When picking a husband or wife, American couples seek out new immune genes, while Africans stick to the ones they've got.
New research shows that American couples of European ancestry go for mates with versions of immune genes that recognise pathogens dissimilar from those their own genes recognise. These genes are part of the major histocompatibility complex, and the more MHC genes a person has, the greater variety of pathogens his or her immune system recognises.
Previous work in fish, lizards and birds has suggested that animals seek out mates with different MHC genes than their own. Yet studies in humans have painted a far blurrier picture of MHC-driven mating preferences.
One study concluded that Hutterites, who live communally, wed people with different versions of the genes, while another found that women prefer the scent of sweaty T-shirts worn by men with similar MHC genes. However, an additional sweaty T-shirt experiment using slightly different methods showed just the opposite trend.
"It seems that body odours can reveal someone's immune genetics, and so through the smell we could be able to distinguish the MHC genes from different potential mates," says Raphaëlle Chaix, a human population geneticist at the National Centre for Scientific Research in Paris, France.
Instead of smelly T-shirts, Chaix and colleague Peter Donnelly of the University of Oxford studied previously gathered genetic data on 30 Caucasian couples from Utah and 30 Yoruba couples from Nigeria. The researchers analysed about 9000 genetic differences within the MHC genes, as well as more than 3 million differences dotted across the rest of their genomes.
This suggests that the American couples are selecting mates, in large part, based on MHC genes.
Not so for Yoruba couples, who seemed to pick mates with MHC genes no more different than would be expected for any two people picked at random from the population.
No pressure One explanation for the different findings could be diversity. Overall, Yoruba people had more differences in their MHC genes than Americans, so there could be less evolutionary pressure to find a mate with new genes.
Culture may also play more of a role in mate choice for Africans. Among Yoruba, marriages between distantly related couples from socially connected clans could be more common than marriage between completely unrelated men and women.
"We can think that maybe two lineages will prefer to exchange their wives," says Chaix. "In several generations, you can create in such a way a pattern where husband and wife are more similar than random individuals."