pathogens and consanguinity

remember the woodley & bell paper about the connections between consanguineous matings and democracy (or lack of) that i said i was going to post a lot about? and then i got distracted? well, let’s get back to it.

by way of introducing woodley & bell’s idea that pathogens (and consanguinity) and democracy don’t go together, the last post in this thread of posts had to do with possible connections between pathogens and individualistic vs. collectivistic societies — i.e. seems like the more pathogens in your environment, the more collectivistic (ethnocentric, conforming, suspicious of outsiders) your society is going to be. now i want to look at one more paper before i get back to consangunity and democracy.

in On the Adaptive Origins and Maladaptive Consequences of Human Inbreeding: Parasite Prevalence, Immune Functioning, and Consanguineous Marriage [opens pdf], hoben, et al., present their finding of a correlation between frequencies of pathogens and frequencies consanguineous marriages in various societies around the world. it’s not a huge correlation — r = 0.39 — but it is there. the more pathogens you’ve got in your environment, the more likely you are to practice cousin marriage. kinda/sorta.

like the paper in the last post, these researchers worked from the consang.net data and the historic pathogen data from this paper. hoben, et al., didn’t include any nifty charts in their paper, and since i like my data served up visually, i made a chart of my own (click on image for LARGER view):

x-axis=their pathogen index. positive numbers (z-scores) indicate higher frequencies of pathogens; negative numbers, lower frequencies. bosnia-herzegovina has a z-score of 0; england has a z-score of -1.01. burkina faso has the highest z-score at 1.16. canada has the lowest z-score at -1.31.

y-axis=weighted mean consanguineous marriage rates.

i think there is a broad, general pattern here. and it makes some sense. as the authors say:

“[I]ncreased homozygosity and other genetic coadaptation that results from inbreeding can facilitate highly specific forms of immunological resistance to local parasites, and … these immunological benefits will be most pronounced under ecological circumstances in which endemic pathogens are more highly prevalent (Denic et al., 2008; Denic and Nicholls, 2007; Fincher and Thornhill, 2008a, b; Shields, 1982)…. [O]ur study indicates that, under certain circumstances (i.e., high pathogen prevalence), inbreeding may have advantages that outweigh its costs.”

but the correlation was not that high. and i think part of the reason for that is “technical” — that is that there are some “glitches” in the consang.net data that the researchers didn’t take into account.

take brazil, for instance. high on the pathogen index (0.930) but pretty low consanguinity-wise (4.348%). but that’s because brazil was fairly recently settled by westerners (and blacks and japanese) who brought roman catholic traditions with them when they settled there — in other words, cousin marriage prohibitions. so of course the consanguinity and pathogen frequencies don’t match very well for brazil. what should be looked at are native brazilian consanguinity rates which are more like 13% (only one study, unfortunately — see page 3** [opens pdf]).

the authors have also calculated the consanguinity data for slovakia as 11.618%, but that includes gypsies [pg. 10 – opens pdf] who are consanguineous wherever they go, so that is undoubtedly skewing the numbers for slovakia. the consanguinity figures for czechoslovakia are more like 0.2% [pg. 1 – opens pdf].

the BIG outliers, though, are the arabs and all their middle eastern/north african/south asian muslim buddies. they are the ones throwing off the correlation completely. i’m talking about most of the dots that are right in the middle of the chart: 0.5000 or lower on the pathogen index (so, not a lot of pathogens) but above the 20% consanguinity rate:

regionconsang. ratepathogen index
uae – 36% – -0.450
kuwait – 51.7% – -0.340
iran – 32.2% – -0.150
oman – 35.9% – -0.140
pakistan – 51% – 0.020
saudi arabia – 38.4% – 0.040
libya – 37.6% – 0.040
jordan – 31.2% – 0.160
afghanistan – 55.4% – 0.230
syria – 31.6% – 0.300
lebanon – 26.6% – 0.360
yemen – 35% – 0.410
egypt – 31% – 0.440

these societies are amongst those that have the highest consanguinity rates, and yet some of their pathogen index scores are very low. the united arab emirates, for instance, scores like france (-0.460) or the republic of ireland (-0.450). qatar has got the exact same score as australia (-0.250). and saudi arabia and pakistan have lower scores than italy (0.160).

clearly having pathogens in the environment is not the whole story when it comes to the push towards cousin marriage. and hoben, et al., don’t claim that either:

“To complement those partial explanations [economic, etc.] for the persistence of consanguineous marriages, in the present research we offer an additional explanation that we label the ‘parasite hypothesis of inbreeding.'”

a complementary explanation. i agree! prolly the most fundamental, underlying one i would guess.

previously: consanguinity and democracy and consanguinity and islam and democracy and pathogens and culture

(note: comments do not require an email. **note to english readers: sorry, no half-naked women on that page 3.)

