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Body Odor and Subconscious Human Sexual Attraction

Research has shown that scent is connected to human sexual attraction. Rather than an individual’s scent being affected by a single gene, it is affected by his or her unique combination of genes and by the level of fertility of the female (2). This connection between scent and sexual attraction is typically related to the passing on of genes to offspring (2). Two parts of the human genome which have been connected with scent are the human leukocyte antigen (HLA) (3) and fluctuating asymmetry (FA) (4). HLA is the human version of the major histocompatibility complex (MHC) which is the part of the genome that codes for an individual’s immunities and for self/non-self-recognition (5). FA is the part of the genome connected to an individual’s body symmetry, which is related to traits such as body size and stress tolerance, etc (4). Studies have not suggested that one scent caused by a particular HLA is more attractive to all individuals but they have demonstrated attraction between individuals with certain types of HLAs (6). Research has, however, indicated that the scent of low FA is universally more attractive (4). In addition, facial attractiveness is connected to scent attractiveness (7).

Relationship between subconscious scent attractiveness and sexual attraction
Scent is connected to sexual attraction, as indicated by researchers for The Journal of Neuroscience. These scientists determined that through scent, humans can subconsciously tell if a potential mate will pass on favorable traits to their offspring. Martie G. Haselton and Steven W. Gangestad (8) researched human mating and found that the effect of scent on males and females differs. They concluded that part of this difference is caused by different motives in each gender for mating. Their research indicated that as far as biological reasons are concerned, males primarily mate to pass on genes, so they subconsciously notice and are attracted to traits that indicate fertility in females, for example a higher pitched voice, a specific hip-to-waist ratio, and a certain body odor. Haselton and Gangestad inferred that females had two main motives for mating; one motive was to pass on favorable genes to offspring and the other was to find a partner who would be invested in the relationship and who would be a good parent to offspring. Their research indicated that as the female reached the fertile stage of her menstrual cycle, the desire to pass on favorable traits to offspring became more important and the female was more attracted than usual to males with favorable traits (8). Many of these traits are subliminally detected through scent.

How scent affects the human leukocyte antigen
One factor detected in scent is HLA. It is unclear how the HLA affects scent. Below are three hypotheses as to how it works. 1.	One hypothesis comes from an article in Science (9). The article explained that the “vomeronasal organ detects social information about gender, status, and individuality.”  After performing research on mice, scientists found that MHC peptides, when introduced in a specific sequence, would induce a specific response in the vomeronasal organs. Based on this information, scientists concluded that different sequences of MHC peptides were detectable by scent and were an indicator of gender, status and individuality. 2.	Researchers for Genetica (10) proposed a different hypothesis. They suggested that certain molecules within the HLA were responsible for an individual’s unique body odor. Using mice and rats, they found that a difference was detectable in odors and that MHC was correlated with attraction, but they were unable to prove whether it was individual molecules within the MHC or if it was the fact that the molecules may have been binding with other compounds, which compounds could have been the cause of the body odor. 3.	Scientists Jan Havlicek and S. Craig Roberts mentioned a third hypothesis in a review over the connection between MHC and body odor (5). They explain that different MHC peptides may allow different organisms to live and that these organisms are what influence the body odor.

Relationship between human leukocyte antigen, major histocompatibility complex, and scent attractiveness
However HLA is affected by scent, it is clear that there is a relationship between the two. HLA is more than only a scent; through it, certain traits which influence sexual attraction are subconsciously detected (3). Different studies have been performed to clarify the relationship between sexual attraction and various aspects of the HLA, or MHC, including dissimilar MHC, heterozygous MHC and rare alleles within the MHC.

Dissimilarity
Dissimilar MHC refers to the difference between the MHC of two individuals. According to the European Journal of Immunogenetics (11), MHC is a codominant trait, which means that all of the dominant alleles inherited from both parents will be expressed. A more diverse MHC, then, should mean a better immune system. Accordingly, females should be more attracted to males with dissimilar MHCs from their own to provide offspring with a good immune system. A study published in Proceedings: Biological Sciences (3) suggests that the latter is indeed true. Researchers had males wear a t-shirt for 2 consecutive nights without any artificial scents, then had females smell the shirts and rate their odor as attractive or unattractive. The results showed that females who were not using contraceptives were more attracted to the scent of males with dissimilar MHCs while females using contraceptives were attracted to the scent of males with similar MHCs to their own (3). These findings indicate a relationship between a female’s conception risk and her desire to pass on favorable genes in the form of a good immune system to offspring (8). The University of Oxford performed a similar study (6). Oxford researchers did not find the same attraction of females to males with a dissimilar MHC but did note the attraction of females using contraceptives to the scent of males with a similar MHC. Also, the Oxford study showed that males were more attracted to the scent of females with a dissimilar MHC.

