MHC-correlated odour preferences in humans and the use of oral contraceptives:
Previous studies in animals and humans show that genes in the major histocompatibility complex (MHC) influence individual odours and that females often prefer odour of MHC-dissimilar males, perhaps to increase offspring heterozygosity or reduce inbreeding. Women using oral hormonal contraceptives have been reported to have the opposite preference, raising the possibility that oral contraceptives alter female preference towards MHC similarity, with possible fertility costs. Here we test directly whether contraceptive pill use alters odour preferences using a longitudinal design in which women were tested before and after initiating pill use; a control group of non-users were tested with a comparable interval between test sessions. In contrast to some previous studies, there was no significant difference in ratings between odours of MHC-dissimilar and MHC-similar men among women during the follicular cycle phase. However, single women preferred odours of MHC-similar men, while women in relationships preferred odours of MHC-dissimilar men, a result consistent with studies in other species, suggesting that paired females may seek to improve offspring quality through extra-pair partnerships. Across tests, we found a significant preference shift towards MHC similarity associated with pill use, which was not evident in the control group. If odour plays a role in human mate choice, our results suggest that contraceptive pill use could disrupt disassortative mate preferences.
It's Open Access, you can read it yourself. I've gotten a few emails inquiring about this paper, and Darwin Catholic pretty much baited me into posting on this. There's been a fair amount of treatment of the results in the media as well. This isn't the first paper of this kind, there's a large genre of work (copiously referenced above) on the relationship between smell, MHC and female attraction to men. To restate the basic findings here: taking the birth control pill seems to have resulted in a shift of women toward greater preference of those with immune profiles similar to themselves. We already know that women have very strong preferences for individuals of the same race as compared to men, so a follow up on male preferences at some point would be interesting. Perhaps men don't care what a woman smells like?
As alluded to in the paper, and emphasized in the popular treatments, there's a good reason why one might get concerned about assortative preference of this sort: more homogeneous immune profiles will be the outcome in the progeny, and those progeny will exhibit greater vulnerability to disease. This insight about the necessity of immunological diversity in the face of the varied pathogens extant is why the MHC loci are among the most diverse within the genome. Nothing ever goes extinct because fitness is negatively frequency dependent. That is, the lower the frequency of an allele within a population, the greater its fitness. Normally there is a turnover of genes as they go extinct and are replaced by neutral variants, but the alleles around the MHC are very old. Many of them even span species, so that you may share an allelic variant with a chimpanzee which you do not with your own sibling.
The character of your immune portfolio is very important, ask the native populations of North America. It also implies that over evolutionary time you would expect that there would be some proximate biological adaptations to foster the maintenance of diverse immune profiles within offspring; something that would not occur as much with a great deal of incest. In many species the proximate adaptation is keyed in to chemical signals. In The Blank Slate Steven Pinker referred to data which shows that Israelis raised in communal environments tended not to marry, while in Taiwan betrothed girls adopted into the families of their future husband tended to have more than expected marital issues. That implies that these proximate adaptations aren't simply a matter of chemical recognition of identity by descent, since these were cases where the individuals were unrelated by living together. But it suggests the general principle and shifts our priors in terms of our expectation of how much "free will" we have in determining our significant others. The die is loaded.
On the other hand, I think you can go a little far in extrapolating in a linear fashion like this. After all, brother-sister marriage persisted for centuries, likely a millennium, in Egypt. Data from Iceland suggests that 3rd and 4th cousins who marry are the most fertile. Then there is always the issue of mother-fetus incompatibility. There is some evidence that too diverse a pairing might increase rates of miscarriage, just as first-degree relative pairing does. Many of the other results in this area have been equivocal; there are results which suggest that women prefer men who carry their paternally transmitted alleles for MHC! Many species have optimal points of genetic distance; you smell too different and you ain't hot, you smell too similar, and you ain't hot.
This is not to say that I don't think this work is invalid or not of worth, I just think we also need to be very cautious because these can be very complicated questions and dynamics we're addressing. The authors know this, they tried to limit the social composition of their study group to eliminate confounds, but they can't be sure what sort of structure lurks in there. All things being equal it might be better to pair up with someone with a different MHC profile, but perhaps all things are rarely equal these days. Perhaps those with different MHC profiles tend to be very different socio-cultural backgrounds, which might cause more tension in a relationship than the relative lack of zing within one where there's some bland homogeneity.
