Is inbreeding like asexuality?

By Razib Khan | July 23, 2012 10:50 pm

The standard argument for why there is aversion to incest among humans as matter of innate disposition is the Westermarck effect, which is a model where aversion to mating emerges if you are raised with an individual of the opposite sex. Some basic illustrations are sketched out in The Blank Slate: The Modern Denial of Human Nature. But some comments below make me wonder if there is are alternative explanations. Robin Fox has made the claim, repeated in many places, that cousin marriage was ubiquitous in the human past:

As an anthropologist I am forced to face the fact that for the vast majority of our existence as a species close cousin marriage must have been the norm, if for no other reason than that most of the time there was no one but cousins to marry. Indeed I have spent much of my professional life analyzing the complexities of systems of marriage that not only allowed but insisted on cousin marriage by rule. Not only was it not forbidden, it was prescribed, often with a particular degree of detail. You were enjoined, for example, to marry your mother’s brother’s daughter but not your father’s sister’s daughter, or required to marry a mother’s father’s sister’s daughter’s daughter, and forbidden to marry a father’s father’s sister’s daughter’s daughter. The details don’t matter. What matters is that in small-scale societies with low mobility, spouses were drawn from a pool of close relatives. Marriage relationships once set up were perpetuated over the generations by the rules of cousin marriage. Even in nomadic societies like the ancestors of the Semitic people, marriage with a close cousin was prescribed. The ideal marriage was between the children of two brothers, and this remains both the norm and the practice in Arab and Muslim societies today.

I’m moderately skeptical because of the problems with pedigree collapse. Additionally, I don’t recall the South African Bushman genome had a much lower mutational load than the other samples, which would be the case of recessive alleles were always being exposed in “small-scale societies.” In fact, cousin marriage seems to increase in some societies, such as Saudi Arabia and Pakistani, with modernity (larger families due to modern health results in more marriageable cousins).

Let’s grant Fox’s anthropological observation. I think there are some genetic problems with this. How we do resolve the two? The comments below got me thinking in functional terms: perhaps groups which promoted consanginuous practices successfully to leverage short-term gains of cohesion tended to go extinct in the long-term because of a mutational meltdown event? This can explain Fox’s observation of the ubiquity of endogamy, and yet still avoid the problems which repeated generations of cousin marriage tend to produce genetically. Human history may have been a balance between the cultural benefits of establishing tighter kin relations through intra-familial/clan marriage, and the genetic benefits of outbreeding. This is analogous to the macroevolutionary patterns with sexual vs. asexual lineages. The latter tend to be found near the “tips” of phylogenies as derived lineages; a strong clue that they are ephemeral, and tend toward extinction. Yet at any given time asexuality can seem common because it is an effective short-term strategy.

Addendum: Just to be clear, I am implying here there may be multiple reasons for incest aversion. A functional model regarding the balance between genetic and anthropological factors would operate at the level of the group and meta-populations. But within the group there might also be individual level tensions, responsible for the Westermarck effect.

CATEGORIZED UNDER: Anthroplogy
  • Solis

    Additionally, I don’t recall the South African Bushman genome had a much lower mutational load than the other samples, which would be the case of recessive alleles were always being exposed in “small-scale societies.”

    I don’t know much about the bushmen’s lifestyle; but if I recall, their bigger genetic diversity is because, since they are one of the most ancient groups, the genetic characteristics have accumulated across the centuries; and not because they don’t practice endogamy.

    I may be wrong though.

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #1, i’m not talking about genetic diversity, though that’s kind of related (inbreeding reduces effective pop. size & genetic diversity). i’m talking of mutational load.

  • http://forwhattheywereweare.blogspot.com/ Maju

    Cousin marriage as described above must be a Neolithic pehnomenon and, while common, by no means universal. The main reason for cousin marriage is to keep the lands (or other properties) in the family as compactly as possible or even increase them; it’s motivation is economic and implies private property of land (or other key resources like cattle).

    Nothing like that existed in the Paleolithic. And in some cases at least, it is known among hunter-gatherer peoples to get visitors to lay with the local women in order to increase genetic diversity, vigor.

    Of course, even with visitor genetic input the genetic pool was surely quite small when the total number of people was small, but cousin marriage was surely avoided in the larger ethnic groups – while in the smaller isolated ones, like the peoples of the Far North it was probably unavoidable at least to some degree.

