Sweaty t shirtIt’s been suggested for a long time that mice select mates by smelling MHC types, perhaps in the urine. MHC is by far the most variable region in vertebrate genomes, so this would offer a way for mice to avoid inbreeding: The more related the mice, the more likely they are to be similar at the MHC, so selecting a different MHC will help avoid inbreeding.

Partly as an argument by analogy, and partly through some rather poor-quality experiments, it’s also been argued that humans select mates the same way — that differences in MHC type make a partner more desirable. These are the notorious sweaty T shirt experiments that most people seem to have at least vaguely heard of.

I started off very skeptical about the human claims, because the quality of the experiments has, as I say, tended to be poor. There have been small numbers of people, indifference to alternative explanations, and a lot of post hoc hand-waving. (If the preferences turned out to be reversed, why, it was because the female was near her period, or something like that.) I think that most people who have actually looked at the data have had similar reservations, but that hasn’t stopped the concept from becoming pretty well known.

MHC & mate choice I became even more skeptical about the human experiments as I learned more about the mouse data. The evidence for MHC as a mechanism for avoiding inbreeding turned out to be relatively weak, or at least inconsistent (see here for my first discussion); and recently a paper that I found fairly convincing (discussed here) suggested that MHC is not in fact used by mice in this way at all — rather, a much more plausible, highly variable family of molecules called “major urinary proteins” (MUPs) are the source of the anti-inbreeding odor in mouse urine.

Much of the interest in human MHC and sex has been driven by the mouse observation, so I think that if mice don’t use MHC to select mates, then likely humans don’t, either. Still, it remains possible, even probable, that difference species use different methods to select mates. And since humans don’t even have variable MUPs (as far as I know) MHC remains in the chase.

A recent paper1 tries to look at this in a more objective manner, using genome-wide data on couples. Unfortunately the numbers are still quite small (just 30 couples each from a European-American subset, and an African subset) and the results remain slightly ambiguous. Their conclusion was that

African spouses show no significant pattern of similarity/dissimilarity across the MHC region … We discuss several explanations for these observations, including demographic effects. On the other hand, the sampled European American couples are significantly more MHC-dissimilar than random pairs of individuals … This study thus supports the hypothesis that the MHC influences mate choice in some human populations.

So, heads we win, tails you lose, because even though their hypothesis was invalidated overall, some post-hoc wiggling (“demographic effects”) lets them dismiss the data they don’t like.

I’m still pretty skeptical about any real effect from MHC on mate choice. I’m willing to be convinced otherwise, but it’s going to take a larger and more rigorous study than this one to make me interested.

  1. Raphaëlle Chaix, Chen Cao, Peter Donnelly, Molly Przeworski (2008). Is Mate Choice in Humans MHC-Dependent? PLoS Genetics, 4 (9) DOI: 10.1371/journal.pgen.1000184[]