The smell of a woman

The attractiveness of a woman's smell was not related to immunity

Daria Spasskaya, N+1

Men really distinguish women by smell and distinguish more or less attractive smells for themselves. However, contrary to the popular hypothesis, the male choice was not associated with the genetic diversity of immune markers. This conclusion was made by scientists from the University of Bern in Switzerland, comparing the genotype of volunteers and their smell preferences. The work is published in the journal Proceedings of the Royal Society B.

Major histocompatibility complex (MHC, in humans this complex is called HLA – human leukocyte antigen, human leukocyte antigen) – a protein complex presented on the cell surface, which is responsible for recognizing foreign proteins and presenting them to immune cells. This complex is encoded by a multitude of genes that are assembled into one large locus.

Genetic variations of this site expand the possibilities of the immune response of the body, therefore, the MHC locus is characterized by great variability within populations. According to one of the evolutionary hypotheses, it is more profitable to mate with partners whose MHC locus differs from their own as much as possible – to increase diversity, and therefore increase resistance to pathogens in offspring.

It is assumed that proteins encoded by genes inside this locus are involved in the recognition and production of odor molecules. Thus, a partner with a "suitable" histocompatibility complex can be found by smell. In general, this hypothesis has been confirmed for many vertebrates. For example, within mouse families, males and females more often have different alleles (i.e. combinations of genetic polymorphisms) of MHC, and, apparently, with free crossing, scent information helps them not only choose a partner, but also avoid closely related mating.

It is believed that the smell of a potential partner is also important for people, but whether the most attractive smell is associated with the genetic diversity of the HLA locus cannot be said for sure, since studies on this topic show contradictory data.

In a new paper, psychologists with biochemists from the University of Bern decided to find out whether the HLA genotype of women affects men's preference for female odors. The study involved 96 male and 49 female volunteers. All women at the time of the experiment were in the same phase of the menstrual cycle (the authors call this condition an important difference from the work of their predecessors). The odors were collected just before ovulation – it is believed that HLA-associated olfactory markers are most pronounced during this phase.

Also, according to the experiment, the participants did not smoke and did not take any medications, including hormonal contraceptives. The researchers tried to exclude all extraneous influences on body odor, including strong-smelling food and even the smell of the participants' partners.

Women collected axillary secretions with cotton swabs for three days. Then, the men were asked to evaluate the participants' smells on a scale from 1 to 100 according to three criteria: attractiveness, pleasantness, intensity, as part of a randomized test. All participants were given a blood test, which was used to genotyping the six most studied sites of the HLA locus, which differ in a wide variety of alleles.

For each potential donor-evaluator pair, a "similarity coefficient" was calculated, according to which the pairs were divided into two groups: the most similar in HLA genotype (HLA-similar) and the most different (HLA-dissimilar).

Preference for a particular smell and genetic variations within the HLA locus in men do not correlate. On the X–axis is the level of attractiveness of the smell, on the Y–axis is the respondent's number (the graph is not fully presented).
Fabian Probst et al / Proc.R.Soc.B 2017

When analyzing the results of the study, the authors proposed to check the influence of three factors on male preferences: the number of common HLA alleles between a man and a woman, the presence of rare alleles in a woman and heterozygosity for HLA. Despite the fact that men did rate some smells higher than others on the attractiveness scale, for all three factors, scientists did not find any correlation with the assessment. Thus, the authors argue that for men, there is no association between the attractiveness of a female smell and the genetic diversity of immune markers of a potential partner.

According to the authors of the work, even if such an association actually exists in the human population, it should be more pronounced for women. From an evolutionary point of view, when choosing a partner, the maximum survival of offspring is more important for the female, while from the male's point of view, the maximum fertility of the partner is more important to increase reproductive success.

We told you that, according to some scientists, the role of smell in human society is underestimated, and in fact people recognize smells no worse than other mammals. In support of this hypothesis, Swedish scientists have proposed to identify criminals by smell.

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