Re: exogamy, kinship, and heterozygosity

Gerold Firl (
3 Oct 1996 19:55:42 GMT

In article <52uo9m$>, (Robert Snower) writes:

|> (Gerold Firl) wrote:

|> >This is an amnplification of a brief note to daniel maltz about how
|> >culture-specific rules of exogamy and kinship exert an influence on
|> >levels of heterozygosity. Many in the soft sciences who loudly profess
|> >their fear and loathing for sociobiology seem to believe that they are
|> >struggling to oppose the encroachment of biological determinism into
|> >the study of culture and the arts; that is a misapprehension. I will
|> >try to explain how incest rules influence human genetics, specifically
|> >the level of heterozygosity.

|> Most sociobiologists believe that the prohibitions against inbreeding
|> which are found so frequently in human societies find their
|> explanation as you are suggesting here--they are adaptations
|> reflecting the benefits of heterozygosity, the injurious effects of
|> homozygosity. The former promotes variability, the latter reduces
|> variability.
|> Anthropologists, again as you suggest, sometimes prefer a less
|> biological explanation, a more cultural explanation.
|> Wilson, in his later works, held for this conventional view, the one
|> about homozygosity and reduction of variability. But in his original
|> _Sociobiology_ he was very much on the fence, pointing out that
|> inbreeding promotes kin selection and altruism, which would work in
|> the opposite direction.

That's a good point. This is a great example of the ecological analogy;
culture, as a complex adaptive system, evolves in ways which show
startling correspondance with biological evolution. This is not just a
consequence of sociobiological co-evolution, where genes influence
social structures and vice versa.

One of the basic questions of biology is: why sex? (I know, I know: sex
is not the answer, sex is the question - yes is the answer). Seriously,
from the evolutionary standpoint of a selfish gene, sex is partial
suicide. Why not stick with cloning, or parthanogenesis, so _all_ your
genes are propagated, instead of just half? If organisms are survival
machines which perpetuate genes, sexual reproduction must offer major
benefits to counteract the 50% loss of meiosis.

The answer is given clearly by bacteria, who do reproduce asexually by
fission; bacteria also exchange extra-chromosomal dna with each other,
thereby engaging in a form of genetic transfer which has the same
result as sexual reproduction: randomly combining genes with other
organisms, gambling that the fitness of your descendants will be higher
as a result.

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.

A good example illustrating these tradeoffs between keeping it in the
family and exogamous mixing is found in the royal incest of egypt and
hawaii. Here we have a situation where there is such a concentration of
resources in a single gene line that the costs of exogamy outweigh the
benefits; the benefits of incest outweigh the costs.

|> Shapiro, whose side I always seem to come down on, states flatly, in
|> _The Sociobiology of Homo Sapiens (1978)_, that heterozygosity and
|> homozygosity have nothing whatever to do with the incest prohibitions,
|> the endogamy/exogamy rules, the kinship constructions, of human
|> cultures.
|> His argument is as follows. Long before human societies came along,
|> the behavioral inclination pro heterozygosity, con homozgyosity was
|> selected for, spreading through the mammalian populations which
|> supplied the precursors for Homo sapiens. That is why there is
|> evidence that incest is rare among mammals, that they have built-in
|> behavior which inclines against it. So, for humans, the problem has
|> already been solved. There is every reason to believe the genetic
|> advantage of heterozygosity had already been selected for, by the time
|> they came along.

To some extent. Two points to consider:

1. Chimps have an incest prohibition on mother-son relations. As far
as I know, that is all they have. Anyone with further data on animal
incest inhibitions?

2. Humans have a pheromonically-mediated attraction for sex partners
with high degrees of genetic distance; one obvious benefit of such a
reaction is in the area of immune response. A higher degree of
variability in immune response confers a better chance of surviving the
next disease.

These are indications that humans have instincts promoting
heterozygosity and inhibiting incest - but how well do they work?

|> Yet the pre-historic human societies found it necessary to solve it
|> again, with fantastically complicated, extensive systems. Why?

Here's a couple of thoughts on that question: first, my earlier
hypothesis - human kinship systems go *beyond* our instincts to promote
exogamy to a greater degree than what existed among proto-cultural

After considering your question however, perhaps a more comprehensive
way to look at the situation is to view the range of human kinship
systems as a continuum of adaptive responses to different environments.
A median solution is cousin marriage, a very common system in a wide
range of societies. It's a compromise between the inbreeding extreme of
royal incest, and the extraordinary scope of incest taboos found in
first millenium europe, where *any* trace of family relation, no matter
how insignificant (or even purely symbolic) was ruled incestuous. A
stable environment, where accumulated resources could be maintained
without high levels of competition, could be expected to favor
inbreding. A more fluid environment, with unpredictable upheavals
necessitating a quick response and requiring social solidarity in the
face of external competition, would be more favorable to exogamy.

|> These systems were eminently successful in, among other things,
|> prohibiting incest. If there is one thing typical of ancient
|> societies, it is that they don't commit incest, except in the most
|> special and regulated of situations.

There are degrees of incest; is cousin marriage incestuous? Different
cultures give different answers.

|> Then why, if the problem was
|> being so successfully dealt with, should incest, and incest
|> prohibition, become a prevailing theme of the myths and literature of
|> the historic culture for which these ancient societies were
|> precursors? Why is it necessary to deal with this problem again, and
|> then again, and in such elaborate fashion?

|> Shapiro's answer is that the temptation of incest, along with its
|> prohibition, was a primordial social construct, originally highly
|> adaptive, accounting for the beginnings of human culture and society.
|> This primordial construct evolved into what we now see displayed in
|> myth, literature. Freudian, but entirely different from Freud's own
|> anthropological theories, which people do not take seriously.

Does that mean that cultures *promoted* incest, against the prevailing
wind of instinct? I'm trying to think of examples, but drawing a blank;
can you give examples? How would such a thing be done? It seems like
most cultures teach children that incest is dangerous.

Are you saying that primordial cultures promoted incest, but later
turned against it? That seems like a difficult proposition to
substantiate; what evidence supports such a view?

Disclaimer claims dat de claims claimed in dis are de claims of meself,
me, and me alone, so sue us god. I won't tell Bill & Dave if you won't.
=-=-=-=-=-=-=-=-=-=-=-=-=-=---- Gerold Firl @ ..hplabs!hp-sdd!geroldf