Birth control pills solve a lot of problems for us, the most obvious being control over if and/or when we have children. They’re useful for a variety of other things too, but that’s not under discussion here today.

What is? How they can alter a woman’s sense of smell *, potentially causing her to screw up when choosing Mr. Right.

A few years ago, researchers gathered together a bunch of ovulating women and had them sniff the sweaty armpits of T-shirts worn by a number of different men, asking them to rate which ones were most appealing.

Let’s pause for a moment and imagine THAT project for a moment.

Ready?

Then they compared the MHC genes of the women to that of the men they preferred – and found women tended to chose the sweaty armpits of men who had different MHC genes from themselves.

I know you’re wondering: “Am I supposed to know what MHC genes are?”

No. Bear with me a moment.

Then they gathered up a bunch of women on hormonal birth control (non-ovulating women) and repeated the experiment. This time, the women preferred the men with MHC genes most like themselves.

Hmm.

Okay. So what does this mean?

You’re all aware you shouldn’t marry your brother, your cousin, and probably not even your second cousins. Why? Because – eww. But also because they have very similar genetics, increasing the chance for certain genetic diseases. So it makes sense that a women would be biologically hard-wired to choose a man genetically different from herself, right?

MHC genes.

Deep breath.

What are MHC genes? Major Histocompatibility Complex… genes that are responsible for displaying bits and pieces of proteins in your body to its own immune system.

If a parasite (or a bacteria or a virus) invades, stuff inside the cell chews it up and… well it doesn’t quite spit it out. It’s more like security grabs it tightly by the wrists and escorts it to the surface of the cell where official White Blood Cells (WBCs) wearing suits appear to ask for I.D. And when that chewed up bit of protein can’t account for its presence, it’s summarily executed – and the WBCs draw their weapons and go looking for any similar enemies that might be hanging out in neighboring cells.**

So why do you want diversity in your MHC gene collection?

The more MHC genes you have that are different from each other, the better you will detect these foreign invaders, and the more likely you are to survive any number of invasions.

So those ovulating women sniffing armpits? They were choosing men, sight unseen, who would make the best father for their baby. (Whether or not they wanted children.) It appears we’re all programmed to look for a mate with the best chances of producing offspring with a wide range of differing MHC genes. It’s hard-wired.

What about the women on birth control pills? Well, the hormones in the pills prevent ovulation by mimicking pregnancy. Those women – whose bodies thought the deed was already done – were choosing men more like themselves, like family. Those men most likely to take good care of them while they brought a child into the world.

And while we are not the sum of our hormones, while we are not controlled entirely by our biochemistry, another study did show that once women stop taking birth control pills, they can potentially find their partner suddenly unappealing. Stinky. As in not-so-sexy-what-was-I-thinking? And such women are more likely to leave the relationship or stray.

As if finding Mr. Right wasn’t complicated enough already.

*Nerve O (nerve zero) is a cranial nerve that travels directly to the brain, bypassing the nose and hooking directly into a portion of the brain that regulates sexual behavior. It may be the nerve through which pheromones are processed and may play a role in such unconscious choices.

**It’s a good thing, setting aside autoimmune problems and donor organ situations. Immunology was its own class in graduate school. It’s a whole degree in which people subspecialize and then subspecialize again. And again. We’ll keep this simple here so our brains don’t explode, okay?

Want to read more? Birth Control Pills Affect Women’s Taste in Men.