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15 Comments

  1. “Our study indicates that, under certain circumstances (i.e. high pathogen prevalence), inbreeding may have advantages that outweigh its costs”

    I think that must be the case – for various reasons – else it couldn’t have been so widespread for so long (at various slightly diluted levels) and only supplanted in modern times by what was effectively a bit of a fluke.

    .
    “the BIG outliers, though, are the arabs and all their middle eastern/north african/south asian muslim buddies.”

    A marriage culture evolved to suit one environment becoming dominant elsewhere through conquest?

    Reply

  2. @g.w. – “A marriage culture evolved to suit one environment becoming dominant elsewhere through conquest?”

    yeah, absolutely. my friend korotayev talked about how people in, for instance, pakistan and afghanistan and iran wanted to emulate their arab conquerors. of course. if what they’re (the conquerors) are doing works so well for them — copy it!

    it’s really weird to be practicing fbd marriage when you’re a farmer though (like some groups in afghanistan and pakistan). it’s not a very good fit.

    Reply

  3. Could the correlation simply be a result of latitude? The real outbreeders are cold-dwellers (NW Euros and the Anglosphere), and of course pathogen load is negatively correlated with latitude. I wonder what would happen if one controlled for latitude, or more specifically, average winter temperatures.

    Reply

  4. gw and hbd chick,

    but the data shows that the arabian peninsul does not have a big pathogen load. so there is no really good explanation as o why the arabs inbreed so much in the first place.

    why does everyone assume interbreeding is the defult condition and inbreeding must be 0explained? it could easily be the other way around. it isnt like cousin marriage is THAT bad for fitness, I doubt it is as bad as the amount of time energy and resources that exogamous people waste finding a mate. suppose the situation is the other way around, humans “normally” breed with cousins, except when certain cultural conditions force them not to. perhaps those conditions only arise in regions with low pathogen load, but not all regions with low path make the transition–again for cutural reasons, or just by blind chance. is it implausible?

    Reply

  5. @jayman – “Could the correlation simply be a result of latitude?”

    yeah, i think that’s probably right. the farther away from the equator you get, the farther away from pathogens you get.

    notably, the other outbreeders seem to be the bushmen and apparently (i read in passing the other day) the eskimos. otoh, the yanomamo in south america and the papua new guineans are inbreeders.

    the arabs and their pals are odd in that they inbreed so much compared to their latitude/pathogen index. saudi arabia and north africa are around the same latitude as northern india or southern china, so you’d think their inbreeding levels ought to be something like those places, but it’s not. obviously something else is at play here in addition to pathogens. still, i really like the pathogen idea otherwise. (^_^)

    Reply

  6. @bleach – “but the data shows that the arabian peninsul does not have a big pathogen load. so there is no really good explanation as o why the arabs inbreed so much in the first place.”

    i think (and this is only an idea i’ve got) that, along with pathogen load, prolly something like population density+resource availability affects inbreeding rates.

    if you’re bushmen or eskimos with fairly low populations and enough resources to go around (i.e. big game, seals), then i think there’s no push towards strong inbreeding (like cousin marriage) ’cause there’s no huge need to keep the resources in the family. otoh, if you’re yanomamo or png’ers with relatively high populations and limited resources, you might start inbreeding closely to keep the resources — i.e. the tribe’s territory — in the family. (the png’ers, btw — or some of them anyway — regularly marry their second cousins. haven’t read up on the yanomamo yet, but a little bird told me that they’re close breeders.)

    the arabs certainly had/have (apart from the oil today) some pretty scarce resources. that may be why they experienced a push towards very close marriage. you really want to keep the goat herds (and water resources?) in the family in that part of the world!

    again — just an idea.

