By Vilhelmiina Haavisto, Deputy Science and Technology Editor
The old adage of “opposites attract” could be true on a genetic level.
As Valentine’s Day rolls around again, love is on many people’s minds. Humans are distinct from many animals in that we tend to choose our partners a little differently - we don’t perform elaborate mating dances like many birds of paradise, or permanently attach ourselves to much-larger females like male anglerfish.
Evolutionarily, behaviours and qualities that signal greater ability to survive and reproduce are ones we should find desirable in potential partners. However, we know from experience that there are other factors that affect our attraction to someone too. Indeed, a 2001 study showed that the values we base our choice of partner on have changed substantially within the last 50 years: love/mutual attraction, physical attractiveness, and good financial prospects have become more important to us, while values like chastity and being a good housekeeper have declined in importance. The relative importances of ambition, a dependable character, and a pleasing disposition were found to have remained stable.
There has been some research in the last 20 years into the possibility that major histocompatibility complex (MHC) genes in humans are having an effect on partner choice. MHC genes are codominant, meaning that both versions of the genes (from the mother and father) are expressed. MHC genes encode for cell surface proteins that are expressed in the cells of the immune system. A man and woman who reproduce will both contribute MHC genes to their children, so if they have dissimilar MHC genes, their children could be expected to be resistant to more diseases and pathogens as they have greater diversity in these genes.
It has been shown that mice avoid mating with others that have similar MHC genes; research into this phenomenon in humans was pioneered by Wedekind and colleagues in their 1995 paper, detailing what has become known as the “sweaty t-shirt experiment”. The research was based on the fact that MHC genotype appears to influence body odour. In the experiment, male students were given a t-shirt to wear on two consecutive nights - then, female students were asked to rate the t-shirt odours based on how pleasant they found them. Both male and female students' MHC gene complexes were also sequenced to determine their MHC genotype.
The study found that the female students generally rated odours from MHC-dissimilar males as more pleasant than those from MHC-similar males. However, this was only for those not taking oral contraceptives; the pattern was actually reversed in the female students taking them, who tended to find the odours of MHC-similar males more pleasant. They did not find any difference in the perceived intensity of the odours in groups who were on or off oral contraceptives. Interestingly, the female students were also reminded of present or ex-partners more often when smelling the MHC-dissimilar males’ t-shirts.
However, the results from the research remain somewhat controversial. For one, the methods included making the male students live an “odour-free” lifestyle for the two days that they wore the t-shirts. Normally, other odours from body wash, deodorant, or cigarettes would also be part of a person’s ‘odour profile’, and so might mask any effects of MHC genotype on body odour. The female students were also given nasal sprays to help them smell better, and were told the aims of the study beforehand. All of these measures point to a confirmation bias, a systematic cognitive error where we look for or interpret results in a way that favours our pre-existing hypotheses.
Photo by Edgar Chaparro / Unsplash
A response to the research also pointed out that the sample sizes were small (just 49 women and 44 men), that the statistical significances of the findings were marginal, and that MHC genes do not, as far as we know, produce any volatile (airborne) chemicals. The latter makes it difficult to understand how they could be influencing partner choice via body odour.
However, the case is not yet closed. Research by Chaix and colleagues from 2008 analysed European-American and African couples’ MHC gene complexes to see whether there was any evidence for couples to be more likely to be genetically dissimilar in the MHC region. They did not find a significant pattern in African couples, who were not systematically similar or dissimilar from one another in the MHC region but did tend to have more similar genomes than random pairs of individuals.
They did find that the European-American couples were significantly more dissimilar in the MHC region than random pairs. The researchers suggest that this could have something to do with odour-based partner choice, as Wedekind thought. Alternatively, it could be to do with the fact that all of the couples in the study were selected on the basis that they had children, and were therefore fertile - maybe MHC-dissimilar couples have lower rates of infertility and/or miscarriage? They also note that the MHC regions of the European-American couples’ children were not any more diverse than would be expected by chance.
All in all, it seems that the jury is still somewhat out on this one. While MHC-dissimilarity preference has been demonstrated in mice, the evidence for it in humans is still inconclusive. Though it might be evolutionarily advantageous to partner up with people who have dissimilar MHC genes, so far it seems that there are more important factors that we consider when choosing a partner.
Featured Image: Everton Vila / Unsplash
Do you think your MHC genotype could be influencing your relationships? Let us know your thoughts!
