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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.

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37 Responses to If opposites attract, why is my sister so pretty?

  1. Mike Blume says:

    Hmm, isn’t MHC the one thing where we have a strong clear story about why lots of genetic diversity would be useful? So it being an exception of sorts doesn’t seem *that* surprising.

    • Charlie says:

      Not just the story is different, it’s a different mechanism than the general benefit of diversity in evolution.

      This is also probably a point to say “adaptation-executers, not fitness-maximizers.”

    • Lucas Slaon says:

      Following up on this, the MHC attractiveness is being mediated through an entirely different sense than the others – it would seem plausible that different people smell sexy than look sexy.

      • Army1987 says:

        Dunno about smell, but my sight and my touch disagree a lot about who is sexy.

        • Rob says:

          I may regret asking this, but when are you assessing people’s sexiness by touching them without having seen them?

  2. Anonymous says:

    #1.) One set of traits we inherit genetically from our parents is whom we find attractive. Presumably, our parents found each other attractive. This has seemed obvious to me for as long as I’ve believed in evolution, bet that’s probably because I only started believing in evolution since I read Red Queen.

    #2.) I read some where on the internet that people were actually more fertile with people who were somewhat more related to them e.i. distant cousins. One hypothesis to explain was the lack of genetic drift making people more genetically compatible.

    • Avantika says:

      #1.) One set of traits we inherit genetically from our parents is whom we find attractive. Presumably, our parents found each other attractive.

      I think that makes a lot more sense than the imprinting.

    • Scott Alexander says:

      The increased fertility thing seems like it’s got to be the answer if true, but why would that be so?

      • Mary says:

        The same process that causes species to become distinct and unable to interbreed — on a very small scale, and so a very small effect.

        • Richard Gadsden says:

          I’ve seen – probably on hbdchick – that peak fertility is with third cousins.

          And that strongly endogamous populations (ie where you never marry / mate with anyone outside the group) average out as third cousins to each other.

  3. Avantika says:

    This seems to depend on whether MHC complex similarity is correlated with similar personality. Apart from a very very general ‘personality is partly genetic’ it doesn’t seem obvious that they are. People might select to optimize MHC difference (for disease-resistant kids) and similar personality (for actually getting along with each other).

    • Kevin says:

      For any given personality, the set of similar personalities and the set of compatible are not necessarily identical, and may not even have significant overlap.

  4. Firedrake says:

    I don’t have references, but I believe that unrelated children raised in the same family rarely marry each other. Whether that’s a social or a lower-level biological imperative is of course unclear.

    • András Kovács says:

      That should be the plain old Westermarck effect.

      • Mary says:

        But then you have to explain the Westermarck effect. How did it arise if siblings are actually more attractive to each other when they did not grow up together?

        • Nestor says:

          It arises because siblings find each other attractive without it, which causes problems, which select for the Westermark effect without weakening the histocompatibility attraction function.

          Think of it as a patch.

  5. Leonard says:

    Humans are intensely tribal, and physical appearance is an important part of that. That is we are naturally “racist” (which is the less flattering interpretation of the studies you cite). I can think of several ways in which looking different might be a problem in a tribal situation.

    If looking different was a survival handicap in average evolutionary arc of a gene, then we should expect to see that people are attracted to people who look like themselves, or rather like some easily known referent person (since people would not know their own appearance due to lack of mirrors). So, Mom or Dad. But this would not negate any potential advantage to outbreeding that was not related to appearance.

    If the above is right, then we’d expect to see opposites attract in all ways except physical appearance, and possibly other traits which humans use to differentiate groups.

  6. Damien says:

    “there are many reasons to want to mate with an organism maximally genetically distinct from you… while the children of two different strains or species are unusually prone to genetic success and well-adapted.”

    Uh, the children of two different species tend to be non-existent, or sterile when they do exist. More true for animals than plants but hey, we’re animals. I don’t know of gold standard studies, but it’s plausible that divergent strains may be fertile but problematically so, e.g. a higher rate of birth defects or miscarriages.

    Leading to “you want to mate with someone similar to you, i.e. has a compatible genetic program, but not so similar as to double up on harmful recessives, also you’d like immune diversity.”

    Which leads to genetic sexual attraction and MHC attraction for cousins and immune-diverse people, minus attraction to people you’re raised with because usually those are your siblings or parents and too related to you, and thus strong attraction to separated siblings or parent/children because they register as “really similar!” and haven’t been blacklisted for familiarity. So apparently the MHC thing isn’t all that strong.

  7. Creutzer says:

    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.

    Couldn’t this just be because the images of parents triggered generic positive and familiarity feelings that were then wrongly associated with the picture of the person being evaluated wrt attractiveness?

  8. jsalvatier says:

    So animals who have the choice choose sexual reproduction during times of hardship and asexual reproduction during times of plenty. It’s a time of plenty, but humans don’t have a choice, they only do sexual reproduction. Maybe choosing similar partners is the closest humans can get to asexual reproduction.

    The MHC has a good story for why you would basically always want diverse MHC genes.

    • Army1987 says:

      That doesn’t square well with Scott’s hypothesis according to which hardship -> conservatism and plenty -> progressivism, since conservatives tend to be more xenophobic than progressives.

