Like multitudes of other women across the globe, I've been using oral contraception for a good few years, but although the doctors who handed me prescriptions for these tiny discs of sexual freedom were always careful to point out the health risks and check my blood pressure, none of them ever told me about the effect of the pill on how I look, how much I fancy my partner, or how jealous I am.
It turns out that the use of hormonal birth-control impacts on our sexual desire, our relationship satisfaction and our attractiveness in a multitude of ways, but before you rashly chuck your contraceptives into the bin, take note that the effects can be positive as well as negative.
Seems confusing? Scientists certainly think so, and studies of pill effects have yielded mixed findings, but new research presented by Stirling University's Kelly Cobey at last month's meeting of the European Human Behaviour and Evolution Association has finally shed light on the puzzle. It turns out that the important issue is not simply whether you're on the pill or not, but whether you change your use of the pill after embarking on a relationship.
Cobey and her colleagues recruited 365 couples from visitors to the Glasgow Science Centre and questioned men and women from each couple individually. Subjects answered questions on the women's use of oral contraceptives both when they met their partners and currently, and they were also quizzed on various aspects of their relationships and sex lives.
The team found that women who changed their pill use after they met their partners reported less sexual satisfaction and less sexual desire for their men than those who continued either using or not using the pill, and this was the case regardless of the direction of change, that is whether they went on to or came off the contraceptives. The researchers have dubbed this "the congruency effect".
There was no issue, though, with other non-sexual stuff; changing pill use didn't have any impact on how fed-up women were with their blokes' ability to do the washing up or how happy they were with his fathering skills.
But how is changing pill use messing with our sexual desire?
Time for a bit of background: When not using the pill, naturally cycling women experience an extra surge of the hormone oestrogen for the few days of the month around ovulation when fertile, and during this time we look better, smell better and even have sexier voices. In a now classic study, Geoffrey Miller and his colleagues demonstrated that lap dancers who were at peak oestrogen earned a lot more than other dancers.
The pill introduces synthetic hormones and knocks out our natural oestrogen which, disappointingly, just flat-lines at a low level without the mid cycle peak in attractiveness. How unfair is that? If you're on the pill and planning to take up lapdancing you'll probably earn a fair bit less than your colleagues. Always something to bear in mind.
Anyway, this peak in oestrogen experienced by non pill users also comes with changes in what we find sexy in a man, and when oestrogen levels are high and women are fertile, they tend to be most attracted to relatively masculine, dominant men.
It stands to reason then if that our hormones are subtly influencing the type of man we find attractive and we then mess with our hormone levels by taking the pill, we're likely to find a change in our sexual desire for our partner, which is exactly what the research found. And in case you're wondering, men's sexual satisfaction didn't change with their partners' pill use, so any changes in women's attractiveness haven't had a big effect.
The researchers think the congruency effect might explain other anomalies of relationship functioning. For instance, Cobey and her team found in a study of Dutch couples that women who changed their pill use during their relationship became more sexually jealous.
There's no doubt that the pill has brought massive benefits, empowering women and giving them much greater control over their lives, but there has been much debate about its effect on libido and sexual satisfaction.
Craig Roberts, who led the research says: "These results show... what seems to be important is whether a woman's current use matches her use when she began the relationship with her partner. We hope our results will help women understand why they might feel the way they do about their partner when they change use."
The study is published this week in Psychological Science.