why fbd marriage amounts to more inbreeding than mbd marriage

*update 07/09 – see bottom of post.*

i keep saying that father’s brother’s daughter (fbd) marriage — the favored type in the arab world — amounts to greater inbreeding than mother’s brother’s daughter (mbd) marriage — the usual type almost everywhere else (including china) — but i haven’t been able to put my finger on exactly why.

i showed before that fbd lineages fold back in on themselves while mbd lineages draw in new brides from other lineages. now i finally figured out exactly what’s so inbred about fbd marriage:

because it leads to more double-first cousin marriages, and double-first cousins share, on average, more genes with one another than first cousins.

*****

first, some illustrations.

here is fbd marriage:

ego, who is a guy of the male persuasion, marries his paternal first cousin, i.e. his father’s brother’s daughter (fbd). ok? ok.

but who does ego’s brother (the triangle to the right of ego) marry? if he lives in a society in which fbd marriage is favored (anywhere in the arab world, for instance), and if there’s a female paternal 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, the kids of ego and his brother should also marry each other, continuing the fbd marriage tradition:

and those kids are not just first cousins but double-first cousins because, not only are their fathers brothers, their mothers are sisters. they share both sets of grandparents in common. if you think about you and your first cousins, provided you’re not from a society where fbd marriage is the norm, then you only share one set of grandparents with your first cousins.

in societies where fbd marriage is preferred, there must be an increase in the number of double-first cousin marriages compared to other cousin marrying societies. it’s not something that happens one hundred percent of the time, but it must happen more than it would have if fbd marriage wasn’t preferred because there is this push to marry within the patrilineage.

*****

and now for a little math (a very little, i promise you!).

first cousins have an inbreeding coefficient of 0.0625. double-first cousins, 0.125.

imagine two societies, A and B — and in each of these societies, half of the marriages are between first cousins (consanguinity rate = 50%) — but in society A, all of the cousin marriages are between first cousins, while in society B, all of the cousin marriages are between double-first cousins. then we wind up with different coefficients of inbreeding for each of the populations:

society A: 0.50 x 0.0625 = 0.03125
society B: 0.50 x 0.125 = 0.0625

society B is more inbred (has a greater coefficient of inbreeding) than society A because of all the double-first cousin marriage.

so, if you look at the consang.net data, for example, china today has a cousin marriage rate of ca. 5%, while syria has a rate of ca. 31.6% (per woodley and bell). that’s a substantial difference in the amount of cousin marriages between the two nations; BUT, 7.8% of marriages in syria are between double-first cousins, so there is an even greater difference in the degree of inbreeding between the two countries than one might first suppose.

*****

so, not only does the arab world have more cousin marriage than most other societies today, it also has a more inbred form of cousin marriage than everyone else. although it doesn’t occur in arab societies one hundred percent of the time, the higher frequency of fbd marriages in those populations must make the arab world potentially even more inbred than populations with no tradition of fbd marriage.

note that mother’s sister’s daughter’s (mzd) marriage ought to be as similarly inbred as fbd marriage — it’s the mirror image of fbd marriage. mzd marriage occurs occasionally here and there, but it is not a favored type of cousin marriage anywhere that i know of. wrong! wrong, wrong, wrong. (thnx, wilhelm!)

update 07/05: see also why mbd marriage amounts to less inbreeding than fbd marriage

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16 Comments

  1. @spandrell – “I wonder why doesn’t it happen 100% of the time. If fbd marriage is good, why doesn’t everyone do it?”

    sometimes it must have to do with numbers — if you have five sons, but your brother only has two daughters, what are you going to do?

    also, i imagine that other times a father might want to make an alliance with someone other than his brother — maybe his second cousin who is better positioned economically/socially. then you marry your kid off to that person/his kid if you get the chance.

    Reply

  2. “If fbd marriage is good, why doesn’t everyone do it?”

    Numbers. The trade-off between creating a group who are extremely closely related is the group has to be small. The standard form of cousin-marriage outside of the Islamic world creates a slightly larger group who are slightly less closely related. The numbers probably relate to the needs of pastoralism vs farming imo with the optimum number of related allies being higher for farmers. If so then the FBD form becoming established as a side-effect of the Arab conquests would have been a backwards step that would have taken a few centuries to show itself.

    Reply

  3. I don’t understand why there can’t be double first cousins with mbd. If a brother-sister sibling pair marries the cross cousins brother-sister sibling pair (Pakistan watta-satta?), their children will become double first cousins and still be eligible to marry each other.
    Why is then fbd particularly bad?

    Reply

  4. @violet – “I don’t understand why there can’t be double first cousins with mbd…. Why is then fbd particularly bad?”

    i replied to that with a post. (^_^)

    also, as an aside, pakistan is one of those fbd marriage countries, so watta satta might often happen between the kids of brothers.

    Reply

  5. note that mother’s sister’s daughter’s (mzd) marriage ought to be as similarly inbred as fbd marriage — it’s the mirror image of fbd marriage. mzd marriage occurs occasionally here and there, but it is not a favored type of cousin marriage anywhere that i know of.

    If the driving force behind this is environmental i.e people adapting to their environment – with exceptions where a particular form has become associated with a religion – then mzd marriage might follow the same pattern of being the optimum for a particular (and seemingly much smaller) niche.

    If so and if FBD evolved as the optimum for patrilocal pastoralism in a particularly harsh environment (low pop. density) and the MBD form evolved as the optimum for most farming environments then i think the elements that might drive MZD could be things like
    – matrilocal
    – marginal terrain
    – low MRR (Marginal Return on Raiding)

    so that gives very marginal farming environments where boosting resources by raiding isn’t an option (which would mean reducing the “band of brothers” effect might not be maladaptive in that case).

    Eskimos?

    Maybe not.

    High-altitude terraced farming relying on high yield from small amounts of land (i.e. not suitable for herds)?

    Sounds possible.

    Reply

  6. hbd*chick, why don’t you plot the family trees for a hypothetical MBD marriage culture, and see if the consanguinity is as high.

    I think you will find they’re at similar risk for double first cousins.

    Reply

  7. @g.w. – “High-altitude terraced farming relying on high yield from small amounts of land (i.e. not suitable for herds)?”

    incas? bhutanese? some other hill-tribes somewhere?

    Reply

  8. hubchik
    “incas? bhutanese? some other hill-tribes somewhere?”

    Yes that’s the sort of thing i was thinking. I recall reading years ago about various odd (in marriage terms) tribes up mountains around the edges of India and China – very marginal with very little space for animals (which i think is important for the rate of return on raiding as a food supplement).

    Reply

  9. […] not an exclusive practice. A blogger who writes on human biodiversity points out that there are two ways cousin marriage leads to inbreeding. Because of how the Y chromosome is passed down, the sons of marriages between men and the […]

    Reply

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