  9. Nestor says:

    According to Sapolsky, second cousins are the sweet spot

    • Scott Alexander says:

      Sweet spot for what? What’s the advantage of inbreeding?

      • Deiseach says:

        Presumbaly, if by “second cousins”, they mean “the children of first cousins”, that is, people who share a common great-grandparent but not a common grand-parent, they are close enough genetically to be attractive but not close enough genetically to share enough recessive traits to be problematic for offspring.

        • Mary says:

          Well, actually, it is close enough to have some problems, but not so much as siblings or even first cousins. I believe you have to go out to third or fourth before it’s no more of a risk than an unrelated person.

          Possibly the point at which the two factors’ combination hits its max.

      • Nestor says:

        The lectures are here, the cousin thing was in the 3rd or 4th video, iirc

        But in a modern context at least, transplants can be hard to come by for people with very mixed parentage.

        Not all recessive genes are bad, and losing the traits connected with them might be a bad idea in some contexts. I hear in the UK they have cases of rickets again in dark skinned kids.

  10. Deiseach says:

    Since I cannot add anything relevant to this discussion, I will instead relate a humorous (depending upon taste) story. Scott, psychologists may have the best job in the universe, but what of psychiatrists (your future profession)?

    May it not be a stressful job which leads to the likes of this behaviour (taken from the “Bog Cuttings” section of “The Phoenix” Irish satirical magazine):

    “Psychiatrist Bit Gardaí in Bar (Limerick Leader)

    A consultant psychiatrist employed by the HSE has been charged with assaulting two Garda officers and a member of staff outside Nancy Blake’s bar in Limerick in a “highly irate and agitated state” last summer. Limerick District Court heard that Dr Seamus Ó Flaithbheartaigh, 51, with an address at Castlemara, Annacotty, would not be contesting the four charges set against him. Sergeant Donal Cronin told the court that at around 1 am on the Saturday, July 7, 2012, a male was observed by gardaí to be in a highly irate and agitated state, and was being restrained by a number of males at the popular Denmark Street bar. Gardaí approached the male, who then bit them both and a member of staff as he tried to assist the arresting gardaí. His case was due to be heard in April next, however the court was told he is out of the country for a number of weeks around that time. Judge Aeneas McCarthy said the accused should not be out of the country when his case is due to be heard, but accepted that he was not a flight risk. The case is due for mention on March 27, and the accused will be excused from attending on that day.”

    Please be careful, Scott! 😉

  11. campofthesaints says:

    Genetic Similarity Theory by Roushton is one of the more interesting studies I have ever read.

  12. Nick T says:

    I continue to maintain that the “imprint on opposite sex parent” thing is dumb


  13. xor says:

    “the top 0.01% of the population for similarity to me” <– memetically similar, not genetically (though they may be somewhat correlated)

  14. AphroditeGoneAwry says:

    It makes sense (regarding your last paragraph), because nature needs to fall somewhere in the middle, or closer to consanguinity; protecting that genes are not too heterogeneous to avoid genetic incompatibilities in offspring, but yet bolster the beneficial similarities that can result by doing that, like intelligence and attractiveness.

    The fact that siblings separated at birth are likely to be sexually attracted to each other might hint that some dysfunction in the MHC mechanism occurs when siblings are not reared together, implying an environmental influence at play in its development.

    Perhaps the best blending is somewhere between distant cousins, as what you might get with extended tribe interaction. Historically, tribes have always done this; come together once a year to meet and forge alliances, but also romantic and sexual interaction between members.

    I’d say that is likely the best mix possible. And it appears that anecdotally, historically, and scientifically (thanks to your awesome article synopsis) that is supported.

  15. Cereus says:

    Another reason for the attraction to similar people –

    In general, we don’t just mate with people and then walk away and never talk to them again. We *typically* have some sort of relationship with them and watch out for any kids we might have (and watch out for eachother). So it would make sense to be with someone that is coming from enough of the same perspective that the two of you can communicate and work as a team well.

    Of course different people can make a good team because they have different strengths, but if you’re very different it takes a lot of effort just to figure out what you’re going to be doing and make sure you’re on the same page. So there’s advantages and disadvantages.

  16. nydwracu says:

    The idea that people are attracted to genetic similarity in general seems really weird to me; I have an abnormally large and diverse social network and have never dated a girl of my… er, subrace? (Out of the ones I’ve even considered dating, only one (out of only four white girls) had substantial Western European ancestry; of the other three, one was Italian and two were Jewish.)

    The observed tendency toward spousal genetic similarity seems like it can be explained in terms of 1. geographic proximity -> genetic similarity and 2. genetic influence in behavior leading toward people with enough shared interests, behavior patterns, etc. to get along well being more genetically similar than the average for the village or whatever, with the motivator being not genetic similarity on a subconscious level but behavioral or thedish similarity on a mostly-conscious level. (Or perhaps subconscious-level genetic similarity-judging processes motivate initial judgments of physical attraction and those results are refined later by mostly-conscious-level judgments of behavioral/thedish similarity…?)

    Do people with more diverse social networks (people in cities, say) end up with less genetically similar spouses? How does that number compare to the level that would be predicted by chance?