When it comes to what attracts one person to another, there are a lot of factors. What they look like, their beliefs and values and the things they are passionate about are all important. But there are also more subtle cues that can lead to attraction, and one that may be important is smell. We know that humans can detect a lot of information from one another’s musk. We can recognise our family members from their odour, and even determine facts about strangers such as age[i] or, amazingly, personality[ii].
In many animals, there is evidence that individuals choose partners on the basis of differences in their immune system[iii]. The major histocompatibility complex (MHC) is a set of genes involved in detecting pathogens and activating the immune system, and it is passed on from parents to their offspring. To have the healthiest offspring, it may be an advantage to have parents with different genes in their MHC[iv]. Doing so would also avoid inbreeding, as those with MHCs similar to your own are more likely to be relatives. And it seems some animals can detect differences in potential mates’ MHC via smell. Each individual is constantly releasing tiny fragments of protein, bound to their MHC, via bodily fluids. These are broken down by bacteria on the skin, to give each animal their characteristic smell. And this, the theory goes, guides mate selection.
But does it work in humans? There was a lot of excitement for the idea in the 1990s when Claus Wedekind, now Professor of Biology at the University of Lausanne, Switzerland, carried out experiments on a group of college students[v]. He gave T-shirts to a group of male students, and asked them to wear them for two nights, while avoiding the use of deodorants or scented products. He then presented these T-shirts to a group of women, and asked them to rate how attractive each scent was. Analysing the results, the team found that the women consistently chose men with MHCs different from their own.
So does that mean we have no control over who we end up with, and pick them solely based on their stink? Of course not. In fact, there have been mixed results when other groups have tried to replicate these findings, with some finding no effect at all, so we can’t treat them as decisive, even in the lab[vi]. And even if it were the case that humans prefer the body odour of others with dissimilar immune systems, or perhaps dislike those with very similar genes, this doesn’t tell us whether we actually use this as a factor in choosing a partner.
To work out whether MHC differences really do affect mate choice in real-world environments, a number of studies have taken couples, sequenced their genomes and compared them to random pairs of individuals from the same population. Again, results are mixed[vii][viii], so it seems the jury is still out on whether sniffing someone’s genes can make you want to get into their jeans.
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[i] Mitro, S., Gordon, A. R., Olsson, M. J., and Lundström, J. N. (2012). The smell of age: perception and discrimination of body odors of different ages. PLoS ONE 7:e38110. doi:10.1371/journal.pone.0038110
[ii] A. Sorokowska, P. Sorokowski, A. Szmajke Does personality smell? Accuracy of personality assessments based on body odour European Journal of Personality, 26 (2012), pp. 496-503. https://doi.org/10.1002/per.848
[iii] Potts, W. K., Manning, C. J. and Wakeland, E. K. (1991). Mating patterns in seminatural populations of mice influenced by MHC genotype. Nature, 352: 619–621.
[iv] Kurtz J, Kalbe M, Aeschlimann PB, et al. Major histocompatibility complex diversity influences parasite resistance and innate immunity in sticklebacks. Proc Biol Sci. 2004;271(1535):197-204. doi:10.1098/rspb.2003.2567
[v] MHC-dependent mate preferences in humans. Claus Wedekind, Thomas Seebeck, Florence Bettens and Alexander J. Paepke. 22 June 1995 https://doi.org/10.1098/rspb.1995.0087
[vi] Havlíček J, Winternitz J, Roberts SC. Major histocompatibility complex-associated odour preferences and human mate choice: near and far horizons. Philos Trans R Soc Lond B Biol Sci. 2020 Jun 8;375(1800):20190260. doi: 10.1098/rstb.2019.0260. Epub 2020 Apr 20. PMID: 32306884; PMCID: PMC7209936.
[vii] Derti A, Cenik C, Kraft P, Roth FP (2010) Absence of Evidence for MHC–Dependent Mate Selection within HapMap Populations. PLoS Genet 6(4): e1000925. doi:10.1371/journal.pgen.1000925
[viii] Chaix R, Cao C, Donnelly P (2008) Is Mate Choice in Humans MHC-Dependent?. PLoS Genet 4(9): e1000184. doi:10.1371/journal.pgen.1000184
