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WEIRD cultures collapse traditional social technologies

In the comments Matt reminded me of this preprint that came out last week, Human Parental Relatedness through Time – Detecting Runs of Homozygosity in Ancient DNA. When I first skimmed it my thought was on how ingenious the methods for generating runs-of-homozygosity (to measure inbreeding) from lower coverage (i.e., low quality) ancient genomic results were. It was ingenious precisely because it had a “well, why didn’t I think of that!” quality.

But as Matt notes, the results on the data are interesting too! The first author shared the r.o.h. table, so you can look at them yourself. The major thing to note is that the estimates suggest cousin-marriage was far less common in the prehistoric and historic past than it is in some modern societies. In particular, it is far less common than it is today in the Islamic world and in India. That being said, the r.o.h values do decrease with agriculture from hunter-gatherer periods, indicating that large farming societies were more exogamous…before a recent shift in some areas to endogamy. Wouldn’t this pose some issues in regards to the arguments The WEIRDest People in the World?

How to make sense of all this?

There are several dynamics going on. First, these results put paid to the notion promoted by some behavioral ecologists and anthropologists that most marriages in the past were cousin-marriages. This is just not true. Even hunter-gatherers tended to be exogamous. So was there a cultural change that led to the shift in r.o.h with agriculture? (and its increase with pastoralism in their dataset) No. I think one of the issues we have to remember are simple structural parameters that have nothing to do with ideology.

Hunter-gatherers were at a much lower density than farmers. Elevated inbreeding was almost certainly a function of this ecological circumstance, as gene flow across hunter-gatherer populations was less common simply due to the lack of regular interaction. In the data, the authors point out that a lot of their elevated r.o.h. samples are from islands. Islands don’t impose ideology, they impose limits to contact. The rise of pastoralism and mobility on the steppe in some ways recapitulated the lifestyle of hunter-gatherers (note that I do believe less habitual contact probably meant more xenophobia, so the structural parameter probably had an ideological consequence which amplified what we’re seeing here).

So what explains the pattern in the modern world, and why is it some different from the past? The authors, for example, had transects from Pakistan, and in the past people in this region were not nearly as inbred. The dynamic that is important to remember is the confluence between population growth and the development of what Samo Burja calls “social technology”. In a modern world where societies are undergoing demographic transition and in the earlier stages of that process, so subject to massive growth, there will be lots of cousins to marry. In Malthusian societies, where families are just replacing themselves, there will not be as many cousins to marry. In other words, when there is a strong ideology of cousin-marriage, the limitation is going to be the number of cousins.

As many Islamic societies undergo demographic transition, I predict cousin-marriage will decrease as a phenomenon simply due to the reality that smaller families produce smaller kindreds from which one can select a mate.

But the second aspect here is social technology. In David Reich’s group’s work on India, it seems clear that strong endogamy as we see today did not really crystallize until ~1,500 years ago. Just as one can think of WEIRD culture as a social technology, so Islamic marriage and kinship norms, or that of Hindu jatis, are similar. The practices of these societies are not from time immemorial but develop due to the exigencies of history and social evolution. Practices that we see as “conservative,” such as arranged-marriage, are actually innovations.

Massive population growth has resulted in massive kindreds in many societies. These kindreds have well developed social technologies which are optimized to increase the group-level fitness. The combination of these dynamics has supercharged and spread practices such as arranged-marriage and consanguinity which in the past may have been rare, or accessible only to elites.

If Joe Henrich’s thesis in The WEIRDest People in the World is correct, between 500 and 1000 AD as other societies become more kindred based, the West became less so. It’s special-path diverged greatly up to the present because the combination of material and social technology is the recipe for the amplification of differences of trajectory. The West did not divergence from the rest, everyone diverged from the starting point in the past…

Addendum: Jim Wilson is not a co-author on this paper, even though it is about runs-of-homozygosity. But he is cited, of course (though only once!)

7 thoughts on “WEIRD cultures collapse traditional social technologies

  1. Good analysis with some new ideas here. As adna technologies hopefully become even cheaper and large sets can be sampled from medieval and early modern graveyards in regions like Pakistan and the Levant, we can hopefully see even when the timing of consanguinity happens in relation to trends in population growth, and corroborate against written records.

    That can test if large – and possibly unevenly distributed? – population growth led to development on close-kin marriage, and whether there was a trend led by elites or not (as far as we can systematically tell on elite status from burials).

