Innate Kin Recognition via MHC and Olfaction

Key Insight

Beyond learned mechanisms, innate biological preferences play a role in optimal mate choice, particularly regarding the Major Histocompatibility Complex (MHC) genes. The Hutterites, an endogamous population with high fertility, show that couples sharing too many HLA (human MHC) antigens experience longer conception times and higher miscarriage rates. For instance, sharing all sixteen MHC genes can lead to a fetal loss rate of 33.3 percent, compared to the average 16 percent, highlighting the immunological incompatibility that depresses fertility in highly related partners.

The problem with shared MHC genes lies in the immune system's need for diversity. A genetically homogeneous community, particularly in its immune system, is vulnerable to rapidly evolving pathogens, as it presents a 'fixed target.' Diverse immune systems, facilitated by MHC variability in offspring, create a 'moving target' that enhances protection against various diseases, a concept known as the 'Red Queen's Hypothesis.' This selective pressure drives the evolution of innate mechanisms for recognizing and preferring mates with complementary immune profiles.

Hutterites, for instance, non-randomly select mates with different MHC genes, possibly through olfaction. Studies show that female mice prefer males with dissimilar MHC genes, detecting differences even down to a single gene via scent. In humans, women's odor preferences are also linked to MHC dissimilarity; one study found women preferred the scents of men with dissimilar MHCs, reminding them of lovers. More precise research revealed that women preferred men who shared an intermediate number (2.3 on average) of MHC genes with them, inherited from their fathers, guiding them toward an optimal amount of outbreeding. Furthermore, genetic markers for olfaction and pheromone receptors are linked to political attitudes, suggesting a deep connection between mate choice and political orientation.