Partner selection ultimately happens in the woman's reproductive tract
Achieving pregnancy has been shown to be more likely between partners who carry dissimilar human leucocyte antigen (HLA) immune genes. Accordingly, humans are expected to choose HLA dissimilar reproductive partners. Earlier studies have demonstrated that HLA dissimilarity preferences are mediated either by body odors or facial preferences. However, it has been unclear whether HLA-based mating preferences could occur after sexual intercourse in the female reproductive tract. Researchers at the University of Eastern Finland have now shown that the women's reproductive tract is capable of mediating post-mating sexual selection (known as the "cryptic female choice") toward the sperm of HLA-dissimilar men. This indicates that the ultimate mating bias toward genetically compatible partners occurs only after mating, at the gamete level.