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Human Biology
Research has attempted to clarify the relationship between sexual attraction and various aspects of human biology and genetics, including: HLA, dissimilar MHCs, heterozygosity of MHC, and rare alleles for MHC. While research is not conclusive, hypotheses suggest such genetic factors play a role in sexual selection.

MHC

See also: Major Histocompatibility Complex and Sexual Selection

MHC (Major Histocompatibility Complex) are a group of genes essential for the immune system. Unique body smells are heavily influenced by MHC, and studies have suggested that these olfactory cues are involved in mate choice and preferences. Similarity or differences in MHC genes can indicate variations between individuals' genotypes.

MHC is expressed codominantly; a more diverse set of MHC genes leads to a stronger immune system. It has been found that women prefer male partners with differing MHC genes from themselves. Therefore, one evolutionary explanation for these differences in attraction proposes that females are more likely to be more attracted to males with MHC alleles different from their own in order to provide their offsprings with a stronger immune system.

A study by Weekend et al., provided confirmatory evidence to show that MHC influences mate preference. In the study, males wore a t-shirt for two consecutive nights without adding any artificial scents then had females smell the shirts and classify the odor as attractive or unattractive. Females not using hormonal contraceptives were more attracted to the scent of males with dissimilar MHCs, whereas females on hormonal contraceptives preferred the scent of men with MHCs similar to their own. Research has also shown that the scent of low FA person is universally more attractive.

The increased attraction between people of dissimilar MHCs is also hypothesised to be a prevention of incest and subsequent birth defects. This inbreeding avoidance hypothesis proposes that biases towards hetrozygotic alleles prevent harmful genetic consequences which can arise from mating with genetic family members. The role of MHC in avoiding potential negative fitness effects from inbreeding has also been supported by Brown and Eklund, who found that olfactory cues can be used to recognise kin.

It has been shown that couples who have many failed attempts at conception share a considerably larger amount of genes than those who are able to conceive without problems. If, for these genetically similar couples, there is a successful conception, the babies are often born early or underweight.

A third hypothesis explaining the function of dissimilar MHC mate preferences, the parasite hypothesis, suggests that MHC heterozygotes may be resistant to rapidly evolving parasites.

Both sexes commonly use artificial scents to enhance their perceived sexual appeal. Many of these share a similar chemical profile with naturally produced body chemicals. A study from Milinski and Wedekind found a significant correlation between an individuals MHC genotype and the ratings they assigned to certain perfumes labelled "for self". This study supports the hypothesis that perfumes used personally enhance body odours which indicate immunogenetics. However, several infertility issues may arise due to the fact that people often use perfumes or scented body washes that erase their natural scent, hindering (women in particular) from being able to detect if their partner is genetically comparable.

HLA

HLA, the human leukocyte antigen is a gene complex which encodes MHC in humans. Men and women are attracted to the pheromones they produce and HLA is related to the perceptions of other peoples odours. Males produce androstenol and androstenone. Androstenol is produced by fresh male sweat and is most attractive to women, while androstenone is produced once the sweat is exposed to oxygen and is highly unpleasant to women.

Women are able to detect a single HLA difference, and are more attracted to the HLA of partners with more matching elements of their genetics. Studies have shown that women who are at the most fertile stage of their menstrual cycle prefer the smell of men that have higher testosterone levels. Heterozygosity of HLA can also be detected through scent: in this case, heterozygosity confers greater ability to recognize a wider variety of antigens. Females, especially when not using hormonal contraceptives, are more attracted to the scent of males heterozygous for HLA.

However, the same attraction and mate preferences are not held by males for heterozygous females. Males are, however, more attracted to the scent of females with rare HLA alleles. Mens arousal can be influenced by a variety of odours. For example; pumpkin pie, liquorice, doughnuts, and lavender can increase penile blood flow consequently causing arousal.