friendship and natural selection (and human biodiversity)

i linked to this paper…

Friendship and Natural Selection

…in this past sunday’s linkfest, but, to be honest with you, i hadn’t actually read it. (yeah, i do that sometimes. ok, ok — a LOT of times!)

since then, a couple of people urged me to read the arXiv blogpost on it (thanks @DKshad0w and @SamoBurja!), which prompted me to read the orig. research paper and … wow! … mind. blown.

now, no, i’m NOT a geneticist — i don’t even play one on the innerwebs — so i’m not your gal for evaluating whether or not these guys — nicholas christakis and james fowler — have done their work right, but if they have, this is very cool stuff:

Pairs of friends are, on average, as genetically similar to one another as fourth cousins, which seems noteworthy since this estimate is net of mean ancestry and background relatedness.”

fourth cousins! note that these are (i believe) all white folks. the data are from the framingham heart study, but the researchers do say that they controlled both for relatives (i.e. that individuals might be “friends” with people who are, in fact, distant relatives) and ethnicity (eg. that italians might just be friends with other italians).

It is intriguing that genetic structure in human populations may result not only from the formation of reproductive unions, but also from the formation of friendship unions. This in turn has relevance for the idea of an evocative gene-environment correlation, proposed more than 30 years ago, which suggests that a person’s genes can lead one to seek out circumstances that are compatible with one’s genotype. Our results suggest that these circumstances could include not only the physical environment but also the *social* environment, and hence the genotypic constitution of one’s friends. As Tooby and Cosmides argue, ‘not only do individual humans have different reproductive values that can be estimated based on various cues they manifest, but they also have different association values.’ People may seek out particular, convivial social environments that affect their fitness.

i’ve been saying for a while now that the family types/social structures in which individuals are located ought to be taken into consideration when thinking about the fitness of those individuals and how natural selection might be working on different individuals living and reproducing in different sorts of social structures (individuals living in nuclear family structures versus large clan structures, for instance). i’ve mostly been thinking about family structures, but christakis and fowler are talking about friendship structures — alliances with non-family members. cool!

“The existence of excess genetic similarity between friends is also relevant to the growing area of indirect genetic effects, wherein the phenotypic traits of focal individuals are influenced by the genomes of their neighbors, in a kind of ‘network epistasis.’ In fact, our results support the idea that humans might be seen as metagenomic not just with respect to the microbes within them, but also with respect to the humans around them. It may be useful to view a person’s genetic landscape as a summation of the genes within the individual and within the people surrounding the individual, just as in certain other organisms.

yeah. just muse on THAT for a while. (~_^) and then try this on for size…

“[T]he human evolutionary environment is not limited to the physical environment (sunshine, altitude) or biological environment (predators, pathogens), but also includes the social environment, which may itself be an evolutionary force. Our finding that positively correlated genotypes are under positive selection suggests that the genes of other people might modify the fitness advantages of one’s own genes, thus affecting the speed and outcome of evolution.

did you get that? the researchers found that, the SNPs that were most common between the friends (the homophilic SNPs), have, indeed, been under recent positive selection:

“To test the hypothesis that homophilic SNPs are generally under recent positive selection, we use the Composite of Multiple Signals (CMS) score. This score combines signals from several measures of positive selection to create a single value that indicates the likelihood a SNP has been increasing in frequency due to selection pressure over the last 30,000 years (see SI). In Fig.3, we show that, after correcting for correlated outcomes due to linkage disequilibrium and for varying precision in the GWAS estimates (see SI), the top 20% most homophilic SNPs have significantly higher CMS scores than the other 80% (+0.07, SE 0.02, p = 0.003)…. In contrast, Fig.3 also shows that CMS scores are not significantly higher for the most homophilic SNPs in the strangers GWAS (–0.00, SE 0.02, p = 0.86). This suggests that the whole-genome regression model we use does not generate false positives…. In sum, it appears that, overall, across the whole genome, the genotypes humans tend to share in common with their friends are more likely to be under recent natural selection than other genotypes.

friendship and natural selection 01

friendship and natural selection 02
_____

with human biodiversity in mind, my question would be: do all peoples everywhere form an equal number of friendships with (technically) unrelated individuals? yes, i have an obsession with clannish peoples — but, seriously — do people who live in clans or tribes make as many “friends” as peoples who don’t? do they even have the opportunity? if not, what might this mean for natural selection in those populations?

(note: comments do not require an email. friends.)

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27 Comments

  1. The first thing that came to my mind was that this was in an outbred society. People in inbred societies simplify the problem: you befriend your cousins. No need to seek out common genes (well, there is, but the task is made a lot easier).

    I’ve said since I’ve seen Rushton’s paper on this that life in an outbred society is all about finding your cousins. Since everyone is your cousin in an outbred society, the task is finding the ones whom are more related to you.

