If marrying cousins is so bad why does everyone want to marry their cousins?

The above figure illustrates the geographic distribution of the prevalence of people marrying people closely related to them. Mostly this involves cousin marriage. Most people know the urban legends around the debilities that occur due to cousin marriage, but traditionally the focus has been on rare recessive diseases (e.g., albinism). Now, a massive new study has been published (more than 400 authors, with sample sizes for 1 million or more for some characteristics) looking at a variety of traits, Associations of autozygosity with a broad range of human phenotypes:

In many species, the offspring of related parents suffer reduced reproductive success, a phenomenon known as inbreeding depression. In humans, the importance of this effect has remained unclear, partly because reproduction between close relatives is both rare and frequently associated with confounding social factors. Here, using genomic inbreeding coefficients (FROH) for >1.4 million individuals, we show that FROH is significantly associated (p < 0.0005) with apparently deleterious changes in 32 out of 100 traits analysed. These changes are associated with runs of homozygosity (ROH), but not with common variant homozygosity, suggesting that genetic variants associated with inbreeding depression are predominantly rare. The effect on fertility is striking: FROH equivalent to the offspring of first cousins is associated with a 55% decrease [95% CI 44–66%] in the odds of having children. Finally, the effects of FROH are confirmed within full-sibling pairs, where the variation in FROH is independent of all environmental confounding.

The offspring of first cousins have on average 0.10 fewer children. On an individual level, this is not that great of an effect. But in an evolutionary population genetics sense this is a serious selection coefficient.

On the whole, the paper is impressive in its scope. There are even sibling analyses to confirm the impact of runs of homozygosity causing problems due to rare alleles (since this paper involved r.o.h, of course, Jim Wilson is involved!).

Rather, I want to ask: if inbreeding is so bad genetically and biologically, why is it so common? One of the consequences of the Protestant Reformation is that the Roman Catholic Church’s strict enforcement of consanguinity rules were dropped, and cousin marriage became much more common among elites (such as the Darwin-Wedgewood family). The material rationale for cousin marriage is actually rather straightforward, in that it keeps accumulated property and power within the extended lineage. Marriages between children of brothers may cement alliances, while matrilocality and marriages between cross-cousins in South India have been associated with lower domestic abuse rates (in contrast, in North India strongly enforced exogamy has been associated with the idea that women marry into an alien household).

I would suggest perhaps that though marriages between relatives are biologically disfavored, there are many cases where it is culturally beneficial. In societies where collective family units engage in inter-group competition, some level of consanguinity may benefit cohesion. Other societies where individualism is more operative may exhibit no such incentives.

Note: I don’t see great evidence of purging genetic load in populations with more inbreeding. The rare variants are probably replenished constantly through mutation?


9 thoughts on “If marrying cousins is so bad why does everyone want to marry their cousins?

  1. I would say that there might be a bias in the study too, because 1st cousin marriages seem to be disproportionally associated with lower social classes and mentally unstable people.

    And a lot of cultures, like some Kurdish clans, which practise some sort of cousin (not necessarily 1st obviously) marriage regularly, do so for many generations.

    Another aspect is the quality of the relatives and the traits they have. Its like it is with dogs if people try to create a new physical standard: They can get there slowly, by constantly caring for a low inbreeding overall, or they can do it fast, and risk side effects of too much inbreeding.

    The same applies to humans. Obviously the best would be, if a family wants to cultivate its traits, if they don’t inbreed but get partners with the traits in the question without particularly close genetic relations. However, they can improve their standard by inbreeding, its just a greater risk and it gets more risky the longer it goes.

    There were always jokes about some inbred aristocratic or researcher families, but they created extraordinarily individuals far above the average population with a lot of their qualities.

    So I’d say it can pay off biologically too, its just a greater risk to take and it needs to be justified by qualities which are of great value and infrequent in the population.

    You can observe this in some Ashkenazi, Amish and Indian caste families to use well-known, important examples. They are not below average in all respects, quite obviously, and there is in a lot of instances no way around close-relative, cousin marriages in their population history. They paid a price by a higher genetic liability, but it might have been worth it. From the cultural perspective for sure, from the biological probably as well.

    Another comparison would be sickle cell anaemia which is an actual disease: It is a huge burden, it results in horrible morbidity on a regular basis, but it was worth it in the face of a specific challenge (Malaria).

    Of course if you have low level cousins intermarrying on a regular base without a goal and selection favouring specific valuable traits, infrequent in other potential partners, it can be only disadvantageous. Because then you get the burden and nothing but short term social and economic advantages in exchange.

    Yet talking about 0,10 less children and comparing that with cultural degradation resulting in childless or low fertility in otherwise healthy people because of cultural instability and disorientation, to keep your pack together, even by these means, might be worth it as well.

