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Opposites attract, goes the saying, and we all know about those sayings. With one exception: That of the major histocompatibility complex, or MHC, the group of genes that determines how our immune systems will deal with disease. In short, the more diverse your MHC, the better your chances of fighting off viruses, bacteria and other intruders.
All of us inherit your MHC variations through your parents, who, while they didn’t quite sequence one another’s DNA before deciding to start a family, did -- however unknowingly -- biologically select for a distinct MHC type during courtship.
How does this happen? If you have an e-mail account, you’ve probably opened a spam message proclaiming that a perfume composed of “pheromones” exists out there to make your gender of choice go crazy over you. While such claims are patently false (pheromones are still a hotly debated topic and no human pheromone has yet been identified), the concept of smell having a bearing on attraction isn’t, and it’s closely linked to the role of selecting a partner with a different MHC.
In her book The Science of Kissing, Sheril Kirshenbaum cites Claude Wedekind’s “sweaty t-shirt experiment,” in which the Swiss zoologist had 44 men wear clean t-shirts and no scented products (including deodorant) for two nights before putting these in boxes with a single opening and asking 49 women to smell the contents of each box and describe their sexiness.
Having tested the MHC type of both the men and women prior to the experiment, Wedekind found that women consistently selected the shirts worn by men with differing MHC types. The results of the study suggested that even without advanced tools, we seem able to distinguish between MHC types.
Many experiments since have explored the topic, arriving at similar conclusions.
“One particularly intriguing result came from a 2006 study conducted at the University of New Mexico, which considered the MHC genes and bedroom behavior of 48 couples,” says Kirshenbaum. “The researchers found that the women who were more genetically distinct from their partners reported a higher degree of sexual satisfaction. Those with similar MHC genes, by contrast, reported having more fantasies about other men, and were also more likely to cheat.”
But a study in 2008 by Craig Roberts at the University of Newcastle in the U.K., found there was just one problem with this internal system. It seems that the birth control pill makes it very difficult to select for men with a different MHC type.
An interesting theory noted in Kirshenbaum’s book is that this is due to how the pill works. Because the pill tricks a woman’s body into believing she is pregnant, a woman’s body is no longer on the look-out for a suitable genetic partner, but for those who will shelter and care for her and her family, such as her kin. For this reason, it is theorized, women are more likely to look for men with similar MHC types.
“If true, it’s possible that going off the pill during a relationship may alter a woman’s perceived level of attraction to the person she is with,” notes Kirshenbaum. “This could explain the divorce rates in young couples. When a woman goes off the pill to start a family, the romantic chemistry with her husband may be affected.”
Kirshenbaum cautions that while this is an intriguing theory, it is still a theory. What’s more, we still do not know just how significant MHC types really are when it comes to mate selection.
In his book, How Sex Works, Sharon Maolem touches on the topic, noting that relationships involve a lot more than selecting MHC types and that if you met your partner while you or she were on the pill, there isn’t much to worry about: “if you’re in love, you’re in love, it’s too late.”
Maolem quotes a conversation with psychologist Rachel Herz, author of Scent of Desire:
Once you’ve fallen in love with someone, once the emotional attachment has been made to an individual, they could smell like garbage truck and you’d be attracted to that smell. If you’re healthy, and I’ve been on the pill and you’ve been wearing cologne, I don’t really know how you truly smell. Once you stop wearing cologne and you’ve been sweating around the house, and now I know how you really smell because we’re past that stage of the courtship, then how you smell now is going to be associated