Another possible reason for attraction to dissimilar MHCs mentioned by both studies was to prevent incest (3) (6). Again, HLA codes for an individual’s immune system and distinguishes between self/non-self (5) so this hypothesis makes logical sense, but because the results of the attraction of females to dissimilar MHCs were unclear in the Oxford study, this hypothesis was neither supported nor rejected.

Heterozygosity
A different aspect on the relationship between HLA and sexual attractiveness that can be subconsciously detected in scent is HLA heterozygosity. Heterozygosity refers to having multiple alleles, or versions, of a gene. Heterozygosity of HLA would mean more alleles to fight against a wider variety of diseases and better survival rates in an environment with diverse diseases. At least one study supports this idea; researchers at the University of Utah (12) studied the relationship between MHC heterozygosity and survival rates in mice infected with different strains of avolotile (non-deadly) diseases. In the study, mice with a heterozygous MHC survived avolatile diseases better than homozygous mice, showing that heterozytosity was a favorable trait in the mice because it gave them a wider-covering immune system.

The relationship between HLA heterozygosity (again, the HLA is the MHC within humans) and sexual attraction has also been researched. In the Oxford study mentioned above (6), researchers found that males were not more attracted to the scent of females with a heterozygous HLA. This is perhaps because males are not primarily concerned with passing favorable traits onto offspring, but rather in passing genes on (8). Contrarily, the Oxford study indicated that females were more attracted to the scent of males with heterozygous HLAs and the attraction increased with her conception-risk. Combined with the study showing that heterozygosity of MHC increased survival rate (12), this supports the idea that as a female goes through the more fertile phase of her menstrual cycle, she is more concerned with passing on favorable traits and is more attracted to the scent of males who possess those favorable traits (8).

Rarity of alleles
Yet another aspect of HLA in relation to sexual attraction is the rareness of an individual’s alleles. The Oxford researchers discovered that males were more attracted to the scent of females with rare-alleles (6). The study did not suggest an attraction of females to males with rare HLA alleles. It was unclear why there was no attraction there, especially given the female desire to pass on favorable genes (8).

Relationship between fluctuating asymmetry and scent attractiveness
Aside from the HLA, FA is also detectable through olfactory senses and is connected to sexual attraction (6). As explained by J. T. Manning, K. Koukourakis, and D. A. Brodie (4), FA is the index that measures developmental instability. FA and certain favorable developmental traits share an inverse relationship, so a low FA is correlated with more favorable traits such as high stress tolerance, larger body size in males and smaller body size in females, and higher facial attractiveness (4). Although there is little research about whether males are attracted to the scent of females with low FA the Oxford study showed that females, particularly highly-fertile females, were attracted to the scent of more symmetrical males, or males with a low FA, presumably to pass on good genes to their offspring (8).

Relationship between facial attractiveness and scent attractiveness
One more quality connected to both sexual attraction and body odor attractiveness is facial attractiveness. Anja Rikowski and Karl Grammer (7) did a study (similar to both the Oxford (6) and the study published in Proceedings: Biological Sciences (3) in that they had individuals rate the scent of t-shirts slept in by test subjects) which included photographs taken of test subjects. The photographs were rated and Rikowski and Grammar found that both males and females were more attracted to the scents of facially attractive individuals (7).

Summary
In conclusion, based on all of the studies, it is clear that scent is subconsciously used in sexual attraction. The HLA and FA as well as facial attractiveness can be detected in an individual’s scent (6). While it remains unclear how HLA affects an individual’s body odor, the correlation is clear and studies have shown the relationship of scent attractiveness to various aspects of HLA such as dissimilarity, heterozygosity, and rareness of alleles. Based on this research, the scent attractiveness of HLA is not universal, or rather, there is no superior and more attractive HLA. Instead, different individuals show a preference for certain HLAs and generally that preference leans towards creating a diverse allele combination, and therefore a genetic advantage for offspring (8). FA is more of a universal factor; the scent of an individual with low FA is more attractive to most individuals, especially females at high conception-risk (4). This trait, too, is connected to passing on favorable traits to offspring. Finally, facial attractiveness is another trait detected in scent. Facial attractiveness is positively related to scent attractiveness. In other words, facially attractive people also have a more attractive scent (7). And what does all of this go to show? The nose knows.