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.

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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.

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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.

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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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