Body odor and subconscious human sexual attraction
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Research has shown that certain body odors are connected to human sexual attraction. Both fluctuating asymmetry (FA) and the human leukocyte antigen (HLA) are connected to scent. HLA is the human version of the major histocompatibility complex (MHC), a protein complex that plays a role in an individual’s immunities and self- versus nonself-recognition. FA affects body symmetry, which in turn affects body size and stress tolerance. 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 of certain HLA types. Research has also shown that the scent of low FA person is universally more attractive, and that facial attractiveness is connected to attractiveness of scent.
Scent and sexual attraction
Through scent, humans can subconsciously tell if a potential mate will pass on favorable traits to their offspring. Research on human mating has found that the effect of scent on males and on females differs. Part of this difference is caused by the different motives each gender holds for mating. Males, in order to pass on genes, subconsciously notice and are attracted to traits that indicate fertility in females, such as a voice of higher pitch, a specific hip-to-waist ratio, and a certain body odor. Evolutionarily, females have two main motives for mating: to pass on genes and to find a partner who can provide adequate support for herself and future offspring. As a female reaches the fertile stage of her menstrual cycle, the desire to pass on favorable traits to offspring gains more importance and the female becomes more attracted than usual to males with favorable traits. Many such traits are subliminally detected through scent.
Scent and HLA
It is unclear exactly how HLA affects scent, though there are various hypotheses:
- The vomeronasal organ, an olfactory bulb used to detect pheromones, can detect social information about gender, status, and individuality. Research on mice has shown that a particular sequence of exposures to MHCs induces a specific response in the vomeronasal organ.
- Certain molecules within the HLA are responsible for an individual's unique body odor. Research on rats and mice has shown that variation in MHC composition can be detected by smell.
- Variations in MHC may allow the growth of certain organisms that influence body odor.[clarification needed]
HLA, MHC, and scent
Research has attempted to clarify the relationship between sexual attraction and various aspects of HLA, including dissimilar MHCs, heterozygosity of MHC, and rare alleles for MHC.
MHC is expressed codominantly, and a more diverse set of MHCs leads to a stronger immune system. Females are thus likely to be more attracted to males with MHC alleles different from their own to provide offsprings a stronger immune system. A study confirming this had males wear 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 contraceptives were more attracted to the scent of males with dissimilar MHCs, whereas females on contraceptives preferred the scent of men with MHCs similar to her own. Further studies, while not confirming the female attraction to the scent of a dissimilar MHC, were able to confirm the attraction of females on contraceptives to similar MHCs, as well as showing that males are more attracted to the scent of females with a dissimilar MHC. The increased attraction between people of dissimilar MHCs is also hypothesized to prevent incest and its possibility of producing birth defects.
Heterozygosity of HLA can be detected through scent as well: in this case, heterozygosity confers greater ability to recognize a wider variety of antigens. Females, especially when not on contraceptives, are more attracted to the scent of males heterozygous for HLA. The same attraction is not held by males for heterozygous females. Males are, however, more attracted to the scent of females with rare HLA alleles.
FA and scent
Fluctuating asymmetry is also detectable through the olfactory senses and affects sexual attraction. FA acts as an index measuring developmental instability. It shares an inverse relationship with certain desired traits; a low FA is correlated with higher stress tolerance, larger body size in males, smaller body size in females, and higher facial attractiveness. Females, especially during their fertile phase, are more attracted to the body odors of males with a low FA (more symmetrical males) and of males whose faces they rate as attractive.
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