    Reply

  2. I haven’t read your blog post either, but: what if friends are genetically similar because people tend to like people who are like them ’cause they’re easier to empathize with? E.g. the reason my roommate and I are roommates is because we have a lot in common: we were both classified as “gifted” when we were children, we both like economics, and we both have a cynical way of looking at the world. So it seems likely that we share genes for high childhood IQ, economics interest, and cynicism. But we’d probably be even closer if we had even more in common (e.g. he likes surfing, I don’t; he’s a neat freak, I keep my clothes in a pile on the floor)–and surfing interest/neatness tendencies are probably genetically determined at least to a degree.

    Imagine how much harder it would be for me, as a social animal, to get my social grooming & empathy needs met if I was living with a radical feminist anarchist who had a hard time with basic algebra. Do we really need to postulate some kind of novel evolutionary argument for why social animals prefer social animals like them? On the other hand, maybe I evolved to need social grooming & empathy from people genetically similar to me mostly so I would develop relationships with my family members who are genetically related to me, and friends who are also genetically related trigger the same instincts?

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  3. This post reminded me of something that happened in my real life. My last girlfriend started dating me *because I smelled good to her*. Specifically when she smelled me it made her want babies. Like “knock her up now” kind of a thing.

    Turns out, there is some science behind this: “major histocompatibility complex”. Apparently a woman can *smell* if she is genetically compatible with a man.

    Another interesting thing is, if you read the articles below, if she had been on the pill when we met *it may have produced the opposite effect* and made her not like me.

    http://www.psychologytoday.com/articles/200712/scents-and-sensibility

    http://www.psychologytoday.com/blog/sensoria/201007/dating-mating-and-olfaction

    http://www.psychologytoday.com/articles/200910/the-smell-love

    “Evolutionary biologists had long hypothesized that humans pick mates by smelling them, but this was only proven in 1995 by biologist Claus Wedekind in a groundbreaking initial study while at the University of Bern. In that study, women were asked to smell t-shirts worn by different men and to rate which ones were most attractive to them. The participants overwhelmingly selected the shirts worn by men whose major histocompatibility complex (MHC) differed from their own.”

    “The 1995 study also included a troubling side note: a woman’s odor preference is compromised while taking oral contraception. Initially receiving criticism for its low number of participants, the 1995 findings related to the pill were upheld by a more extensive 2008 study “MHC-correlated odour preferences in humans and the use of oral contraceptives” conducted by S. Craig Roberts and The Evolutionary Psychology and Behavioural Ecology Research Group based at The University of Liverpool, UK. In short, the same pill that gave women peace of mind and the perception of control over their reproductive destinies also dismantles one of evolutionary biology’s greatest gifts- the capacity to literally sniff out a suitable partner. And yet the emotional and physiological impact of this troubling side effect of hormone-based contraception has received very little attention.”

    How is this fucking with human evolution? (pun intended)

    Reply

  4. I remember looking at Rushton’s stats for best friends and spouses, and this backs it up.

    My closest friends are two DZ twin black Trinidadian-Canadians, a black Jamaican-Canadian, a white German, three Indian-Canadians, a Sri Lankan-Canadian, three Chinese-Canadians, and a whole plethora of white Canadians who descend from Western Europe generations ago.

    The only person I share phenotypic similarities with is the Sri Lankan-Canadian guy — the Indian-Canadians hail from Punjab and are therefore roughly 35% “white” from the old ANI admixture event; and other than having black hair, we don’t look alike.

    So this is interesting to me. Regardless of generations of distance between my non-brown friends, we get along so, so well. Are we still fourth cousins when it comes to behavioural genetics alone?

    Reply

  5. My best and really only close intimate friend — we’re real bosom buddies, no secrets — is 100% Italian, Sicilian in fact, from Buffalo. Me, I’m 100% NW European. Go figure

    Reply

  6. There are Norse connections with Sicily, I know, but my friend is swarthy with one of those big Italian noses, full of hand gestures. We were both defensive linemen in high school however.

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  7. “Turns out, there is some science behind this: “major histocompatibility complex”. Apparently a woman can *smell* if she is genetically compatible with a man.

    Another interesting thing is, if you read the articles below, if she had been on the pill when we met *it may have produced the opposite effect* and made her not like me.”

    Yes, as you note, the Major Histocompatibility Complex genes can influence human sexual behavior in the opposite direction as well.

    The group of genes that most differentiates human populations also happens to be the group that is most involved with your body’s ability to differentiate between self and non-self — friend or foe identification. This is because different populations tend to have different diseases — and some populations — particularly those in old habitats or those in high traffic trade centers, will have more diseases. These genes are the Major Histocompatability Complex genes or MHC genes.

    Experiments conducted on college age students have shown that females are unconsciously sexually stimulated by the mere presence of men with MHC genes different from their own.

