitment.
But the twist in the tale comes when women are ovulating and therefore more likely to get pregnant, and, as it happens, more likely to desire sex. Then women prefer the looks of a high-testosterone masculine male. And, of course, this is also the time when a woman's oestrogen is in full flow and she's at her most attractive to men. Like I said, hormones have a lot to answer for.