The Pill and Mate Selection

This has been bouncing around the Catholic blogsphere due to being picked up by the blog at First Things, but I post it here in hopes of perhaps drawing a comment out of Razib or someone else with some more serious biological knowledge. It would seem that evolutionary psychologist Stewart Craig Roberts has a paper coming out in the current issue of the Proceedings of the Royal Society: Biological Sciences in which he presents data that women show different odor-based preferences in regards to men when they are pregnant and when they are on The Pill (which uses hormones to reproduce some effects of pregnancy, thus surpressing ovulation.)

While several factors can send a woman swooning, including big brains and brawn, body odor can be critical in the final decision, the researchers say. That's because beneath a woman's flowery fragrance or a guy's musk the body sends out aromatic molecules that indicate genetic compatibility.

Major histocompatibility complex (MHC) genes are involved in immune response and other functions, and the best mates are those that have different MHC smells than you. The new study reveals, however, that when women are on the pill they prefer guys with matching MHC odors.

MHC genes churn out substances that tell the body whether a cell is a native or an invader. When individuals with different MHC genes mate, their offspring's immune systems can recognize a broader range of foreign cells, making them more fit.

Past studies have suggested couples with dissimilar MHC genes are more satisfied and more likely to be faithful to a mate. And the opposite is also true with matchng-MHC couples showing less satisfaction and more wandering eyes.

"Not only could MHC-similarity in couples lead to fertility problems," said lead researcher Stewart Craig Roberts, an evolutionary psychologist at the University of Newcastle in England, "but it could ultimately lead to the breakdown of relationships when women stop using the contraceptive pill, as odor perception plays a significant role in maintaining attraction to partners."

The study involved about 100 women, aged 18 to 35, who chose which of six male body-odor samples they preferred. They were tested at the start of the study when none of the participants were taking contraceptive pills and three months later after 40 of the women had started taking the pill more than two months prior.

For the non-pill users, results didn't show a significant preference for similar or dissimilar MHC odors. When women started taking birth control, their odor preferences changed. These women were much more likely than non-pill users to prefer MHC-similar odors.

"The results showed that the preferences of women who began using the contraceptive pill shifted towards men with genetically similar odors," Roberts said....

"When women are pregnant there's no selection pressure, evolutionarily speaking, for having a preference for genetically dissimilar odors," Roberts said. "And if there is any pressure at all it would be towards relatives, who would be more genetically similar, because the relatives would help those individuals rear the baby."

So the pill puts a woman's body into a post-mating state, even though she might be still in the game.

”The pill is in effect mirroring a natural shift but at an inappropriate time,” Roberts told LiveScience.

Obviously this is just one factor in relationship dynamics, but it does strike me as interesting in that it seems http://www.blogger.com/img/gl.link.gifto me that birth control is a fairly culturally disruptive technology which generally speaking was taken up without a whole lot of thought about anything other than the obvious benefits.

It's also an example of the ways in which things we don't think of affect our feelings and actions. No one, I'm sure, would think, "Boy, my boyfriend just doesn't smell alluring anymore." (Unless, perhaps, she was about to tell him to go take a shower rather than plopping down on the couch next to her after returning from the gym.) But a thought of, "He just doesn't seem exciting anymore" or "We just don't seem to have a spark these days" might well include a response to senses that we do not actively think about.

UPDATE: Razib puts up a good post on the question here. And provides a link to the original study here.

Worth noting is the confluence of interests that gives this story so much play. In the mainstream press, it's a quirky result about something which nearly everyone takes -- probably good mostly for a laugh. "Hey, did you hear the one about how your girlfriend is more likely to dump you for her brother when she's on the pill?"

Meanwhile, in the small subculture of those who have rejected birth control, it serves as a bit of an "I told you so".

In the end, it strikes me as a bit interesting -- more as an example of how a physical reaction can unconsciously affect our personal choices than as a proof that women on the pill will form bad relationships. (After all, there's nothing that would necessarily make a relationship with someone who happened to have a more similar immunity profile a "bad relationship".) Much more concerning, if one is listing off reasons to be cautious of the wide use of birth control, is that having fertility be strictly optional removes the biological incentive from a lot of ancient social structures that we pretty much take for granted, and don't want to see go away.