endogamy and genetic relatedness

this is just a short note — a little food for thought.

the following quote comes from catherine linley day’s FABULOUS ph.d. thesis, “Marriage Patterns in Two Wiltshire Parishes 1754-1914: Geographical Mobility, Consanguinity and Illegitimacy” [opens pdf]:

“In a theoretical isolated population of 500 people, after six generations all potential marriage partners would have been related to each other as 3rd cousins or closer (Fox 1967).”

the fox 1967 reference is to robin fox’s Kinship & Marriage: An Anthropological Perspective. i haven’t looked through fox’s book, yet, to see where linley day got this from.

in any case, it’s interesting to see how quickly endogamous mating patterns can lead to everyone in the population being quite related to one another (like in iceland or ashkenazi jews), genetically speaking. of course, no human population is totally isolated (right??), so you won’t find this exact scenario out in nature. but it’s interesting — and important — to keep in mind.

update 7/10 – calculating the inbreeding coefficient (see comments below):

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historic mating patterns in japan

readers (luke & jayman) request: what about the japanese? well, we aim to please… (^_^)

the japanese definitely have a history of cousin and endogamous marriages. i’m not sure, yet, how far back it goes (although i’m going to guess pretty d*rn far), but between 1912 and 1925 the consanguinity (first-/second-cousin) marriage rate for japan was 22.4% [pg. 29]. compare that to italy toward the beginning of the twentieth century or to some of the arab countries today. compare it also to the first cousin marriage rate amongst rural english folks in the 1870s: 2.25% (4.5% for the peerage).

but it’s been decreasing ever since (looks like a stock market crash – pg. 30):

by wwii the rate was only about 12.3%, and nowadays it’s like 4% (3.9% in 1983).

imaizumi, the author of the article to which i’ve linked above, also found in the early 1980s that 27% of recently married japanese folks had married endogamously, while amongst the oldest folks studied, 40% had married endogamously [pg. 39]. so endogamous marriages have also declined in japan over the course of the twentieth century. still, more than 1 in 4 japanese entered into an endogamous marriage in the ’80s (or maybe the late 1970s).

seems like the shintoists practice cousin marriage most frequently, followed by buddhists, and is lowest amongst catholics. farmers/fishermen, blue collar workers, the self-employed and people working in services (like transportation) inbreed the most, whereas white collar workers, salesmen and professionals inbreed the least.

note: the type of cousin marriage practiced in japan is mostly mother’s brother’s daughter (mbd) marriage as in china. more on that in the next post on japan. that’s important because mbd marriage amounts to less inbreeding (i think) than the arab type of cousin marriage (father’s brother daughter or fbd marriage) since all of the marriages do NOT occur exclusively in one lineage. in mbd marriage, at least more than one other lineage is involved.

the events of the meiji period obviously shook up the social structures in japan a LOT, but i wonder if cousin marriage/endogamy was officially — or even unofficially — discouraged in any way during that time period. i’m wondering if what happened in europe starting in the early medieval period regarding mating patterns has sorta been repeated in japan, only starting in the nineteenth century. -?-

goes to show, too, how rapidly cousin marriage rates can drop — within one generation in japan cousin marriage rates halved. maybe this could happen only amongst east asians who are big into conformity, but it’s something to keep in mind when trying to imagine what happened in europe in the medieval period, i.e. that things could’ve moved pretty quickly.

more anon!

previously: on the non-violent japanese of today

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