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #3, i don’t know the anthropology well enough, so i won’t dispute fox. rather, i express my skepticism because of my intuition from the genetic consequences over the long term (e.g., much higher Fst across human populations due to limited gene flow and more drift). but, i think that the existence of small groups which limited population size through extended weaning, etc., may actually argue against cousin marriage. that’s because the pool of partners who are cousins shrinks. one reason cousin marriage is feasible even more in neo-trad societies like modern pakistan are the huge families due to modern medicine.

  • Steve Gunnell

    How many low population societies had moiety rules such as: (http://en.wikipedia.org/wiki/Australian_Aboriginal_kinship) that minimised the effect of having a small pool of potential mates?

  • wijjy

    You have to worry about how informed Prof Fox is about the effects of inbreeding. A quote from his article:

    Students ask: “Why then did our ancestors not all die out as the result of genetic inbreeding?” Good question. The answer is on the one hand that those supposed bad effects only are dangerous if there is bad stock to begin with. On the contrary, if the genetic stock is good, then close inbreeding would perpetuate it. (Think of thoroughbred horses.) On the other hand, it has been shown that even if genetic diversity is somewhat lowered by inbreeding, small periodic doses of out-breeding rapidly restore it and even speed up evolution. Also, if there are true deleterious effects then those carrying them would rapidly die out and the bad genes with them. The problem would be self-correcting. (See my Red Lamp of Incest.)

    He appears to be almost completely misunderstand the genetics of inbreeding. Also he does not make any distrinction between marriage between first cousins (bad) and between second and further cousins (much less harmful).

    Pakistani first cousin marriages in the UK are boon for whole exome sequencing studies of disease.

  • wijjy

    #2 Are you sure inbreeding reduces effective population size?

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #7, first page of chap 4 of falconer 1996:

    Ne = 1/(2 [change in F]), where F = inbreeding, and change is evaluating over generations.

    i think the intuition is simple: inbreeding reduces the set of your ancestors in the recent past. so it’s like a bottleneck. a bottleneck is just a form of inbreeding. some of these issues is what i was trying to get at in the previous post re: inbreeding: inbreeding is often just an epiphenomenon of stochastic processes in pop gen.

    a simple way to think about it. someone whose parents are cousins have fewer distinct great-grandparents than someone whose parents are not related.

  • Justin Ma

    #6…What exactly is wrong about the quotation? The only part I see that I may disagree with is that it may speed up evolution, but even then I understand the point. The thouroughbred horses anaology is apt. As a breeder (plants), I certainly understanding the ill effects of inbreeding – but you have to remember, it only takes one generation of outbreeding to return to an inbreeding coefficient F of 0. Hybrid vigor occurs in both plants and animals, and I’m sure Fox was alluding to the fact that using inbreds can enhance heterosis.

    Let’s also remember, selection reduces effective population size and genetic diversity. Genetic diversity isn’t good per se, it is only good to the extent that continued evolution requires it.

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    As a breeder (plants)

    don’t think you think that the effects of inbreeding on plants (and their ability to purge genetic load through selfing, etc.) is operationally different from what occurs with complex metazoans? from what i know about mammalian breeding programs which make recourse to inbreeding, it’s always a difficult balancing act.

  • Justin Ma

    Yes, they are certainly different, and there is certainly a balancing act. Line 1 Herefords (http://www.ars.usda.gov/is/AR/archive/mar10/hereford0310.htm) are a great example that demonstrate the balance. I only wanted to emphasize that not everything is a movement toward outbreeding. Inbreeding is easily remedied by a little bit of outcrossing. Also, a little bit of migration from an outgroup can have a pronounced effect on diversity.

  • Sandgroper

    What Maju and Steve Gunnell said.

    Not only were Australian Aboriginal kinship laws ubiquitous throughout the continent (when some other deeply ingrained cultural practices were not), but very strictly and severely enforced.

    I have to entertain at least some skepticism about Fox’s certainty on this.

  • wijjy

    #8

    Students ask: “Why then did our ancestors not all die out as the result of genetic inbreeding?” Good question.

    Begging the question. It assumes that his explanation of ubiquitous cousin marriage is correct

    The answer is on the one hand that those supposed bad effects only are dangerous if there is bad stock to begin with.

    Everyone is bad stock – some people perhaps worse than others, but there will be few individuals without serious deleterious mutations that inbreeding can help to express

    On the contrary, if the genetic stock is good, then close inbreeding would perpetuate it. (Think of thoroughbred horses.)

    Thoroughbred horses can run fast, They are also extremely temperamental and are prone to a number of genetic diseases. Do we have records of the horses with severe defects that enabled us to breed out any bad traits?

