If opposites attract, why is my sister so pretty?

My girlfriend and I went on a beautiful three-day hike all alone through the romantic Marin Hills. And the conversation, as it so often does in such cases, turned to asexual reproduction and incest.

She brought up the example of heterogamous organisms to alternate between sexual and asexual reproductive strategies, preferring parthenogenesis in times of plenty and sex when during times of tougher competition.

And this makes sense. If you expect all your offspring to survive, then it’s evolutionarily advantageous to load these offspring with your own genes, so you can send 100% of your genome to the next generation instead of just 50%. But if you expect heavy competition, then in the end only a few highly-optimized organisms that take the best characteristics of all their predecessors will survive. You can either contribute some portion of your genes to these future uberbeings, or be left out entirely.

Obviously you don’t want to mate with an organism identical to you; if you did, this would be very similar to asexual reproduction – not exactly similar, since you’d still randomize allele distribution – but similar enough that going through sexual reproduction seems sort of like a waste of time. So you want to mate with an organism that is at least a little genetically distinct from you. And it seems that there are many reasons to want to mate with an organism maximally genetically distinct from you. This increases the rate at which you achieve the offspring-optimizing capabilities of sexual reproduction, and is best for confounding parasites, who prefer their hosts to have high levels of genetic uniformity for about the same reason that human pharmaceutical researchers prefer the parasites they target to high levels of genetic uniformity.

She supported this position empirically, noting that the children of incest (especially high levels of genetic similarity) are unusually prone to genetic problems and maladapted, while the children of two different strains or species are unusually prone to genetic success and well-adapted.

More interesting, there’s a set of genes called the major histocompatibility complex (MHC), which to somewhat oversimplify things are involved in the immune system. The more diverse your combination of MHC genes, the more pathogens your immune system can recognize and the harder it is for pathogens to fool your immune system. Some animals seem to be able to smell MHCs and deliberately select mates with different MHCs than themselves in order to give their children new and exciting MHC combinations.

A famous experiment by Wedekind et al suggested humans may do this as well. Women were asked to smell different men’s sweaty t-shirts and rate their attractiveness. The women were most attracted to t-shirts worn by men with dissimilar MHC genes. A further study found that couples had much more different MHCs than would be predicted of two randomly selected members of the population. This has since been replicated a few times with moderate success.

So she concluded that animals probably choose mates to maximize genetic dissimilarity.

I made the opposite argument. We know that close relatives (especially siblings) separated at birth who meet as adults very often have unusually strong sexual attraction to each other, to the point where it seems hard to explain by coincidence or even by the people’s general kakosexual tendencies. Well-intentioned attempts to come up with a proper social explanation of this by attributing it to “imprinting on the opposite-sex parent” seem to ignore that most of these people were, in fact, separated from their opposite-sex parent at birth.

In another one of those experiments that make it obvious that psychologists have the best job in the universe, a group of scientists asked subjects to rate the sexual attractiveness of various people in photographs, but secretly flashed subliminal pictures of the subjects’ opposite-sex parents during some of the photos. Sure enough, people found the photos associated with their opposite-sex parent more attractive.

Yet another study shows that people’s spouses are about as similar to them as their fraternal twins – and further, that spouses are more similar on highly heritable traits than on less heritable traits (warning: this study was done by Jean Rushton, who is most famous for studies claiming that between-race differences are genetic. You may wish to use this information when considering how much to trust his work).

Finally, homogamy passes the ocular trauma test in a way that heterogamy doesn’t. Most people tend to marry people who are a lot like them. It seems highly unlikely that I will marry an !Kung bushman. Part of this is geographical (I don’t meet many !Kung); part of it is social (I’m from a society that has given me standards of beauty and expectations for interpersonal relationships which most !Kung don’t satisfy). But the allele chart confirms that the !Kung are the human population most genetically different from me; it still seems relevant that I am not the least bit attracted to them. On the other hand, I do find myself dating a woman who takes long hikes through the mountains while talking about the role of histocompatability complexes in natural selection, which probably puts her in like the top 0.01% of the population for similarity to me already.

But there’s not a whole lot of good theoretical explanation for this (I continue to maintain that the “imprint on opposite sex parent” thing is dumb). One possibility I proposed was that this maximizes the number of your genes that continue to the next generation. That is, suppose a male lab mouse mated with a female lab mouse who shared 50% of his genes. Now each of his offspring has 75% of his genes rather than just 50%, so more of his genes are passed on to the next generation. Whether or not this works seems to depend on whether or not this is funging against both partners having kids with someone else instead; most likely it would for the female and wouldn’t for the male. This produces weird predictions about sex-specific Westermarck effects which as far as I know have never been observed.

So right now as best I can tell the status of evidence on this question is that people are attracted to other people who are genetically similar to them except in the case of MHC complexes where they’re attracted to other people who are genetically different from them. This seems very weird and not parsimonious and I have no logical explanation for it.