    The best theory to explain this instinctive sexual arousal by women is the fact that if a foreign male is allowed into the physical presence of fertile females of her tribe, it is likely that the diseases of his tribe will not be far behind and will, without the foreign male’s immunities, possibly exterminate the female’s tribe. The further removed the male’s tribe is from the female’s the less likely the female’s tribe will have immunity to the foreign diseases.

    Further, if the foreign male is from an older population, particularly with high population density, the need to acquire the foreign male’s immunity may be quite urgent since both the variety and sophistication of his tribe’s pathogens may be greater than her tribe’s.

    In the modern environment, where we have medical treatments for many forms of disease, the female’s instincts may not be as adaptive as they once were, but there are examples of diseases, such as TB, where multi-drug resistant strains are evolving and to which certain blood groups, such as type B, are known to have greater immunity than others.

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  8. I was much interested when I read the report and was a little surprised you did not write anything up on it.

    But I want to see the study reduplicated before I put too much trust in it. Among similar populations and different ones. Won’t speculate on the ‘how’ until this has been done.

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  9. What could this be suggesting that there would be two types of genetic similarity
    the racial (genotype)
    and behavior (phenotype)?

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  10. amazing. It would have been nice to know why the nations of Europe worked the way they did _before_ messing with the formula. The “social engineers” were of course working from a kind of Eurocentrism in which anything nice about Europe is actually something natural about humanity as a whole, while anything bad about Europe is uniquely European.

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  11. @andrew – “My closest friends are two DZ twin black Trinidadian-Canadians, a black Jamaican-Canadian, a white German, three Indian-Canadians, a Sri Lankan-Canadian, three Chinese-Canadians, and a whole plethora of white Canadians who descend from Western Europe generations ago.

    The only person I share phenotypic similarities with is the Sri Lankan-Canadian guy — the Indian-Canadians hail from Punjab and are therefore roughly 35% ‘white’ from the old ANI admixture event; and other than having black hair, we don’t look alike.”

    you guys all need to get your genomes sequenced so we can find out how alike (or not) you are! (^_^)

    Reply

  12. @john – “what if friends are genetically similar because people tend to like people who are like them ’cause they’re easier to empathize with?”

    yeah, maybe.

    i have to say that i find the other end of this process far more interesting — not why do some individuals befriend each other, but what happens, evolutionarily-speaking, once they do (over and over again)? if christakis and fowler are right, then the very traits that the circles of friends have in common get selected for! that is neat!

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  13. @puzzle pirate – “My last girlfriend started dating me *because I smelled good to her*.”

    yes. i have heard a lot of women — a LOT — say that about the men they’re attracted to. (~_^)

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  14. @luke – “My best and really only close intimate friend — we’re real bosom buddies, no secrets — is 100% Italian, Sicilian in fact, from Buffalo. Me, I’m 100% NW European. Go figure”

    (^_^)

    my best friend for years (high school and beyond) was from the same ethnic background as me, and we were always — always — taken for sisters. and we used to hang out with two sisters who came from a big family (7 girls and 2 boys), and they were also from my same ethnic group, and my best friend and i used to also always be taken for some of their sisters. (~_^)

    now, to quote the grumpy grandfather from Hope and Glory (should be read in a very cranky tone): “I don’t have any friends — only relations.” (~_^)

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  15. @jim – “The group of genes that most differentiates human populations also happens to be the group that is most involved with your body’s ability to differentiate between self and non-self — friend or foe identification.”

    that’s neat!

    Reply

  16. @t.greer – “But I want to see the study reduplicated before I put too much trust in it.”

    oh, absolutely! Further Research is RequiredTM.

    @t.greer – “Among similar populations and different ones.”

    yes, please!

    Reply

  17. I have an intuition that the genetic similarity of friends is due to related people having relatively more similar psychological phenotypes and people assorting on psychological similarity (personality, class, intelligence, interests, hobbies, and other human qualities) in friendship relationships.

    Assortativity for any given individual personality variable is, from what I can remember of examinations of them, quite weak, but perhaps composites of many personality variables may be strong.

    Trouble would be that testing this is very hard. We don’t even know if the personality variables we have (big five) are that good as true measures of personality, and even if so those are only a fraction of a person.

    Re: the test of selection comparing friends and strangers, I wonder if this measure would be influenced by comparing strangers of similar relatedness and friends, rather than the set of all strangers (as I think was done here). It would be more interesting if people shared selected SNPs with their friends net of their relatedness rather than if it was simply explained by increased relatedness of friends versus strangers (i.e. the of course they share selected SNPs you idiot – they’re related! explanation).

    On this friends stuff, I wonder what kind of patterns people and their enemies / competitors would show (rather than the stranger / friend comparisons here). Enemy groups are obviously much uh… harder to recruit.

