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Major histocompatibility complex (MHC) is a genotype found in vertebrates including humans. MHC is thought to contribute to mate choice in animals and humans. In sexual selection, females opt for mates with MHC which differs from their own, optimising genes for their offspring. The ‘heterozygote advantage’ and ‘Red Queen’ explanations for these findings fall under the ‘pathogen hypothesis’. Due to differences in MHC alleles’ resistance to pathogens, preference for mates with a dissimilar MHC composition has been argued to act as a mechanism to avoid infectious diseases. According to the ‘heterozygote advantage’ hypothesis, diversity within the MHC genotype is beneficial for the immune system due to a greater range of antigens available to the host. Therefore the hypothesis proposes that MHC heterozygotes will be superior to MHC homozygotes in fighting off pathogens. Experimental research has shown mixed findings for this idea. The ‘Red Queen’ or ‘rare-allele’ hypothesis suggests that diversity in the MHC gene provides a moving target for pathogens, making it more difficult for them to adapt to MHC genotypes in the host. Another hypothesis suggests that preferences for MHC-dissimilar mates could serve to avoid inbreeding.

Body odor can provide MHC information. Although less is known about how odor is influenced by MHC genes, possible explanations have been that microbial flora or volatile acids are affected by the gene, which can be detected in body odor. Female mice and humans have both shown odor preferences for males with MHC-dissimilarity. Research has shown that women prefer the scent of men with dissimilar MHC genes. In a study, women rated the scent of t-shirts, worn over two nights by men, as more pleasant when smelling those of MHC-dissimilar men. It has also been found that women were reminded more of current or prior partners when smelling odors from men whose MHC was dissimilar to that of the smeller. A study of married couples found that MHC haplotypes differed between spouses more than chance expectations. Taking oral contraceptives has been found to reverse the MHC-dissimilarity odor preference.