    Reply

  7. @bleach – “why does everyone assume interbreeding is the defult condition and inbreeding must be 0explained?”

    i haven’t been assuming that interbreeding or outbreeding is the default condition. in fact, i’ve said many times that most peoples everywhere at most times seem to be/have been inbreeders (marrying cousins or at least endogamously).

    having said that, in the last week or so i’ve been reading about the bushmen and am wondering if i need to change my assessment a bit. (^_^) i’m wondering if inbreeding becomes default with larger population sizes. (plus the pathogen thing.)

    @bleach – “suppose the situation is the other way around, humans ‘normally’ breed with cousins, except when certain cultural conditions force them not to. perhaps those conditions only arise in regions with low pathogen load, but not all regions with low path make the transition–again for cutural reasons”

    yes, that certainly makes sense for europeans (esp. nw europeans) if what i’ve presented so far on the ol’ blog here is at all correct.

    nw europe — relatively low pathogen load — so when the cultural (religious) push for outbreeding came, it was not that big a deal to shift.

    interestingly, a couple of nw european groups who i think (given their clannish/extended-family social structures in the late medieval/early modern period) kept inbreeding for longer than the rest were the frisians and the east anglians. both groups lived in marshy areas and were at risk for malaria (i do believe) — until those swamps were drained and/or modern times with pesticides/modern medicine. on the other hand, maybe the fact that they didn’t alter their mating patterns so quickly just had to do with being unable to adopt the new farming techniques. dunno.

    it’s interesting, too, that southern european groups that are/have been at risk for malaria — southern italians, greeks — have kept right on inbreeding up until modern times.

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  8. @bleach – “but the data shows that the arabian peninsul does not have a big pathogen load. so there is no really good explanation as o why the arabs inbreed so much in the first place.”

    teh anthropologists think that the cousin marriage form that is preferred by the arabs — father’s brother’s daughter’s (fbd) marriage (which, i think, pushes populations towards the highest degrees of inbreeding) — originated in the levant and then was introduced to the arabs by jewish tribes migrating down into the arab peninsula. the logic of it is that the frequency of groups practicing fbd marriage is highest in the levantine region, therefore it must’ve originated somewhere there.

    greying wanderer has questioned that and wondered if the spread wasn’t in the other direction.

    now i’ve got an idea — one of those friday-evening ideas that may/will prolly amount to nothing (~_^) — an idea to try to tie in pathogen load to fbd marriage. if fbd marriage started with the arabs, maybe it started down in yemen (or somewhere along the coast there) which has a higher pathogen index score than the arab peninsula. they’ve got a bit of a semi-tropical climate in parts of yemen — along the coast anyway — which must account for their higher numbers of pathogens. maybe fbd marriage started down there somewhere? (i’ve got y-chromosome haplogroup j1 in the back of my mind here.)

    just some wild speculation!

    Reply

  9. bleach
    “why does everyone assume interbreeding is the defult condition and inbreeding must be 0explained?”

    I think on here we mostly think inbreeding (in various forms) is the human default and outbreeding was a NW euro anomaly – but an anomaly that had a massive impact.

    Other than on here i guess it’s because of W.E.I.R.D

    http://neuroanthropology.net/2010/07/10/we-agree-its-weird-but-is-it-weird-enough/

    “The article outlines two central propositions; first, that most behavioural science theory is built upon research that examines intensely a narrow sample of human variation (disproportionately US university undergraduates who are, as the authors write, Western, Educated, Industrialized, Rich, and Democratic, or ‘WEIRD’).

    More controversially, the authors go on to argue that, where there is robust cross-cultural research, WEIRD subjects tend to be outliers on a range of measurable traits that do vary, including visual perception, sense of fairness, cooperation, spatial reasoning, and a host of other basic psychological traits.”

    i.e. most of the research has been undertaken by people descended from NW Europe who naturally assumed their “normal” was the same as the rest of the world’s when in fact their normal was very weird compared to the rest of the world.

    Also, i personally tend to take an idea, assume it is true and then see how far i can run with it until it hits a wall. However in this case, as you say,

    “but the data shows that the arabian peninsul does not have a big pathogen load.”

    in this particular case it hits a wall almost straight away.

    .
    “so there is no really good explanation as o why the arabs inbreed so much in the first place.”

