Re: exogamy, kinship, and heterozygosity

Gerold Firl (
4 Oct 1996 21:07:10 GMT

In article <>, Shannon Adams <> writes:
|> Gerold Firl wrote:

|> > But lets get back to culture and kinship. You pointed out that
|> > inbreeding promotes kin selection and altruism, which then facilitates
|> > the development of social/cultural systems based on higher levels of
|> > cooperation. That creates a mutually reinforcing selection pressure
|> > in favor of inbreeding. But just as in the biological case, there is a
|> > balancing pressure in the other direction favoring exogamy, both from
|> > the biological realm and the cultural: exogamy produces not only more
|> > physically vigorous individuals, but also promotes political alliances
|> > with neighbors. This latter factor has been somewhat overemphasized by
|> > many anthropologists, who choose to highlight artifact and symbol at
|> > the expense of a more balanced view which would also weigh the issues
|> > of survival and adaptation, both cultural and biological.

|> One point I need to make first is that what you term "exogamy" means
|> different things itself. Cross-cousin marriage is "exogamous" in
|> relationship to the kin groups of a society. BUT cross-cousin marriage in
|> "endogamous" in that the two come from the SAME CULTURE/SOCIETY.

You lost me there. It seems to me that until *very* recently, *all*
people married within the same culture/society. Anthropologically,
looking at human kinship and marriage, that should be a given.

Cousin marriage is very common in many societies. It could even be
considered the norm. We need to think statistically, in terms of
*degrees* of exogamy; if marrying a sibling (common inherited gene ratio
of .5) has an exogamy rating of 0, and marrying a total stranger with no
known common relations has an exogamy rating of 1, then cousin marriage
would rate as .25 and marriage with a half-sib would rate .5. Cousin
marriage is moderately exogamous.

|> In fact if
|> you really think about it the gene pool in non-western cultures/societies in
|> remarkably small. When non-western peoples practice exogamy weren't not
|> talking about pulling someone from another state<U.S.>. We're talking about
|> pulling someone from another village (most-likely a village that alreay
|> contains some genetic relations even though those relations may not be
|> expressed culturally) With this in mind I think it is understandable that
|> anthropologists have "overemphasized" the political alliances because, truth
|> be known, the person is probably marrying a genetic relative anyway.

Yes, but how close? How many recessives do they share? Recessive genes
which confer a slight fitness benefit when heterozygous, but impose a
large penalty when homozygous, need to be kept dispersed throughout the
population. The ratio of fitness costs/benefits for homo/heterozygous
recessives will determine the optimum level of exogamy.

|> Although marriage rules of a given culture do (usually) promote (immediate)
|> kin group exogamy, overall an endogamous marriage pattern shows itself. If
|> Barry from kin group A marries his cross-cousin, Susan from kin group B
|> (common ancestors already)and then Susan's younger brother,kin group B,
|> marries Barry and Susan's daughter, kin group A (in many cultures this is an
|> exogamous relationship) can you really argue that exogamous marriage rules
|> promotes "more physically vigorous individuals" based on a widening gene
|> pool?

First of all, the assertion that progeny issueing from parents descended
from genetically diverse populations will be more vigorous is a general
rule observed in a wide variety of species. We observe this at all
levels of biological taxa.

The example you give, of a culture practicing cousin-exchange marriage,
is not what I would consider a cultural adaptation for artificially high
heterozygosity. It may be higher than what is found in our closest
relatives, the chimps and bonobos however. On the human adaptive
continuum it would appear to be a pretty standard strategy.

Lets look at that human continuum again. At one extreme, we have the
royal incest of the egyptian pharoes; at the other, the roman catholic
incest prohibitions of the late first millenium. Cousin marriage falls
in between. As noted previously, a kin-selection analysis of incest
prohibitions shows a trade-off between keeping it in the family (deep
roots for deep pockets) vs. sharing the wealth (hedge your bets). Look
at the pharoes: here was a family with something like a million personal
servants, and all the resources of the kingdom at their disposal.
Infinite wealth, by the standards of the time. That kind of accumulated
wealth makes exogamy very expensive, and makes the cost of inbeeding
appear comparitively lower. Note the tendancy of the european royal
houses to inbreed; the costs do eventually mount.

The phenomenon of royal incest suggests a pattern for the degree of
exogamy mandated by kinship rules: the more stable the society, the less
exogamy will be required. Societies which have settled into a stable,
predictable pattern of existance will gravitate toward endogamous
marriage, while societies embroiled in unpredictable change will find
higher degrees of exogamy to be more adaptive. The pattern seems to
hold. There may well be other factors which are important (such as the
specialized adaptations for low population density discussed earlier)
but the correlation between social stability and endogamy makes a lot of

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me, and me alone, so sue us god. I won't tell Bill & Dave if you won't.
=-=-=-=-=-=-=-=-=-=-=-=-=-=---- Gerold Firl @ ..hplabs!hp-sdd!geroldf