Current methods of contraception have awarded women unprecedented control of their fertility. Yet some form of oral birth control pills are nearly universally consumed by women of childbearing age, and often for reasons other than sexual activity. Oral contraceptives can greatly reduce ovarian cancer rates, even over long-term periods. The estrogen in many over-the-counter birth control pills also decreases testosterone, which helps clear the skin by eliminating the oil testosterone would otherwise produce. Oral contraceptives also reduce the buildup of the uterine lining, which means that women on the pill typically have considerably lighter periods with less bleeding and fewer cramps, and some women may have fewer periods altogether.
With all these clear benefits, it may be hard to convince both women and men of how profoundly oral contraceptives affect partner selection, as well as what those implications mean for couples who plan to stay together with the possibility eventually having children.
Birth control’s effects on potential offspring center on the genes that control the major histocompatibility complex (MHC), which in turn plays a fundamental role in how the human immune system develops. Over the thousands of generations of human evolution, women have evolved to prefer a major histocompatibility index in a mate that is significantly different—though not vastly different—from their own. This ensures that potential offspring have the greatest chance at developing a healthy immune system. Differences in a couple’s MHCs also indicate that familial relation is unlikely, a kind of in-born defense against inbreeding that would further weaken the development of the immune system.
During pregnancy, however, a woman’s MHC preference shifts to favor partners with considerably more similar major histocompatibility complexes. Scientists have hypothesized that this is because during pregnancy and its attendant hormonal changes, women in humanity’s ancestral environment benefited from the care of available relatives. Preferring a similar MHC might have made women more receptive to their relatives’ care, in addition to shifting women’s genetic preferences away from high-testosterone males and toward more nurturing caregivers.
That’s all well and good for ancient humans, but how does the MHC problem affect modern dating and long-term relationships? Essentially, oral contraceptives prevent pregnancy by “tricking” the body into thinking it’s already pregnant. This means some of the attendant hormonal changes that occur during a natural pregnancy are continuously present in women who take birth control. A woman selecting an MHC-similar partner while on birth control, for example, is more likely to report sexual dissatisfaction if she stops taking the pill during the couple’s relationship. Women were also found to be more likely to cheat on MHC-similar partners. Hence, both for the emotional stability of modern relationships and for the biological health of future generations, it may be prudent to investigate birth control methods that do not cause such dramatic shifts in partner preference.