A loaf of bread, a jug of wine, and dissimilar major histocompatibility complexes

The candlelight is gleaming, soft jazz is playing, the fireplace is crackling, and you’re snuggled on the couch with the woman you love. You finger the diamond ring in your pocket. There’ll never be a better time to pop the question. You look deep into her eyes and say, “My darling, why don’t we get tested to find out how similar our major histocompatibility complex genes are?

It might be a mood-spoiler, but it might save you a lot of grief down the road, according to new research by a team led by Christine Garver-Apgar, a psychologist at the Universityu of New Mexico in Albuquerque, and recently reported by New Scientist.

The researchers analyzed the genetic makeup of 48 heterosexual couples who had been in an exclusive relationship for at least two years, focusing on specific genes that are part of the major histocompatibility complex (which I will from now on, you will be pleased to read, refer to simply as MHC), genes that help the immune system differentiate between the cells that belong in the body and pathogens. Then the researchers asked the couples a series of questions about their relationship, including their sexual responsiveness towards each other and how faithful each had been (presumably, each member of the couple was questioned separately).

If it seems a bit of a leap from immune-system genes to fidelity and sexual responsiveness, you haven’t been keeping up.

A number of studies have shown that MHC genes play a major role in how attracted we are to certain individuals. Evolutionarily speaking, this is probably because mates with dissimilar MHC genes produce healthier offspring with stronger immune systems.

Research shows that couples tend to be less similar in their MHC genes than you would expect if they had simply paired up randomly: in other words, somehow we’re able to detect dissimilar MHC genes in potential mates. How? We don’t know for sure, but smell definitely plays a role.

A few years ago Claus Wedekind, a scientist then at the University of Bern, Switzerland, asked a group of women to sniff T-shirts that had been worn by a group of men, and decide which smelled best. The women (unless they were on oral contraceptives, which reversed their preference) preferred the T-shirts worn by men whose MHC genes were dissimilar to their own. When we talk about two people having “chemistry,” it turns out we may be speaking literally.

And without that chemistry? Well, Garver-Apgar and her colleagues found that as MHC genetic similarities increased, the women involved were less sexually responsive to their partners, more likely to have affairs, and more attracted to other males, particularly during the fertile periods of their menstrual cycles. (That combines dangerously with the fact that men find women more attractive when they are near ovulation.)

The fraction of shared MHC genes correlated directly to the woman’s likelihood of cheating on her partner: if the man and women had 50 percent of the MHC genes in common, the woman had a 50 percent chance, on average, of cheating with another man.

Men didn’t seem to be affected by these genetic differences at all. Increasing MHC similarities didn’t change their sexual interest in their partners or increase the likelihood they would be attracted to woman outside their relationship.

And where does love fit into all this cold biological calculation? Well, some scientists suggest that feelings of love are the “stop code” for our built-in mating program, serving the evolutionary purpose of causing us to stop searching for a mate when, genetically, we’ve already found a good one.

How hard is it to find a good one? According to a computer simulation developed by evolutionary psychologists Peter Todd at Indiana University in Bloomington and Geoffrey Miller at the University of New Mexico, all you need to do is examine nine percent of available mates to find a good match.

In other words, if you’re at a party with 100 eligible members of the opposite sex (and if you are, you go to some pretty amazing parties), you only need to consider the first nine that cross your path at random. Examine fewer, and you won’t have enough information to make a good choice, but examine more, and you increase the chance you’ve already met the best choice and passed him or her by.

Searching for the perfect soulmate just might leave you with no mate at all–and that would be a tragedy: with whom, then, would you share your major histocompatibility complex genes?

Permanent link to this article: https://edwardwillett.com/2007/01/a-loaf-of-bread-a-jug-of-wine-and-dissimilar-major-histocompatibility-complexes/

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