In a review article in the journal Trends in Ecology and Evolution, University of Sheffield researchers Alexandra Alvergne and Virpi Lummaa [1] present a range of evidence that using oral contraceptives influences women’s preferences for mates, and men’s sexual interest. Some of the research shows that women’s preference for human odors while taking the pill are for odors from men who are more likely to be close relatives, with similar variants of the majorhistocompatibility locus (MHC), but that women who are ovulating prefer odors from men who are less similar. There is speculation that the degree of similarity in the MHC locus may affect fertility.

Further evidence comes from a study by Geoffrey Miller and colleagues, looking at men’s willingness to pay professional lap-dancers for sexual access over the menstrual cycle. This was a difficult study to do, and assumed that ovulation occurred on a standard day during the menstrual cycle, which we know is not valid, because women vary in ovulatory timing and may not ovulate in otherwise normal menstrual cycles. Nonetheless, the average curves for women selling sexual access through lap-dancing show a menstrual pattern in profits that is strikingly similar to the pattern of estrogen over the menstrual cycle. Women taking oral contraceptives both made less money, and did not show the same pattern as those who were naturally cycling.

Men's willingness to pay for sexual access to lap dancers by menstrual cycle day. From Miller (2007)

There is further work that needs to be done here to connect all of the dots, but the information is interesting and points to an unsuspected role of the menstrual cycle in women’s (and men’s) lives.

What I would like to know is this:
1. How much of a difference does the degree of MHC similarity make to the average couple’s fertility? Are those who are similar in MHC overrepresented in fertility clinics?
2. Is there an effect of odor preferences on what we think of as “chemistry” in a relationship? On conflicts and marital harmony? On sexual behaviour? I am thinking of some of the excellent work done by John Gottman in Seattle, analyzing couple interactions and predicting relationship success. Do couples who start on the pill differ from those who don’t? Does that relationship dynamic change when pill use changes?
3. What happens when a woman stops using oral contraception? Do her preferences change? How (if at all) does that show up in the relationship?
4. And what about other relationships where odor-based mate choice is blocked, such as internet dating? Are there similar issues?

The bigger picture for me is this – science knows very little about the many roles and effects of the naturally cycling menstrual cycle, and replacing that naturally occurring cycle with oral contraceptive pills is likely to be having other effects that we will come to know in the future.

Control over our own reproduction is a very important part of women’s rights, health and well-being. However, oral contraceptives are not the only effective method of birth control. In addition to a variety of rare but serious side effects, they also leave women at risk of sexually transmitted infections, interfere with peak bone acquisition in younger women, and are the highest risk factor for young men not using condoms during intercourse. Perhaps it is time to rethink the wisdom of widespread and routine use of oral contraceptives, and to recognize the value and the unexamined effects of the menstrual cycle.

References:
1. Alvergne, A, Lummaa, V: Does the contraceptive pill alter mate choice in humans? Trends in Ecology and Evolution, to appear in 2009.
2. Miller G, Tybur J, Jordan B: Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior 2007, 28:375-381.