    On the other hand, it has been shown that even if genetic diversity is somewhat lowered by inbreeding, small periodic doses of out-breeding rapidly restore it and even speed up evolution.

    I suppose this is Sewall Wrights shifting balance

    Also, if there are true deleterious effects then those carrying them would rapidly die out and the bad genes with them. The problem would be self-correcting. (See my Red Lamp of Incest.)

    For deleterious recessives then inbreeding will increase the speed of selecting out variants. But this is not quick for cousin marriages. I hate to be the person who says “think of the children” but I cannot regard diseased individuals as a fair price to pay for breeding out recessive traits.

  • Chad

    I actually find it more interesting how much attention/replies the topic of inbreeding gets in comparison.

  • AG

    Condition for multational meltdown is small population. Most bacteria reproduce asexually. But it depends on incredible large population to sustain its future. With enough number, a few with zero mutational load are possible and serve as progenitor.

  • wijjy

    #14

    For human geneticists inbreeding interesting because
    With next gen sequencing we can see deleterious mutations have effect. When agreggated we can indirectly observe the effects of mutational load, and get an insight into genetic variation.

    It is controversial because we hear from someone like Fox that it’s no big deal because cousin marriage increases the proportion of birth defects from 2% to 4%, and also hear from clinicians who are trying ameliorate the effects of these birth defects on real people.

  • Chad

    #16, I understand why it is interesting scientifically. I am a plant geneticist. You do not have to explain to me why inbreeding would be of interest to a geneticist.

    My comment was on my surprise on the interest this particular subject seems to garner from reader audience as a whole compared to other topics, which I would have suspected to be of greater general interest.

  • wijjy

    #18
    Well inbreeding lies on major fault lines between science and religion/culture, between pseudoscience and science (you can find lots of stuff about how inbreeding isn’t that bad), between freedom and the good of society, in the UK the cousin marriage debate has to do with immigration and forced marriage. I am sure there are other reasons.

  • Chad

    #18

    I am sure this is a reflection of cultural difference then. In the US, inbreeding is generally something to be mocked, used as a slur against rural and southern states and lower class citizens. I can’t say there is anything in the way of a national debate on the issue. Perhaps it is a local one with large immigrant populations, but certainly not one that has risen to the national level. It is also an issue amongst certain sects, such as the Amish, but not necessarily due to a cultural tradition where such marriages are encouraged, but because of the fact that few outside that population are willing to live such lifestyles. Because of the apolitical nature of those groups, debate on the issue doesn’t really extend past the community.

    Because of the cultural stigma against inbreeding in the US, I never would have guessed it to be a subject of larger social interest in the West, let alone a national debate. Rather I would have assumed it to be more of a passing social interest. But then I don’t get worked up much about social issues, having a rather Libertarian bent. Furthermore, while the relevance to human evolution is obvious, I have seen many other relevant topics garner less interest than this one, hence the nature of my reply. Had I been aware that this was considered a matter of actual debate in other Western nations like the UK, it would have made more sense why there would be a strong interest in it.

  • wijjy

    #19

    There is a problem with Pakistani immigrants from some communities and inbreeding. There is a claim is that this is done to bring the whole family over to the UK. A couple of press cuttings below.

    700 children born with genetic disabilities due to cousin marriages every year

    It’s my choice…

    It really is OK to fancy your cousin

    breeding antagonism – It’s islamophobia

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #20, follow up: the main problem in my eyes is not the increased problems of the offspring of first cousin marriages in relation to outbred populations. the problem is that customary/traditional preference for this sort of match can lead to very inbred lineages very quickly. i.e., the samaritan problem.

  • http://www.twitter.com/theogonia Shashi

    “You were enjoined, for example, to marry your mother’s brother’s daughter but not your father’s sister’s daughter, or required to marry a mother’s father’s sister’s daughter’s daughter, and forbidden to marry a father’s father’s sister’s daughter’s daughter.”

    In Hindu society there is a concept of a “Gotr” which is basically a clan of people who have descended from a common male ancestor (sometimes mythical). I think the above statement fits pretty well with how the orthodox population sees matrimonial alliances on a greater scale. In North India, you hear about honor killings of sons/daughters often over the fact that they were of the same ‘gotr’. Something similar is popular as well in South India (at least in Telegu and Tamil anecdotally) where marrying cousins from the matrilineal side is a ‘good’ thing. Actually, it’s a lot more complicated than that, and I don’t know if anyone cares to know.

  • Solis

    #2

    I see, sorry about that.

