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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consanguinity + corruption = correlation

the awesome epigone has found a correlation of .44 between the amounts of (mostly) current consanguineous (first-/second-cousin) marriages in various societies as indicated by the data available on consang.net and perceptions of corruption by the people in those societies as found by transparency international (thnx, a.e.!). that’s higher than i would’ve guessed beforehand — i gave a bunch of reasons for that over @m.g.’s place which the epigone included in his post, so i won’t bother repeating them here.

i know that correlation is not causation, but it does “waggle its eyebrows suggestively and gesture furtively while mouthing ‘look over there,” so i’ll bet anyone a nickle — no, a dime! — that there is a connection here (and that connection is altruism/other innate social aptitudes [pg. 329+]).

i think audacious’ correlation would be even larger if there was some time depth to the inbreeding/endogamy data. what i’d like to see is:

– all the genes for altruism (and other innate social aptitudes) in man discovered so we (meaning teh scientists) can see the hbd differences in altruism, etc., in different populations and trace the evolutionary histories of all these genes in different populations. then someone could check for correlations between the gene frequencies and corruption (and other neat behaviors like nepotism).

in lieu of that, what i’d like to see is:

– all, or at least lots, of the people on the planet getting their dna sequenced so we (meaning teh scientists) can work out the degrees of relatedness within different populations so we (meaning teh scientists) could at least guess at the evolutionary histories of all these genes for altruism. then someone could check for correlations between the actual degrees of relatedness in different populations and corruption (and other neat behaviors like nepotism).

in lieu of that, what i’d like to see/do is:

– what the audacious epigone did but just with some time depth added to the inbreeding/endogamy data. plus, also, some consideration given to the fact that some forms of cousin marriage (i.e. fbd marriage) amount to more inbreeding than other forms of cousin marriage (e.g. mbd marriage).

for example, maybe two points could be awarded for each (likely) generation in which consanguineous marriages were common (haven’t considered what the cut off oughta be), and one point for each (likely) generation of endogamous marriage. zero points for marrying out. bonus point for fbd marriage. add ’em all up and then compare/contrast with corruption, et. al.

the problem is figuring out exactly how much inbreeding happened at any given point in the past for a population. i know there are ways to get at it by looking at dna — maybe what should be looked for are any correlations between runs of homozygosity in populations and corruption, etc. that would still be looking at a sort of proxy for the presence/frequencies of different sorts of genes for altruism, but it might be interesting anyway.

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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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inbreeding and the evolution of altruistic behavior ii

in Understanding Human History, michael hart did a real nice job of explaining how kin selection or inclusive fitness works and how “genes for altruism” could be selected for [pgs. 37-38]:

“For about a century after Darwin proposed his theory of evolution, the origin of altruistic behavior in animals remained a puzzle. It was not until the 1960s, when William D. Hamilton proposed his theory of kin selection, that a satisfactory explanation was given. That theory can perhaps best be explained by an example:

“Suppose a man sees his identical twin drowning in a river, and estimates (correctly) that if he were to jump in and try to save his brother the probability of success would be 80%, while the probability that he would die in the attempt would be 20%. Consider these two alternatives:

“a) Some of the man’s genes strongly dispose him to rescue his brother, and he therefore jumps in and tries to save him (‘altruistic behavior’).

“b) The man does not have genes that dispose him to rescue his brother, and he therefore stays on the shore and lets his brother drown (‘selfish behavior’).

“In case (b), exactly one copy of the man’s genes survives, and may later be replicated. However, in case (a), if the rescue attempt is successful, two copies of the man’s genes survive (one in his own body, one in his brother’s). As this will happen 80% of the time, on average 1.6 (= 0.80 × 2) copies of the man’s genes will survive. In this situation, therefore, genes that dispose a person to altruistic behavior will — on average — have more surviving copies than genes that dispose a person to act selfishly and will be favored by natural selection.

