When choosing a partner to have children with, it is only natural to desire “Prince Charming” or “Cinderella,” who may pass on their beneficial genetic qualities to future kids. Given that better genes increase the offspring’s survival and reproduction chances, mechanisms that detect “genetic quality” should have evolved to lead people to be sexually attracted to “knights in shining genes.”

One such cue for mate suitability is odor, which signals compatibility between potential mates’ immune systems. Specifically, odor indicates the extent of overlapping between potential mates’ immune systems, such that more attractive odor signals less overlap between mates’ immune systems. The larger the dissimilarity between mates’ immune systems, the more threats the immune system can combat.

Offspring would thus benefit the most from having parents with dissimilar immune systems, which results in an enhanced immune system. Accordingly, women have evolved to be sexually attracted to men with a dissimilar immune system, primarily during high-fertility cycle phases. Using the “sweaty T-shirts paradigm”, in which female participants sniff T-shirts recently worn by males, studies have demonstrated that women indeed tend to prefer the scent of partners with dissimilar immune system over that of partners with similar immune system1.

Unfortunately, contraceptive pill use interferes with mate selection and reverses the natural preference for mates with dissimilar immune system, such that women prefer the odor of partners with similar immune system over that of partners with dissimilar immune system while on contraceptive pills2. This shift in preferences corresponds to the one occurring across the menstrual cycle. In particular, naturally cycling women experience male preference shift throughout their menstrual cycle that helps them obtain resources relevant to their current fertility status (fertile versus infertile).

During the fertile phase of the menstrual cycle, women seek genetic benefits for their offspring and are thus more attracted to men whose features indicate such benefits (e.g., masculine faces, bodies, and voice). In contrast, during the infertile phase of the cycle, women seek cues of high investment in parenting and partnership, which are typically associated with fewer masculine features3.

Women tend to prefer immune system similarity during the infertile phase for similar reasons: Its association with the smell of genetic relatives who may assist them during pregnancy. Because contraceptive pills introduce hormones that prevent ovulation and lead to temporary loss of fertility, women’s natural preference fixates on similarity as in natural infertile phases of the cycle. In particular, normally cycling women (women who are not using contraceptive agents) tend to rate men with dissimilar immune system as more physically attractive, whereas women using contraceptive pills tend to rate men with similar immune system as more physically attractive2.

The shift in women’s mate preferences, which occurs because of contraceptive pill use, and involves switching to preferring poorly-fitting partners, may adversely affect the adaptability of their children’s immune system. A recent study4 examined whether children born to couples who met during regular contraceptive pill use would display more symptoms associated with a weaker immune system (e.g., being infection-prone, needing more medical care) in comparison with children whose parents met when the mother was not using contraceptive pills.

One-hundred-ninety-two women aged 22 to 48 (M = 33.51, SD = 5.21) participated in the study. All participants reported being the biological mother of a child 1-8 years old. This age range was selected because children in their early years are frequent users of health services. Sixty-one women reported that they met their children’s father while using contraceptive pill, whereas 119 women reported that they were not using contraceptive pill while they met their children’s father. The participants provided information about their child’s health, including their children’s tendency to get sick, their overall health, overall relative health (in comparison with other children their age), child’s relative speed of recovery when sick, number of visits to a medical institute to receive medical treatment, and number of hospitalizations.

Results have revealed that children to mothers who were on the pill are more infection-prone, require more medical care, suffer from a higher frequency of common sicknesses, and are perceived as generally less healthy than children whose parents met on non-pill circumstances. These findings indicate that a key factor in securing children’s future might be traced to a choice people made years before their children were born: the decision to use a contraceptive pill.

The implications of these findings are profound as the use of contraceptive pills is widespread and still growing. Sixty-two percent of all US women in their reproductive ages are currently using a contraceptive method. The aftermath of these numbers is gloomy: The immune system of current-generation children might be more fragile than that of our ancestors, leaving the current and future generations more susceptible to pathogens and more dependent on medical care as its effective line of defense.

1Saphire-Bernstein, S., Larson, C. M., Gildersleeve, K. A., Fales, M. R., Pillsworth, E. G., & Haselton, M. G. (2017). Genetic compatibility in long-term intimate relationships: partner similarity at major histocompatibility complex (MHC) genes may reduce in-pair attraction. Evolution and Human Behavior, 38(2), 190-196.

2Wedekind, C., Seebeck, T., Bettens, F., & Paepke, A. J. (1995). MHC-dependent mate preferences in humans. Proceedings of the Royal Society of London B: Biological Sciences, 260(1359), 245-249.

3Thornhill, R., & Gangestad, S. W. (2008). The evolutionary biology of human female sexuality: Oxford University Press.

4Birnbaum, S., Birnbaum, G. E., & Ein-Dor, T. (2017). Can contraceptive pill affect future offspring’s health? The implications of using hormonal birth control for human evolution. Evolutionary Psychological Science, 3(2), 89-96.

Prof. Gurit Birnbaum works at the Baruch Ivcher School of Psychology, the Interdisciplinary Center (IDC) Herzliya (Israel). Her research focuses on the underlying functions of sexual fantasies and on the convoluted role played by sexuality in the broader context of close relationships.