    I think it makes perfect sense for low pop. density desert pastoralists because i think it will have significant benefits in the context of small-scale, low-tech violence through a “band of brothers” effect. The costs, in that context, may well be lower than the benefits. However most farmer populations around the world seem to have extended that model a bit to create a slightly less tightly-knit but larger network of related allies which i assume has a more optimal cost-benefit in those environments. If so then the extension of the desert pastoralist marriage model into various farming populations via the arab conquest might partially explain the differences between the muslim and non-muslim parts of the mid-latitude countries e.g. India and Pakistan.

    Reply

  10. hubchik
    “having said that, in the last week or so i’ve been reading about the bushmen and am wondering if i need to change my assessment a bit.”

    I’m thinking hunter-gatherers, pastoralists and farmers will have different pressures. From my reading hunter-gatherers often have complex moiety type systems that prevent inbreeding which i think may have developed because their pop. density was very low so without those systems they’d inbreed very close, very quickly.

    Once farming is developed you have both higher population density and inheritance so the farmers could afford to inbreed a bit to keep the extended family unit as cohesive as possible.

    It may not just have been nutrition that made hunter-gatherer skeletons bigger and stronger than farmer skeletons.

    .
    “if fbd marriage started with the arabs, maybe it started down in yemen (or somewhere along the coast there) which has a higher pathogen index score than the arab peninsula. they’ve got a bit of a semi-tropical climate in parts of yemen — along the coast anyway — which must account for their higher numbers of pathogens. maybe fbd marriage started down there somewhere?”

    Yes, i was wondering that. If the pathogen idea is correct it may have started further south and travelled up changing into the more common paternal / maternal split form *except* where it suited a particular niche e.g. desert pastoralism, for some some other reason not related to pathogens.

    I think the “marriage-forms evolve to fit the environment except when modified by accidents of history and religion” idea is simpler though.

    (Yemen is a particularly interesting place for a lot of reasons

    http://en.wikipedia.org/wiki/Ancient_history_of_Yemen

    )

    Reply

  11. the bushmen and eskimos could also fit JayMan’s “latiudinal hypothesis”. bushmen live in the one region of Africa which has a non-tropical climate, and eskimos live way up north of course. their low inbreeding rates relative to other Africans and Indians might have somethingto do with that.

    Reply

  12. @bleach – “the bushmen and eskimos could also fit JayMan’s ‘latiudinal hypothesis’. bushmen live in the one region of Africa which has a non-tropical climate, and eskimos live way up north of course.”

    oh, absolutely! that’s what i meant, in fact — i mean, that’s why i brought them up. both groups are clearly not in the pathogen-filled tropics (lucky them!). (^_^)

    Reply

  13. oh I see. I got confused when you started talking about pop. density and other stuff (I’m inbred s I’m a bit slow, sorry!)

    Reply

  14. @bleach – “I’m inbred s I’m a bit slow, sorry!”

    heh! (^_^) no, that’s ok. my fault. i was rambling a lot in those comments — lots of ideas but none of them formulated so they all just came out in a goopy mess. (~_^)

    Reply

  15. quoting myself

    “I’m thinking hunter-gatherers, pastoralists and farmers will have different pressures. From my reading hunter-gatherers often have complex moiety type systems that prevent inbreeding which i think may have developed because their pop. density was very low so without those systems they’d inbreed very close, very quickly.”

    What i mean by this is if (early) farming increased pop. density over hunter-gathering by a factor of say 5 then the same valley that supported 400 hunter-gathers might support 2000 farmers. So if the farmers had a system where the valley’s extended familes formed into 5 marriage-based alliances i.e. clans, of 400 people each then those endogamously marrying farmer clans would still have the same total breeding population as the 400 maximally exogamous (through a moeity system) hunter-gatherers.

    So it’s the combination of pop. density and marriage culture that gives you your total breeding population.

    Although, even if the resultant total breeding population of two very different marriage systems comes out as the same number i wonder if the different marriage systems may have varying effects in terms of the the three things that i’ve (probably wrongly) been lumping under the term inbreeding.

    1) first cousin double recessive large-effect gene
    2) double recessive large-effect gene becoming common in (very?) small populations where everyone is a close cousin from multiple different directions. (this wouldn’t happen if it was too harmful i’d guess – at least not too harmful in the context it developed)
    3) genetic load – accretion of lots of dented genes of small effect

    If the above list is correct then marriage-systems have been selection for a long time and potentially in quite a big way.

    Reply

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