Finally, there's the issue of Playing with Nature. Darwin alluded to the popularity of this paper in the Catholic blogosphere. I assume part of the reason is that Darwin and his fellow travelers, traditionalist Catholics, reject birth control for normative reasons and appreciate that there may be other more directly utilitarian implications in the choices they make. That's fair enough, and the bigger picture at the heart of this is I think conservatism and respect for the organic development which precedes us historically and which we take for granted. First rule, do no harm! This sort of rhetoric is generally assumed to be Burkean conservative, but it really isn't. A great swath of the modern Left embraces this principle in the form of environmentalism and a general predisposition to not meddle with Mother Nature's handiwork. In the United States conservatives tend to be cautious about society, liberals tend to be cautious about the environment, while libertarians are cautious about neither. In much of the rest of the world where classical liberalism is less robust a force a more generalized cautiousness and conservatism is not unknown (remember that it was the Left-wing Communists who caused the greatest environmental catastrophes, while Romantic inspired reactionaries were prominent in back-to-nature movements).1
I suspect that the birth control pill and a host of other medications we take affects the choices we make. I also think that the tendency toward obesity and the changes in diet probably affect us in ways we don't understand. The chemicals we are bathed in, the rapidly shifting social matrices in which we are thrown into at a young age, etc. There's a lot of free variables floating around out there, and I think you can pick your poison if you want to be truly cautious. So I suppose I acknowledge that we're riding a tiger, but I really don't see any point in being a trad of either the Left or the Right because I don't think we have any ability to turn around, at least as a whole culture. Of course, there will always be a place for Naturals....
1 - The prominent eugenicist and white nationalist theorist Madison Grant was a prominent conservationist, and after his America First period Charles Lindbergh turned to environmentalist causes as well.
From FuturePundit:
But doesn't the MHC attraction mechanism work for men as well? They won't be on the Pill. Are men finding that funny-smelling women who are on the Pill are hitting on them? Someone should do a study on this.
Libertarians have more "respect for the organic development" of society than conservatives do.
I've gotten a few emails inquiring about this paper, and Darwin Catholic pretty much baited me into posting on this.
When it comes to getting a sane take on something having to do with the evolutionary aspects of mate selection, you're the first person I think of. (Though I'm disappointed there wasn't a way to tie this in with some good picture of inter-racial ancestry babes.)
Darwin alluded to the popularity of this paper in the Catholic blogosphere. I assume part of the reason is that Darwin and his fellow travelers, traditionalist Catholics, reject birth control for normative reasons and appreciate that there may be other more directly utilitarian implications in the choices they make. That's fair enough, and the bigger picture at the heart of this is I think conservatism and respect for the organic development which precedes us historically and which we take for granted. First rule, do no harm! This sort of rhetoric is generally assumed to be Burkean conservative, but it really isn't. A great swath of the modern Left embraces this principle in the form of environmentalism and a general predisposition to not meddle with Mother Nature's handiwork.
It seems to me (speaking from the biased vantage point of the inside) that there may be something of a difference between these two -- though there are also some conscious blendings as with Rod Dreher's "Cruncy Conservatism" thing.
Looking at the intellectual justifications for Catholics sexual morals -- they're generally rooted in a concept of teleology: sex is meant to involve a procreative aspect; marriage is meant to be between members of the opposite sex and thus capable of reproduction, etc. Pop environmentalism, on the other hand, seems to be rooted in more of a romantic attachment to "untouched nature" and is thus arguably more conservative.
However, a lot of individual people's attachment to traditional morality is probably conservative in the Burkean sense, in that many successfully convince themselves (with some truth and a great deal of illusion) that "back in the old days" everyone followed this approach to sexual morality, and the rebellion against them is "new".
There's a lot of truth to arguing that the US and Europe have moved from being a Christian culture to a post-Christian culture over the last 200 years, but looking at the wider swath of world history I think it's reasonable to argue that few cultures have (if looked at in detail rather than by what they profess their ideals to be) actually lived up to those precepts to any great degree. So I'm not sure that asserting them is ever exactly "conservative". (I seem to recall some interesting primary source stuff in a book called Tom Brown's World -- about the context of Tom Brown's Schooldays -- from the 1840s, when the modern trend which led towards Evangelical Christianity was in its infancy, essentially making a Burkean argument that it was unreasonable to expect people to actually live according to what is now called "traditional" morality.)
