The latest study, published on September 12, examined the genomes of fertile Mormon and Yoruba couples to see how exactly their MHCs differed. The results were both fascinating and conflicting. Chaix and her coauthors found that the Mormons were significantly more MHC-dissimilar than randomly chosen pairs of people. Even more interestingly, although the Mormon mates were genetically very close to each other (they're Mormons, after all), the genes in their MHCs were often very different. But (as usual in this area of study, there's a "but") they couldn't find any pattern at all in the MHC-dissimilarity of the Yoruba couples. This might be because Yoruba society is highly patriarchal and stratified, producing a lot of social pressure for genetically similar men and women to mate. Or it might be that in Africa, where modern medical care is less widely available, there is much more selection pressure for antibodies that will protect their carriers against a few especially devastating diseases. More research here is definitely necessary to tease out the precise relationship between histocompatibility and mating choices, and how social and environmental factors might affect this relationship. But if you're a European American woman, sniff hard next time you're at the gym; the best-smelling guy might just be your immunological soulmate.