Thursday, February 14, 2013

It's not you, it's my birth control

So, Valentine's Day, what better time to question the foundations of your relationship?

It's my brain that loves you (source)
Well, part of your relationship may be based on your Major Histocompatibility Complex (MHC) compatibility. The MHC is a cluster of genes that define which antigens get expressed on white blood cells. It is thought to control the ability of the body to recognize pathogens as 'other.' It is also thought that the more varied the genes in your MHC are, the more resistant to pathogens or parasites you are.

So what does the MHC have to do with your love life?
Well the most popular theory goes as such: If you want to have a healthy baby, you want to give it a varied MHC, therefore you want to find a man who has an MHC that is very different from your own.

And... Maybe you can detect whether a man has a MHC that is the same or different from yours through smell (maybe vision too). In 2005, a paper came out explaining that the Major Histocompatibility Complex (MHC) can be detected through smell, and (importantly) that women prefer the smell of men who have an MHC that is different from their own. (However another paper in 2008, did not replicate this preference)

Possible new fragrance? 

Now here's the real kicker: Taking oral contraceptives (birth control pills) might mess this preference up. Roberts et al., 2008 show that in an armpit sweat test (like this one), women on birth control showed more of a preference for the MHC similar men than the women not on birth control. If true, this could have implications for women starting relationships when they are either on or not on birth control. To take this to the greatest sensationalist extreme, you might pick the WRONG GUY because you were on birth control. However, just like I don't believe in destined, fated true love, I don't believe you need to have opposite MHCs to have a good relationship or healthy children.

Roberts et al. 2008 Figure 2C: Odor desirability ratings.
And not only that, I have somewhat of a problem with this graph and their data. As far as I can tell (I found the description to be pretty confusing), the white bars represent 'session 1' in which NO ONE was on the pill and then the gray bars represent 'session 2' when the women labeled 'pill' were actually on the pill, but the women labeled 'non-pill'  were still not on the pill. (following this?)  AND, 0 means that they liked the similar MHC and the dissimilar MHC guys equally, negative means the like the similar guys more and positive means they like the dissimilar guys more... (I told you this was confusing).

So my question is, why are the non-pill and pill users so different to begin with? Unless I am completely misunderstanding this graph, I would think the white bars should be similar, as they represent 'women who are not on birth control.' The huge difference between groups before the 'experimental treatment' should be a red flag: Something is already different between these women.

However, the pill session 1 (white) and pill session 2 (gray) bars are indeed different, and that is their 'main result.' Basically, women on the pill had an overall slight odor preference for MHC similar men, and the same women not on the pill had an odor preference for MHC dissimilar men.

So should you worry this Valentine's Day? Should you break up with your boyfriend because you were on birth control when you met? Should you spend a lot of time smelling your boyfriend's worn shirts analyzing how 'desirable' a scent the give off?

Probably not (unless you really like smelling sweaty shirts). There is more to relationship compatibility than histocompatibility, and making life-changing decisions based on possible olfactory disruptions due to birth control is just not a good idea.

Though if you are worried, you can read more about it at:

Context and Variation "will the pill mess up my ability to detect my one true love?"


First Nerve "pill goggles"

© TheCellularScale
Roberts SC, Gosling LM, Carter V, & Petrie M (2008). MHC-correlated odour preferences in humans and the use of oral contraceptives. Proceedings. Biological sciences / The Royal Society, 275 (1652), 2715-22 PMID: 18700206


  1. Do they have a mechanism of action for this? Enhanced activity in the VNO or something? It'd be interesting if someone could give an explanation for what exactly is happening...

    (And yeah, that difference between the initial 'control sessions' is weird, I suspect it has something to do with either the fact that they're using Z-scores or that they are using a dis-sim metric that somehow obscures the result? But I don't know, I'm too buzzed on coffee right now to think clearly ;)

  2. I don't know of any mechanistic explanation for this yet, but that doesn't mean there isn't an attempt out there.
    The authors do say that they didn't have control over which women went on birth control and which did not. They simply found women who were already planning to go on birth control at some point. But that doesn't mean the bars should be different. This study could have 'found' that women planning to go on birth control prefer dissimilar MHC scent while women NOT planning to go on birth control prefer similar MHC scent.

  3. Hello, I think you describe the figure wrong. The 'white' (or unfilled) bars represent the first test/session and the (grey) filled bars represent the second session. So the appropriate comparison would be across the colored bars. And you can see overlapping error bars for the non-pill but not the pill group.