tribes and types of cousin marriages ii

in the previous post on tribes and types of cousin marriages, i posted a couple of graphics illustrating father’s brother’s daughter (fbd) marriage and mother’s brother’s daughter (mbd) marriage, showing how fbd marriage is a closed system while mbd marriage enables alliances with other lineages/clans.

here now are the last two forms of cousin marriage: father’s sister’s daughter (fzd) marriage and mother’s sister’s daughter (mzd) marriage. they’re not very different from the other two — fbd and fzd marriage are both closed systems, and mbd and mzd marriage both enable alliances with other lineages/clans — it’s just that the connections are slightly different, i.e. ego marries the daughter of his one of his aunts (fzd & mzd) versus his one of his uncles (fbd & mbd).

note that the coefficients of relatedness in cousin marriage are highest for mzd marriage; next highest for mbd marriage; and lowest for both fbd and fzd, which are the same (if i’ve done my maths right!). mbd marriage is the most common form of cousin marriage around the world; fbd marriage is the most common in arab countries and places like iran, afghanistan and pakistan.

here we go. father’s sister’s daughter (fzd) marriage over several generations (triangles are boys, circles are girls. compare with fbd marriage):

and mother’s sister’s daughter (mzd) marriage over several generations (compare with mbd marriage):

the thing with either mbd or mzd marriage is that the bride is always being brought in from an outside lineage or clan. fbd and fzd marriage is diametrically the opposite — the bride is from the same patrilineage as ego.

also, in mbd and mzd marriage, in each generation a bride can be brought in from a different lineage or clan, so ego’s lineage can build up many alliances with many other lineages/clans over time. some societies apparently have, traditionally, had arrangements in which several lineages would swap brides over several generations in a cyclical system, thus building up extended familial connections and, therefore, alliances. i imagine that very large tribes could be built up this way, whereas the fbd/fzd system results in segmented lineages, i.e. clans and sub-clans and sub-sub-clans that always seem to be squabbling.

previously: tribes and types of cousin-marriage and genealogical terminology and what is a tribe?

(note: comments do not require an email. happy hour, anybody? (^_^) )


  1. Something I would like to see is a tribe and clans breakdown of some small areas in a country like, say, Iraq: who are the chiefs, how many clans, total population of, how do they make their livings, and how do they fit in to the new “representative” form of government we are trying to establish there. Anthropology on the county level in other words. No doubt this kind of fine-grained data is hard to come by, but, Christ, we’ve been there for ten years already.


  2. @luke – yeah, that would be interesting! i betcha some studies like that have been done by anthropologists attached to the military. will have to keep an eye out for any such reports.


  3. Very interesting. If one accepts the premise that there is a balance of benefit and disbenefit to endogamy (that varies with the organisational scale you’re looking at) then it starts to get easy to see this as different people’s trying to find the sweet spot for their environment and their highest potential scale of co-operation. The balance of benefits might change when the environment changes or their highest potential level of co-operation changes but by that point the marriage system is locked into the culture.

    “note that the coefficients of relatedness in cousin marriage are highest for mzd marriage; next highest for mbd marriage; and lowest for both fbd and fzd, which are the same (if i’ve done my maths right!).”

    If they’re closed systems though won’t there be compounding effects?


  4. I believe you have mislead yourself with your own diagrams.

    Specifically, in the case of the mzd diagram you fail to consider the likely genealogy of the husbands brought in at the left-hand side of the diagram. You claim they will always be from an outside lineage, but in fact this is only the case because for some reason the patrilineage you consider produces only male children (whereas in the fzd case the lineage under consideration produces both male and female children).

    The same error affects your previous analysis of fbd vs. mbd marriages.

    All you have produced is an argument in favour of killing female offspring in the interests of better genetic diversity in societies with certain forms cousin marriage. (There is an equivalent argument in favour of killing male offspring in the other cases.)

    I do not think that your diagramming technique is a particularly good one for comprehensively considering the effects of different forms of intermarriage, because it is too easy to inadvertently draw a diagram which happens to support whatever argument being made. If you are going to use diagrams, it would be wise to make sure you include all combinations of offspring as well as look at the matrilineages (it should be obvious, for example, that – absent other factors – matrilineal genetics in a msd-favouring society will be equivalent to patrilineal genetics in a fbd-favouring one).


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s