    Anyone knows if there’s statistics on how “inbred” countries are? All I have been able to find is statistics of consanguineous marriages.

    http://www.consang.net/index.php/Summary

  • http://dispatchesfromturtleisland.blogspot.com ohwilleke

    Usually, indeed almost always, Razib’s posts are a model of clarity. Somehow, this particular one just isn’t clicking for me. I feel like there is a leap of logic or two that I’m missing somehow.

    “This is analogous to the macroevolutionary patterns with sexual vs. asexual lineages. The latter tend to be found near the “tips” of phylogenies as derived lineages; a strong clue that they are ephemeral, and tend toward extinction. Yet at any given time asexuality can seem common because it is an effective short-term strategy.”

    A little bit more of a foundation to establish in what kinds of situations you see asexual v. sexual lineages, how they are related to modern human reproduction in inbred communities, and what implications are associated with asexually reproducing population have would be helpful in understanding this analogy.

    I recall Razib talking about some of this many months ago in the context of genetic load dropping that might have parallels to mixed mode reproduction (perhaps in the context of Game of Thrones analysis?) and found it to be fascinating and thought provoking. It was a new idea I’d never considered.

    But, I’m having trouble remaining confident that I’m still really getting the point being made as I try to make the leap in this fairly short post from thinking about the Westermarck effect and to reaching back in my memory to actually recall the points made in the earlier post about the implications that arise from animals with mixed mode reproduction.

    “Addendum: Just to be clear, I am implying here there may be multiple reasons for incest aversion. A functional model regarding the balance between genetic and anthropological factors would operate at the level of the group and meta-populations.”

    If I’m I understanding this addendum correctly, the gist of it is that community-wide mutational melt down wipes out incest tolerant communities thereby continually thinning out the ranks of incest tolerant communities a manner distinct from the Westermarck effect.

    If I have understood this statement correctly, are there any anecdotal examples of this happening? I can recall a couple examples where this arguably happened in elite superstates (Hapsburg inbreeding in Europe associated with hemophilia, and ancient Egyptian royal inbreeding leading to health problems like those seen in King Tut).

    But, I’m hard pressed to recall even a single historic example of a community experiencing mutational meltdown in the community as a whole (with the possible example of post-isolation by water Tasmania, but that seemed more like a simple loss of critical mass population leading to technological degredation thing).

    If I haven’t understood this statement correctly, what am I missing?

  • http://dispatchesfromturtleisland.blogspot.com ohwilleke

    Just to note one counterexample of tendencies towards there being lots of cousin marriage societies, or at least lots of culture specific “weird” rules governing inbreeding, the traditional incest rule in Korea prohibits marriage between people of the same surname, which seems like not such a big deal until you realize that about 95% of Koreans share about twenty surnames.

    Then again, the need to a very expansive on average (albeit prone to false positives and false negatives) rule to control inbreeding may be appropriate in a relatively genetically homogeneous population at the tail end of multiple serial founder effects.

  • Grey

    I think the existence of moiety type systems tends to point to closer marriage being the norm beforehand else why were they invented?

  • Sandgroper

    Seriously, have you seen the guy’s hat?

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #24,

    re: sexual vs. asexual. there is a big short term gain in asexuality. so that lineages are always speciating toward this lifestyle. but it seems that over the long term these asexual lines go extinct fast. asexual lines tend to be derived, indicating they’re young. the connection to the model i’m positing for incest is that there may be short term gains for human groups which are hyper-endogamous in terms of cohesion. but over the long term they aren’t genetically sustainable. e.g., a disease could wipe them out really quickly.

    in terms of mutational meltdown, the samaritans seem to be close that. they’re changing their attitudes to intermarriage with non-samaritans because of this. the parsis too.

  • muhr

    I don’t think it’s true that cousins were often the only people around to marry. A recent paper (Co-Residence Patterns in Hunter-Gatherer Societies Show Unique Human Social Structure) looked at the degree to which HGs live with kin and they found that primary kin represents around 10% of a band. They elaborated with Ache and Ju/hoansi and found that primary and distant kin (up to 5th degree relatives) make up 25% of adults in a band. If those numbers were representative of the past than there were plenty of non-relatives to marry and I imagine non-relatives would have been favored for marriage.

    The authors favorably mention Bernard Chapais’ idea that reciprocal exogamy was important to the emergence of cooperation between groups, as such cousin marriage likely didn’t occur among their study populations.

    I can’t imagine an endogamy practicing family faring too well when within a greater group of HGs that they were unrelated with.

  • http://dispatchesfromturtleisland.blogspot.com ohwilleke

    @28 Thanks. That is helpful.

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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