“Now consider a slightly different example. Suppose that the man on shore is a brother — but not a twin — of the person who is drowning. Case (b) will still result in one copy of his genes being preserved. However, since ordinary siblings share only 50% of their genes, if the man on shore succeeds in rescuing his brother then (on average) 1.5 copies of the man’s genes will survive. Since 80% of the attempts will be successful, case (a) will on average result in 1.2 (= 0.80 × 1.5) copies of the altruistic genes surviving. Since 1.2 is greater than 1.0, the altruistic genes will be favored by natural selection in this case too.

“Suppose, however, that the two men were not brothers, but merely first cousins. First cousins, on average, share only one-eighth of their genes. In this case, altruistic behavior results in only 0.9 (= 0.80 × 1.125) copies of the man’s genes surviving, and natural selection will therefore favor the genes for selfish behavior.

“The upshot is that a gene that disposes its bearer to behave altruistically toward a close relative can have a selective advantage over one that disposes its bearer to act completely selfishly. Furthermore, this can occur even though the relative never returns the favor, and even if the survival of the relative does not increase the group’s chances of survival. It is not necessary that either reciprocal altruism or group selection operate for kin selection to result in the spread of genes that dispose their bearer to act altruistically toward close relatives.”

what’s missing from these examples is, of course, inbreeding. and depth of time.

take michael’s second example up there…

“Suppose that the man on shore is a brother — but not a twin — of the person who is drowning.”

…but let’s add that the parents of these brothers were first-cousins. that makes these two guys: brothers AND second-cousins (i.e. the children of two first-cousins). so they probably share not only 50% of their genes in common as brothers, but also 3.13% of their genes in common as second-cousins. so the “push” to jump in the water to save the brother/cousin must be somewhat stronger in the inbred pair than for the brother to save just a plain ol’ brother.

now let’s take this example of michael’s…

“Suppose, however, that the two men were not brothers, but merely first cousins. First cousins, on average, share only one-eighth of their genes.”

…but let’s make them double first-cousins rather than just first-cousins. what happens then?

well, while first-cousins probably share 1/8th or 12.5% of their genes in common, double first-cousins share … well, double that! … or 1/4 or 25% of their genes in common.

what happens to michael’s calculation then?

“In this case, altruistic behavior results in only 0.9 (= 0.80 × 1.125) copies of the man’s genes surviving, and natural selection will therefore favor the genes for selfish behavior.”

in the case of double first-cousins the calculation becomes 0.80 x 1.25 = 1.0. that’s just breaking even using michael’s example, but what if the odds of saving the cousin from drowing are better than 80%?

or what about the depth of time i mentioned above? what if the family of my double first-cousins has been inbreeding for a very long time. a very, very long time. like for fifty generations or more. then the relatedness between all the family members, including these double first-cousins, will be even closer. natural selection ought, then, to favor such double first-cousins jumping in to save each other.

as wade and breden showed (see also previous post), inbreeding can help to accelerate the rate of the evolution (or frequency in a population) of altruism genes [pg. 846]:

[T]he increase in matings between homozygous parents decreases the genetic variance within families, because these matings produce genotypically homogeneous arrays of offspring.”

repeated inbreeding in a family reduces the diversity (whoa!) of the allele types within that family, and if we’re talking about “genes for altruism” here, then the variety of those must get reduced within inbred families, too. in a population that consists of, say, ten inbreeding families, the one that has super-duper altruism genes that lead all of its family members to help each other out more than the members of the other families will have the advantage (provided selection favors that advantage for whatever reasons). and those super-duper altruism genes will no doubt eventually spread to the other families since, in reality, no family groups inbreed 100% of the time anywhere — there will pretty definitely be gene flow between families. so then you’ll get a whole population of super-duper family altruists (note that these people are NOT altruistic to unrelated individuals).

the human populations on earth today that inbreed most closely (within patrilineages) and often practice double first-cousin marriage — AND have been doing this for prolly at least a couple of thousand years (time depth) — are the arabs (who later spread these mating practices to the maghreb, the mashriq and far off places like iraq and afghanistan and all the other ‘stans) and some peoples in the levant like the druze. i think that, because of their long-standing mating practices, they are the prime human examples of wade and breden’s accelerated evolution of altruism thanks to inbreeding.

previously: inbreeding and the evolution of altruistic behavior and more on inbreeding and the evolution of altruistic behavior

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