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Have you ever heart of the “sweaty T-shirt test?” No, not the quick pit-check you do after a morning at the gym to see if it’s still socially acceptable to run into the coffee shop or grocery store on your way home. I’m talking about the study performed by Swiss scientists in the nineties in which women literally sniffed a whole bunch of men’s sweaty T-shirts. Why did these women subject themselves to such an experiment (and unique form of torture)? For science, of course!

What the Swiss researchers found was that when it comes to the immune system, opposites attract. The women in the study were more attracted to the scents of the men whose major histocompatibility locus (MHC) genes (“one of the immune system's key markers of identity”) were more dissimilar to their own.

As it turns out, there is an evolutionary advantage to such an adaptation: the offspring of mice with genetically dissimilar immune systems tend to have stronger immune systems—and are therefore healthier—than the offspring of two individuals with genetically similar immune systems. Thus, it made good evolutionary sense that the rule of opposite attraction was observed in the Swiss sweaty T-shirt study: the scent of a man who had MHC genes that were too similar to the woman who sniffed his T-shirt was perceived as less appealing.

What stood out to researchers is that this was the case for all women, except for women who were on birth control.

For women on the Pill, the medication actually altered how they perceived the smell of a potential romantic partner, making them more likely to be attracted to men with genetically similar immune systems, rather than to those with immune systems that differed from their own. This has to do with the way that hormonal birth control “tricks” a woman’s body into believing she’s in a perpetual state of pregnancy, which (evolutionarily speaking) makes her more attracted to men with genetically similar immune systems—i.e., men who are more likely to be related to her—because of the “protection” such relationships can provide in her more vulnerable state.

Future effects on children

Recently, a Danish study made headlines by declaring that there was no evidence for birth defects in the children of mothers who had been on hormonal contraceptives immediately prior to, or during the first few months of, pregnancy. But another recent study went even farther back, before those children were in the picture, to determine whether women who were on birth control when they met their romantic partner produced less healthy offspring than women who met their partners while they were not on any form of hormonal contraception. The findings, reported in the journal Evolutionary Psychological Science, were astounding:

“Our findings show that children of women who were on the Pill during relationship onset are more often reported by their mothers to be infection-prone and require medical care, suffer from a higher frequency of common sicknesses, and are perceived as generally less healthy than their peers as compared with children of women who were not on contraceptive pills during relationship onset.”

Put simply, what researchers found was that, not only can being on the Pill alter your preferences for a romantic partner, but it might actually have downstream effects on the health of your offspring, too—even if they don’t come in the form of obvious birth defects.

Similarly, there is also some recent evidence that being on the Pill within six months of conceiving a child may also raise that child’s chances for developing leukemia. And the more we find out about the effect of hormones on the gut, the more we may also learn about the impact the Pill’s gut-wrecking effects may have on our future children.

It turns out that a little pill contains quite a lot of power over a woman’s future, her children’s futures, and the future of human evolution.

Safe and effective alternatives

Thankfully, there are ways for women and couples to plan their families that won’t mess with their (very helpful!) evolutionary adaptations—and which can be just as effective at pregnancy prevention as the Pill. Fertility Awareness-Based Methods, also known as modern methods of Natural Family Planning, have effectiveness rates comparable to those of hormonal birth control, especially when women and couples take the time and effort to learn a method from a certified instructor.

Women are increasingly turning to natural methods of family planning and cycle charting apps, not only to avoid birth control side effects, but to have greater confidence in their healthcare management. With this latest research in mind, they might also make the switch to protect their relationships and children as well.