No, it’s not your personality that really matters, and neither is it your looks. The way that you smell is what really makes it or breaks it for you when it comes to attracting the opposite sex. There’s also no cheating – you can cover yourself with as much cologne as you like, but there is something in your body that is going to give you away: it’s called the MHC.
The Major Histocompatibility Complex (MHC) is a cluster of genes that tell your body what is and isn’t you. Our immune system relies on the MHC to recognise foreign material in our cells and to provide a warning that our body is under attack. MHC alleles vary hugely between individuals, so unless you are closely related, your MHC and that of the person sitting next to you will be very different.
This is a good thing if you’re thinking of asking them out, because humans don’t fancy people who have an MHC too similar to their own. In a monogamous mating system like ours, the pressure to find a partner with ‘good genes’ is huge. Get it wrong and not only could you be stuck with them for a very long time, but your kids might not be as good at surviving as someone else’s. Avoiding MHC similar mates will mean children with a greater variety of MHC genes and a better immune system overall. It also means that you avoid inbreeding and all the health problems that can go with it.
So how do we actually detect MHC differences? Well, studies have shown that we can smell them. Researchers performed t-shirt tests where men were asked to wear the same top for a few days, with no showers or deodorant allowed. A group of women had to smell the t-shirts and rate them in order of attractiveness. The results showed that the more dissimilar a man’s MHC was to their own, the more attractive that woman found them.
Yet, preferring a certain smell does not automatically lead to preferring a particular person. So does the MHC really affect who we choose? The clearest results, so far, have come from studying an isolated religious group in the US, known as the Hutterites. The Hutterites have a high risk of inbreeding because of their tiny population sizes. Studying the MHCs of married Hutterite couples has shown that they share fewer MHC alleles than would be expected had random pairing occurred.
The MHC can also affect your sex life: women who share a high proportion of MHC alleles with their partner tend, on average, to be less sexually responsive to them, finding it more difficult to feel aroused by them in general. In addition, the researchers found that high MHC similarity may make women more likely to cheat: as the number of shared MHC alleles increased, so did the number of other men the women had slept with during the relationship. The idea is that if your boyfriend isn’t going to give you the good genes you need, then you should probably look for them elsewhere.
But don’t panic; it’s not all as straightforward as it sounds. In fact, even the study above found that, although women might have a better sex life with MHC dissimilar men, this had no effect on how happy they were with their relationship overall. There are other unseen factors going on that have nothing to do with our biology. Our culture, personality and personal history all affect who we choose and why we stay with them. Society also plays an interesting role; it’s hard to know just how useful the MHC is in an environment of bewildering scents, deodorants and frequent washing. We humans live in a complicated world and, although the MHC clearly plays some role in human mate choice, the jury is still out as to how important it really is.