T-shirt Named Desire?

An example of an hypothesis cited enough times that it has become accepted lore suggests that one criterion upon which humans select mates is dissimilarity at the major histocompatibility complex (MHC) of genes. The MHC is a region of immune genes that vary greatly within species that have them (all jawed vertebrates), used in self/non-self recognition (and discovered because they're involved in tissue rejection in transplant surgery). The mate-selection idea is that offspring will be better equipped to combat infectious diseases with a broader spectrum of immune genes, and to ensure that, natural selection has built MHC dissimilarity into the way we choose mates.

The association between MHC dissimilarity and mate choice has been found at least in mice, as well as in humans. An apparent correlation between body odor and MHC genes (a correlation that is difficult to explain, but may have to do with the clustering of olfactory receptor genes near the MHC) has led to studies in which tests (in humans) were done by asking females to smell T-shirts worn by males for 2 nights, and to state which are more pleasant, and indeed which reminded them of current or former mates.

In one of the seminal studies (no pun intended?) on this subject, females not on oral contraceptives chose males with a very dissimilar MHC to their own, while women on the pill chose males that were much more similar to themselves. In another study, the preference for dissimilarity wasn't found in single women. Even so, this work has led to the founding of a company that will help you to find your perfect match -- based on science. (Though, if it can really lead you to your perfect mate though, why are they charging $1,995.95 for a lifetime membership -- shouldn't you need their services only once?)

But anyway a new paper in PLoS Genetics by Derti et al. refutes all this.

A recent study by Chaix et al. sought signs of mate selection in the genotypes of HapMap Phase 2 (Hap2) parents by comparing the genetic relatedness of mated and unmated opposite-sex couples, both at the MHC locus and overall (using all common autosomal variants). Yoruban mates (N = 27 couples) were reported to be slightly more similar than expected (nominal two-sided P<0.001) overall, but no significant difference in MHC relatedness was detected between mates and non-mates. By contrast, European-American mates (N = 28 couples) did not differ significantly from non-mates in autosomal relatedness, but were less similar at the MHC locus than were non-mates (P = 0.015).

The latter result was interpreted as supporting a role for the MHC locus in mate choice, and outliers were excluded as an explanation. However, a visual comparison of mate and non-mate pairs suggests a weak effect that may derive from a few extreme pairs. Furthermore, adjusting the significance threshold for the fact that multiple hypotheses were tested (two sets of SNPs in each of two populations) would have rendered the results insignificant.

Thus, with minor modifications of the methodology and analysis (excluding one extreme outlier couple, and correcting for multiple testing, e.g.), Chaix et al.'s findings become from marginally significant to non-significant. Derti et al. further suggest:

If mates were found to differ from non-mates in MHC relatedness, in these or other populations, we note that this phenomenon need not stem from mate selection alone, particularly if only couples with children are considered. If offspring with certain MHC allele combinations survive preferentially, exclusion of mated couples without children could yield a non-random MHC similarity distribution amongst the remaining couples. This idea is supported by the increase in MHC heterozygosity of mouse embryos following viral infection of the parents.

So, the seemingly solid relationship between sweat, MHC variation and mate choice is at least as rocky as the relationship between Blanche and Stan in Streetcar Named Desire. But why was it so seductive anyway? The idea that the kind of variation in MHC that is found in modern large out-bred populations existed and was a strong selective force when hominids lived in small bands is not even convincing on the face of it. And, if the results of different studies have been so equivocal that single women and women on oral contraceptives don't show the preference for men with dissimilar MHC, then what does that say about early hominids? During most of human evolution, adult females were not often cycling, because they were malnourished, pregnant or lactating -- meaning that it's unlikely that their hormonal profiles looked reliably like those of college students on which the T-shirt tests were carried out.

We have written a lot about the seductiveness of selective scenarios. But, this tenuous hypothesis has a questionable aroma about it, that of something too good (or too pat) to be true. Whatever the mechanism is, if it's actually there, and not just a manifestation of collegiate horniness, which is not at all certain, it may be incidental to the functions being tested, and may have evolved for entirely separate and unknown reasons.  (Of course, it's possible that this whole story has been misunderstood -- it could be that this sweat/attraction thing is a remnant of the ancient admixture with Neandertals that everyone's talking about.  That would explain why it's not found in Africans.)

Now whether nice arm muscles inside the T-shirt, or a tough-guy way of mumbling, can trigger your mating corpuscles, is a separate question. Brando does look pretty good in a t-shirt.