    There’s actually a bit more of a reason, from my point of view, for these large medieval to early modern samples now. If you’re looking at understanding deep population structure for insights mainly into medical genetics but also human history (or vice versa), then that’s a less interesting era – people then are mostly ancestral to people there today, big whoop. But understanding how kin biased marriage norms evolved and grew and under what social and economic conditions, adds some more impetus. Probably engaging for a traditional archaeological point of view focused on characterising particular sites and samples as well.

    In terms of elites, if an model where elites pioneer the practice, I wonder if there’s also the Greg Clark option where it’s not just emulation, but literal demographic replacement of poorer by richer that could also spread.

    Re; site structure, there are already a few hints of site structure in the prevalence of close kin offspring in the samples identified in their analysis, at least among the agricultural populations (where its less likely to be simply due to population constraints, though may be). The two England_C_EBA samples identified are both from same site, I2597 and I2457, approximately 2100-2000 BCE, Amesbury Down, representing 2 of 30 graves, at a site with “A number of the early Beaker burials (with) rich and important grave assemblages. The site has some of the best evidence from Britain for early Beaker non-local connections indicated by isotopic (strontium/oxygen) analysis and material culture providing direct links with mainland Europe –in particular the ‘Amesbury Archer’.”. Also, a cluster of identified samples around the Polish Globular Amphora sites at Koszyce and Ksiaznice. The Koszyce site was the site of an unusual apparent massacre mass burial, and showed some evidence of a polygynous marriage and family, I think actually the *only* evidence of a such we have ever seen in the European adna record! Finally, the three Roman Republic era samples (noted in their original paper to have long RoH) were R1015, R473, R474, came from the following sites: R473 and R474 from “La Mattonara is an Iron Age Etruscan necropolis near the coastal town of Civitavecchia, on the Tyrrhenian Sea” and R1015 from Veio Grotta Gramiccia a large Etruscan city, located about 18 kilometers north of Rome. Etruscii?

    I wonder a bit about co-occurance of polygyny and cousin marriage. It’s obviously not universal, but a few males monopolizing the local marriage pool a bit more is obviously going to constrain or allow there to be more cousins about (if one man marries 4 women, each of those have 2 kids, obviously they’ll be a lot of cousins about, any they’ll be less related than is the norm under monogamy). Then if that collides with classical patriarchy in land ownership, and dowries, and a norm of universal marriage (and sham for unmarried women), then cousin marriage could become an attractive means to avoid the old polygynists’s lands and resources being “drained” off by marriage outside the family? (I get the impression a lot of our ideas about why cousin marriage exists are preconditioned by the Afghan War and the Pashtuns – there’s sometimes seems like a bit of an assumption that cousin marriage exists in order to “strengthen” family based clans among “fierce and clannish” Greater Middle Eastern peoples (making the implaceable warrior foes that so sadly frustrated all those neocons’ dreams back in the 2000s!). But what if it’s most commonly just an economic consequence of these sorts of factors?)

  2. I have a speculative theory that in societies that tightly constrain women’s roles and power, cousin marriage is a jiu-jitsu move encouraged by women without men really understanding the reason. If a woman is independently related to her husband’s family, then she or her family can use their family connections with the husband’s family to influence the husband.

    Maybe there’s a well-worked out version of this idea, I don’t know.

  3. Hindu societies don’t practice cousin marriage because the definition of incest is even broader for them. Two persons belonging to the same gotra can’t marry despite having a very distant common ancestor. Some groups wanted to enshrine this broad concept of incest in Indian law after independence but the state considered it too harsh.

    In the UK, 70% of all children born with congenital defects have Pakistani parents. That’s the result of Islamic cousin marriage

  4. Hindu societies don’t practice cousin marriage because the definition of incest is even broader for them.

    cousin marriage is common among south indian hindus moron

  5. Few visualisations of some of their data here, restricting to samples within West Eurasia (simply selected samples >25 Lat, -30 to 60 Long) and between 12000 YBP to present :

    Few quick models of time against probability of close kin offspring. Some of the polynomial fits suggest at 12000 YBP probability of close-kin about 8-10%, then went down with until 6000 BCE to about 3%, then declined again between after some point in 1st millennium BCE (at a point 1500 BCE / 1000 BCE onwards).

    Probably overfitting and biased by sampling in within West Eurasia though.

  6. @Razib Khan

    He is right, he pointed out those with same gotra do not marry – This system is preferred in South India.

    That’s why cross-cousin marriages are allowed in south India (Cross-cousins do not share same gotra) but parallel cousins share same gotra, which is considered incest.

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