How many times have you heard this and rolled your eyes?
“We just have so much chemistry.”
“There just wasn’t any spark.”
“It’s like we were made for each other.”
All clichés aside, chemistry has indeed been shown to play a crucial role in romantic attraction. However, these natural mechanisms underlying attraction may be in peril. Recent studies suggest that the use of hormonal contraception is fundamentally changing the chemistry of attraction between men and women.
How Hormonal Contraception Works
In recent years, the use of hormonal birth control has increased dramatically and is now used by over 12 million women in the United States alone. Hormonal contraception works by manipulating the release of hormones in the body, thereby preventing the release of the egg into the uterus. Normally, estrogen and progesterone levels fluctuate cyclically in a woman’s body; these hormone levels surge around the time of ovulation and drop significantly during menstruation. Upon fertilization of an egg, however, the levels of estrogen and progesterone remain high. Hormonal birth control exploits this phenomenon by inducing the release of estrogen and progesterone and artificially keeping hormone levels high, thus tricking the body into thinking it is pregnant and causing cessation of ovulation.
How We Choose Sexual Partners
Natural selection dictates that “fit” genes are the driving force behind evolution. Yet, when people decide which individual at the club they will grace with their amazing dancing abilities, and possibly sexual trysts, they are not likely to be contemplating whether that individual is a genetically sound partner. Yet everyone unconsciously looks for good genes in a partner. Heterosexual men tend to be attracted to women with large breasts and hips, full lips, and a clear, youthful complexion. These features are all nature’s none-too-subtle ways of displaying to others that a woman is healthy and fertile.
Like females, males also have many ways to advertise favorable genes to potential partners. Male peacocks, for example, are known for their large and brightly colored plumage, which they use to attract mates. The heavy plumage, however, makes peacocks easy pickings for predators. Scientists term this type of display as “costly signaling.” In taking the huge cost of making himself more vulnerable to predators, the male is essentially telling the female that his genes are so good that he can survive despite the handicap.
Surprisingly, human males employ a variant on this exact same tactic. The qualities women tend to find attractive in men, such as prominent brow ridges, jaws and cheekbones, are signs of high amounts of testosterone. And like the peacocks’ feathers, a high amount of testosterone is actually a handicap. Despite having many vital functions, testosterone is also an immunosuppressant — it lowers the body’s ability to fight disease. Men and women need certain levels of testosterone to function properly; however, excessively high levels of testosterone bring with them an excessively high amount of immunosuppression. Men with high testosterone are displaying to women what wonderful genes they have: if they can sacrifice immunoincompetence for these exaggerated secondary sexual characteristics, the rest of their genetic complement must be very strong.
In addition to physical appearances, women have other, less obvious mechanisms of identifying men with genes to complement their own. For example, it is ideal to have a highly-varied immune system that is well-equipped to fight many different types of disease. One of the important components of the immune system is the major histocompatibility complex (MHC). In one revealing study, women smelled T-shirts that had been slept in by men and were asked to rank the attractiveness of the individual purely based on T-shirt smell. Interestingly, women consistently ranked males with MHC genes that differed most from their own as most attractive, thus further supporting the role genetics and chemistry play in attraction.
Natural Changes In Preferences
A study published in Nature showed that women’s facial preferences in men actually change in a predictable fashion throughout the menstrual cycle. In the study, heterosexual women were asked to pick whether a masculinized or a feminized version of a male face was more attractive. During times of high probability of conception, women generally chose the more masculine face; however, during times of low probability of conception, women generally chose the more feminized face. One evolutionary explanation for this phenomenon is that in order to produce the most viable offspring and thus increase reproductive success, a woman will seek a partner with more evolutionarily advantageous genes, as indicated by masculine secondary sexual characteristics. However, in order to have a better chance at successfully raising the child, when she is not trying to conceive, she will seek a long-term partner who will stay with her and be a good father, which is associated with a more feminized face.
On the other end, men may also be able to determine when women are most likely to conceive. Many species have an estrus cycle, in which a female displays some signal to males that she is able to conceive. Researchers at the University of New Mexico recently showed that this phenomenon is also displayed in humans through a study of professional lap dancers. Intriguingly, researchers found that professional lap dancers earned significantly more in tips when they were most fertile, with an average of $185 per night during menstruation, $260 during the luteal phase, and $335 during estrus. These studies suggest that men may be able to unconsciously sense when women are most fertile and that women may unconsciously advertise this fact, possibly by flirting and primping more.
How Contraception Changes The Chemistry Of Attraction
Shockingly, these studies also found that hormonal birth control seems to throw off the biochemical basis of attraction. In the facial preference study, experimental subjects no longer showed the cyclic variation in preference for faces with more masculine or feminine features when they were using hormonal contraception. More dramatically, in the MHC T-shirt study, women on hormonal contraception no longer preferred the more MHC dissimilar males; conversely, they preferred men with MHC more similar to their own. Scientists speculate that this shift is due to the preference of pregnant women to be around kin, who are more likely to assist them, and similarity of MHC could indicate kinship. However, in terms of mating, the offspring of two individuals with highly similar MHC are expected to have weaker immune systems. Finally, in the study of estrus in lap dancers, the women using hormonal contraception showed no estrus earnings peak, suggesting that hormonal contraception shuts down the body’s natural estrus signaling. It would seem from these findings that hormonal contraception is stopping and sometimes even reversing our natural tendencies in sexual attraction.
Hormonal Contraception And The Evolutionary Process
In light of these findings, it is worthwhile to inquire whether the changes are so evolutionarily harmful that women should stop using hormonal birth control. Responding to this inquiry is Laurie Santos, Associate Professor of Psychology at Yale University who teaches the immensely popular psychology course “Sex, Evolution and Human Nature.” Santos is highly distinguished in the field of psychology for her work on comparative cognition with non-human primates. When asked what the evolutionary impact might be on the human species as a result of differing mate preference in women using hormonal contraception, Santos asserted, “The honest answer is that we just don’t know the impact hormonal birth control will have on human mating strategies … birth control hasn’t been around long enough to assess what, if any, real impact it’ll have on our species’ mating preferences down the line.”
Furthermore, there are conflicting opinions about the extent to which the biochemical basis of attraction plays a role outside the laboratory. Certain data suggests that these signals are likely to be largely insignificant in our ultimate choice of a partner. In a variant of the facial preference study, women were asked to pick which face they preferred for either a one-night stand or a long-term relationship. Researchers found that when asked to pick the face of a man with whom she would like to have a one-night stand, the degree of feminization of the face differed depending on where the woman was in her menstrual cycle. However, when asked to pick the face of a man for a long-term relationship, preference was not subject to this variation. Other studies would imply that biochemistry actually has a large impact. In the MHC T-shirt study, for example, women often commented that the smells they rated as more attractive reminded them of the smell of current or past boyfriends.
Santos sums up these results very well: “As scientists, we’ve only realized relatively recently that these biochemical signatures have any effect at all, so it’s still an open question how much these cues are really controlling our mating preferences. I’d say there are still lots of big, open questions that we’ll be wrestling with about the future of human preferences.”
It seems that for now, the jury is still out. However, in terms of reproductive success and natural selection, your offspring are much more likely to thrive if you have them at the point in your life that you are most prepared to care for them. In other words, it might not be time for women who favor hormonal birth control to throw away their pills just yet.