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  18. If life in an outbred society (as JayMan puts it) is about finding your cousins, then there should be another reason added to the 100 reasons NOT to go to grad school: http://100rsns.blogspot.com/

    One of the reasons that the modern academic world is so miserable is that it (like other industries and quasi-industries) moves individuals around almost arbitrarily, mostly on account of the bizarre academic job market. Academics do not choose where they live; the market chooses for them.

    In the most elite institutions, there probably is a great of deal of rudimentary harmony among much of the faculty (and, more importantly, between the faculty and their surrounding communities). Everywhere else, however, academics commonly find themselves living like fish out of water among people with which they have little in common.

    It would not be surprising if the differences in question turned out to be more fundamental than differences of intellectual interest. You can’t find your cousins if you have no freedom of movement.

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  19. bingo

    .

    “with human biodiversity in mind, my question would be: do all peoples everywhere form an equal number of friendships with (technically) unrelated individuals? yes, i have an obsession with clannish peoples — but, seriously — do people who live in clans or tribes make as many “friends” as peoples who don’t? do they even have the opportunity?”

    One factor might be do different populations have different numbers of 4th cousin equivalents?

    Not entirely sure if this makes sense but does a clannish population comprise of clusters of closely related people i.e. 1st to 2nd cousin equivalent, with a big dip in the number of 3rd to 5th cousin equivalent people with the bulk of the rest of the population being 6th to 9th cousin equivalent.

    A population like that which later becomes exogamous (within an endogamous limit) ought (i think) to gradually average out their relatedness in the mid-range somewhere (with the final average level of relatedness depending on total population size and time spent in the exogamous environment).

    Reply

  20. @grey – “Not entirely sure if this makes sense but does a clannish population comprise of clusters of closely related people i.e. 1st to 2nd cousin equivalent, with a big dip in the number of 3rd to 5th cousin equivalent people with the bulk of the rest of the population being 6th to 9th cousin equivalent.”

    yes! that makes total sense. peaks of extended families spread out over the topography of the population with fairly deep valleys between them, yes.

    look here from henry harpending (from this post) at the kinship relatedness between members of the druze population (bottom chart) compared to the kinship relatedness between the japanese or french (bottom chart again). the druze are all over the place — you can imagine, relatively far away from each other in their population’s topography.

    people in really inbred societies might not be able to find “fourth cousins” to be friends with, even if they wanted to!

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  21. Turnpike
    “It would not be surprising if the differences in question turned out to be more fundamental than differences of intellectual interest. You can’t find your cousins if you have no freedom of movement.”

    Including female freedom of movement.

    Reply

  22. You referenced some of this in your counter currents interview but this study makes things more clear.

    Friends are as close to each other as fourth cousins.

    Jews are also as close to each other as fourth or fifth cousins.

    source: http://www.nytimes.com/2010/06/10/science/10jews.html?_r=0

    Therefore it is easy for Jews to make close friends by hanging out with other Jews.

    Meanwhile, it is harder for gentiles to make close friends in mass societies, as people move around and no longer live in a village near a bunch of closely related people.

    Certainly seems like it might explain a lot without implying a malicious intent on the part of certain parties.

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  23. this is a really interesting article, I wonder specifically how it relates to interracial best friends? For myself, I am asian and my best friends in life (at different points in time) have been white, black, mexican, and asian. Although ironically I prefer my group of friends itself to be asian, but on a one to one, best friend kind of basis, race doesn’t factor in as much. That being said, even when I had best friends of different races, it still wasn’t quite the same as having a same race best friend. I mean, on an unspoken level it just feels different. I’m not sure if its because of genetic factors, cultural factors, or my own subjective perspective, etc. (its probably all of them) Anyways, I’m not saying interracial best friends are necessarily better or worse, its just different. Although to me, in my opinion an ideal best friend would probably be one that I have stuff in common with, and is also the same race.

    Another interesting tangent I want to shed light on is; since becoming more aware of the commonality of multicultural messages in the mass media, I have begin to wonder just how much of my own thoughts and perceptions are really my own, at least with regards to my interactions with people of other races. I seriously wonder if my past “best friend” relationships with people of other races were actually as close as I thought they were at the time. Honestly…In my opinion its a little bit of both. I mean, my past interracial best friends were authentic friendships, but now that I think about it, they were severely lacking in depth. In a way, it was kind of a twisted charade where me and the (interracial) friend put on the motions of being best friends, but actually didn’t have an extremely meaningful connection. That being said, I now think the best approach when making friends of a different race is to accept that both parties are different from the get go and to work from there, as opposed to stupidly assuming that race doesn’t factor at all into a friendship. (LOL) I think that approaching interracial friends this way will ultimately lead to a more profound friendship later on.

    Oh crap, I wrote all this and I just realized this post is already two weeks old >_<

    Reply

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