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!)