why mbd marriage amounts to less inbreeding than fbd marriage

in response to my post on why fbd marriage amounts to more inbreeding than mbd marriage (short answer: it results in more double-first cousin marriages), violet asks:

“I don’t understand why there can’t be double first cousins with mbd.”

good question. well, there certainly can be double-first cousin marriages in mbd societies, it’s just that the structure of mbd marriage doesn’t give the same push towards double-first cousin marriage as fbd marriage does.

what i should’ve done in the previous post on fbd (father’s brother’s daughter) marriage was to include diagrams of mbd (mother’s brother’s daughter) marriage along with the diagrams of fbd marriage, but to be honest i just got lazy (sorry!). so, without further ado, here is mbd marriage (you might want to have open the fbd marriage post at the same time):

ego (red triangle guy) marries his maternal first cousin, i.e. his mother’s brother’s daughter (mbd).

but who does ego’s brother (the triangle to the right of ego) marry? if he lives in a society in which mbd marriage is favored (china, for instance), and if there’s a female maternal cousin available, he’ll marry her (maybe/probably). in this case, that’s ego’s wife’s sister:

then what? well, in the next generation, unlike in fbd marriage, the kids of ego and his brother should NOT marry each other. the kids should marry their mothers’ brothers’ kids:

these kids are all first cousins, but they’re not double-first cousins (unlike in the fbd marriage scenario). the four kids do not share both sets of grandparents in common, whereas double-first cousins do.

also, you can see that there’s an additional party brought into this family tree — the yellow mom/aunt. she is not from ego’s patrilineage. she’s an outsider to some degree or another, and these wives that are brought in from the outside are why mbd marriage is often described as alliance building — different patrilineages build ties with one another.

so, the mbd marriage system doesn’t have the same push towards double-first cousin marriage as an fbd marriage system does. you can see this (i think) if you browse through the consang.net tables — there are more incidences of double-first cousin marriages recorded in fbd societies (arabs et al.) than in mbd societies (just about everybody else).

the reason any of this is good to know is because it is important to bear in mind that not all cousin marriage systems are the same — some result in more inbreeding or closer genetic relatedness between family members than others — and that should affect the evolution of “genes for altruism/other innate social aptitudes” in these populations.

previously: why fbd marriage amounts to more inbreeding than mbd marriage and tribes and types of cousin-marriage

(note: comments do not require an email. contrarian!)

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11 Comments

  1. Thanks for taking time to answer this. I am wondering what if “yellow circle” is “red triangle”‘s sister. It is more likely to happen with watta-satta, and we end up with double cousins again. Perhaps, mbd+ watta-satta = fbd in terms of inbreeding?

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  2. Violet: There’s probably a way to prove that that the MBD marriage rule would have been violated at some point if yellow circle was red triangle’s sister.

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  3. @violet – “I am wondering what if ‘yellow circle’ is ‘red triangle’‘s sister.”

    if “yellow circle” is “red triangle’s sister,” then the guy she married married his father’s sister’s daughter (fzd) and so the mbd marriage pattern wasn’t followed (all these marriage systems name everything from the point-of-view of a male “ego” — heh. males and their egos (~_^) ).

    you certainly do wind up with double-first cousins again, though. but then it happened in a mixed system — one side got to marry mbd, but the other side got stuck with fzd. it seems to me like it’s hard to get double-first cousins if everyone sticks to the mbd rule. but none of these systems are pure 100% systems.

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  4. @violet – “Thanks for taking time to answer this.”

    no problem! (^_^) it’s always good to re-think and re-re-think things (especially to make sure one hasn’t screwed up royally — been known to happen (~_^) ).

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  5. Oh hey look, there it is.

    Ahah, I see why this works, it’s because there’s a sex differential in the preference. Father’s Brother’s Daughter follows the chain “male-male-female”, where as Mother’s Brother’s Daughter follows the chain “female-male-female”. Ergo, then, a society where the (female) convention is to marry your Mother’s Sister’s Son should experience similar inbreeding to the FBD society, whereas a society that practiced marriage to your Father’s Sister’s Son should experience a similar amount of inbreeding to MBD societies.

    Of course, this all gets much more interesting as you add more sons, daughters, and multiple wives.

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  6. @wilhelm – “Mother’s Sister’s Son should experience similar inbreeding to the FBD society….”

    if you mean mother’s sister’s daughter (mzd) marriage, then yes. by convention, anthropologists work from the point-of-view of a male ego. (presumably some feminists somewhere have complained about this! (~_^) )

    @wilhelm – “Of course, this all gets much more interesting as you add more sons, daughters, and multiple wives.”

    starts to get like a big bowl of spaghetti!

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  7. hbd*chick,

    The male convention is preferred for good reason, of course, but I used the female convention as an illustration. I’d have to check if MZD marriage resulted in similar inbreeding by sketching the tree, but I’m lazy and don’t want to.

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  8. hbd*chick,

    In constructing the tree, it occurs to me that the sex of the progeny doesn’t matter, does it? Only the sex of the first relations in the chain matter. If they’re alike, double first cousin marriage is encouraged, if they’re unalike, it’s less frequent.

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  9. @wilhelm – “I’d have to check if MZD marriage resulted in similar inbreeding by sketching the tree, but I’m lazy and don’t want to.”

    you know what? it doesn’t. i said so in my post on fbd marriage, but i was wrong.

    i worked up some diagrams on the four basic cousin marriage forms in two previous posts (here and here), and mzd marriage looks a lot like mbd marriage, so it’s NOT as inbred as fbd marriage. sorry ’bout that — and thanks for being persistent with your comments! always good to check and be double-checked. (^_^)

    fzd marriage looks awfully similar to fbd marriage. i’ll have to draw that out to see if it, too, pushes towards double-first cousin marriages.

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  10. ” mzd marriage looks a lot like mbd marriage, so it’s NOT as inbred as fbd marriage”

    ??

    I can make the exact same chart as your FBD chart with MZD just by switching around all of the circles to triangles and vice versa. It works – it follows the MZD rule. Same degree of consanguinity between both, just from the maternal line instead of the fraternal. I can even boost the consanguinity by adding some lines and making a bunch of MZD double cousin marriages.

    What I would be interested in plotting is whether the fact that only in women (except fertile Klinefelter’s) do the X chromosomes have a chance to recombine and thus diversify. To what extent would MZD marriages decrease X chromosome diversity compared to FBD, FSD, MBD? There would be a decrease, but I’m guessing it’s only around the order of 25% – 50% depending on the comparison.

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