    Of course, its better to make positive traits more frequent without excessive inbreeding from a purely biological perspective. But then again, like I know from villages in Central Europe, people in the past preferred to know the potential partners and their family very well. They wanted to keep the family homogeneous and to know the partners background, his family. If a family had a lot of idiots, to say it blunt, they might have been excluded or having a hard time getting partners at all, even the better ones were associated with the worse.

    So people were looking not just at the individual, but the whole family for qualities or burdens they might have had. Its not they were always that stupid biologically, even on the contrary, unless the perspective of money and wealth through marital relations made them so.

  2. Marriages between children of brothers may cement alliances

    Ortho-cousin marriage systems like that are quite rare in the global history of the mankind, and may have only been spread with the Islam. Most of the cousin marriage, especially in the past, has been of cross-cousin variety (between the children of a brother and a sister), as evidenced by equivalencies in the systems of terms of kinships even in the peoples who long ceased practicing cousin marriage.

    Among the culturally acceptable and widely practiced consanguineous marriage types across the world, there is an even closer uncle-niece marriage allowed by the Halacha traditional law (aunt-nephew is proscribed though)

  3. Most of the cousin marriage, especially in the past, has been of cross-cousin variety (between the children of a brother and a sister)
    and it probably makes an event better sense, as it allows to leverage sibling ties to strengthen community alliances to the maximum, while at the same time enabling exogamy and patrilocality?

  4. I would say that there might be a bias in the study too, because 1st cousin marriages seem to be disproportionally associated with lower social classes and mentally unstable people.

    they accounted for this in their study in the regression. also, sibling comparisons replicated 80% of their effect so they got the effect right…


  5. I remember watching Robert Sapolsky talking about evolutionary bio and casually dropping the fact that optimal cousin marriage is between 3rd / 4th cousin’s for maximum fertility. I think that comes from looking at Icelandic historical records. So not sure what to make of that fact but I guess it makes some sort of sense from a germ theory point of view.

  6. I heard that uncle-niece marriage is common in Hindu India and that it is even worse from a biological perspective than cousin marriage. Is this true?

  7. The reduction in IQ due to consanguineous breeding are well-known. This paper is the first time I’ve heard of fertility rate decline due to the same. Two possibilities come to mind.

    1) Perhaps MENA and South Asia will have the lowest fertility rates in the world by, say, around 2040.

    2) Perhaps such dysgenic breeding was the REAL cause for the collapse of Rome.

  8. In Albania in the Kanun law which was the law till the beginning of the 20 century in remote areas of North Albania, you could married your first cousin from the milk lineage which is from your mother but not the 20th cousin in the blood lineage which is the paternal side.

  9. Razib: In societies where collective family units engage in inter-group competition, some level of consanguinity may benefit cohesion. Other societies where individualism is more operative may exhibit no such incentives.

    This is a topic where ancient dna could probably tell us something quite useful. In the modern day we tend to have lots of modernized groups, and even groups which are rural tend to be impacted by these changes. Ancient dna will tell us how changes we see in consang relate to features of ancient societies much more directly? Aka “What are the circumstances for the emergence of consanguinity?”.

    From what we have so far, I don’t think there is much a signature of consang in ancient dna from essentially anywhere.

    This may be because capture adna is not very good for this and is the most productive method?

    Particularly I can’t think of much that has gone on looking at this question in ancient dna from the Near and Middle East. Ancien Bactrian-Margianan, Levant Bronze Age, Iranian Iron Age, Swat Iron Age consang would all be interesting, and ancient genomes from the steppe where some postulated extraordinary degree of clan competition (is there any?). Poss. limitations of data though.

    Re; family unit level competition vs individualism, I would be pedantic (and appreciate you’re trying to keep it fairly pithy and I’m telling you something you know!) and say this may in practice be more about transition from clan family level competition to narrow extended family or nuclear family competition, and other forms of collective competition that are not family based (e.g. village based under “feudal” orders?). Rather than moving from clan based directly to recent, radically individual societies (“I put the I in WEIRD”).

    This is possibly fairly pedantic as you can argue that a narrower extended or nuclear family level of competition is still a tradeoff towards individualism, but I thought it was worth making the comment.

    (Clan competition seeming pretty possible as a driver for inbreeding – https://www.nature.com/articles/s41598-018-27047-3“Close inbreeding and low genetic diversity in Inner Asian human populations despite geographical exogamy”. Turko-Mongolic groups geographically exogamous, but actually about as inbred as more localized geographically endogamous Indo-Iranian speakers (who of course today are less long range pastoralist, though this is a distinction which is dependent on circumstance) “This suggests that in Inner Asia, geographical exogamy is neither efficient in increasing genetic diversity nor in avoiding inbreeding, which might be due to kinship endogamy despite the occurrence of dispersal.”).


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