Connect these dots with the reports of biological impacts from estrogen-mimicking chemicals in the environment, and we get, um, total breakdown of the family unit in mad incestuous orgies!
Therefore, ah -
A) Extrapolation is fun!!!
B) For the first time, I find reason to regret being an only child...
Looking at the intellectual justifications for Catholics sexual morals -- they're generally rooted in a concept of teleology: sex is meant to involve a procreative aspect; marriage is meant to be between members of the opposite sex and thus capable of reproduction, etc. Pop environmentalism, on the other hand, seems to be rooted in more of a romantic attachment to "untouched nature" and is thus arguably more conservative.
there is something to this, but if you scratch the surface many "new agers" are pretty teleological. i think this is probably due to the fact that humans are naturally teleological; browse the cultural creatives for what i'm talking about.
I have always felt that smell has very little to do with sexual attraction in the sense that the basic shape and nature of sexual attraction has been robust to absolutely massive changes in the smells of humans over the lifespan of people now alive.
As recently as 80 years ago many people virtually never bathed, were louse-ridden, pickled in woodsmoke, were sewn into their clothes each year etc - yet now in the same cultures (maybe the same people) most shower daily, change clothes daily (or more often) shave their armpits and wear scent and antiperspirant.
Even in the past couple of decades, the massive decline in tobacco smoking including bans has made smells of all sorts much more obvious to many people.
Yet over this timespan men are attracted to visual cues of youth and health; women attracted to status etc. No change there.
Maybe smell has a minor role in sexual attraction, maybe not, but even if it does I suspect the effect is swamped in the real world. And with such a small effect it is hard not to be measuring confounders by mistake.
I would say that smell most definatly has a large part in the choice of a partner, both consciously and unconsciously. However, one big thing seems to have been left out in the wild worried extrapolations from this survey (oh noes! woman will marry their brothers! populations will divide! quick, get them off the pill etc) which is that women who go on the pill tend, on the whole to be already *in* a stable relationship.
Woman who just want to have fun in nightclubs, or who are single and searching tend not to be on the pill as a) it takes a week of use for it to start working properly b)condoms are easier and c)if they're single there's usually no need. A more useful follow-up survey would therefore be to look at how many women change partners after getting on the pill, and if so who they change in preference for.
In a study done by Dr. Weisfeld (Olfaction based mechanisms in human kin recognition and inbreeding avoidance), mutual olfactory aversion occurred only in the father-daughter and brother-sister nuclear family relationships. Recognition occurred between opposite-sex siblings but not same-sex siblings. Thus, the results indicate that olfaction may help mediate the development of incest avoidance during childhood (the Westermarck effect).
Birth control may change female olfactory preference but to what degree and is it strong enough to change the westermarck effect? In other words, will sisters find there brothers odor so desirable that they will resist years of incest avoidance development? It seems to me that since "data from Iceland suggests that 3rd and 4th cousins who marry are the most fertile," birth control may mediate female attraction towards there distant family and could be a good thing in a world were diversity is the norm and differences in MHC are far too great for a perfect match anyways. As weird as it sounds, birth control may be a good thing in a diversified country. Lucky for me i am not on birth control, but maybe I should look out for any changes in my adolescent cousins behavior towards me..hum..lol
taking the birth control pill seems to have resulted in a shift of women toward greater preference of those with immune profiles similar to themselves
Sorry for the late comment, but before generalising from this study, please look at the methods more closely. The much-publicised effect only brought the test group more or less to the level of the control group! In other words, many women naturally had the same preference as the pill group! In addition, the study failed to replicate the findings of most previous studies about women preferring (moderately) MHC-dissimilar men:
"In contrast to some previous studies, there was no significant difference in ratings between odours of MHC-dissimilar and MHC-similar men among women during the follicular cycle phase."
Very interesting discussion. I just have one question though, what does all this have to do with Natalie Portman and why is her picture posted here?