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