the idea is that greater inbreeding ought to lead to greater “clannishness” — i.e. a greater prevalence of kindreds in the case of the germanics, and kindreds for longer the longer the inbreeding happened — while outbreeding ought to lead to less “clannishness.”
this map maybe kinda/sorta shows that (i think).
if you look at, for instance, the region from france through belgium and up through the netherlands towards dithmarschen (the black square on the map and “ground zero” for clannishness amongst the medieval germanic populations), the pattern does seem to hold: where there are lower ibd rates (i.e. suggesting lower inbreeding), as in northern france, the kindreds disappeared earlier (1300s) than where there are higher ibd rates, namely friesland (1400s). and the ibd rates increase the closer you get towards dithmarschen.
germany, too, has low ibd rates (relatively small green circle centered on berlin there — smaller than friesland’s circle), and, according to phillpotts, kindreds were pretty much gone in central/southern germany by the 1200s.
and norway has lower ibd rates than sweden, and the kindreds disappeared there sooner (1200s) than in sweden (1300s).
i would’ve predicted lower ibd rates for england, especially now given what phillpotts said about the kindreds in anglo-saxon england being gone by the 600-700s — although perhaps that had to do with their migration over water like she suggested and wasn’t related to whether or not they were inbreeding or outbreeding at the time. on the other hand, lorraine lancaster argued that kindreds were actually still around in england into the 1000s, so perhaps that explains the ibd rates a bit better — or vice versa, rather (but see below).
btw, i looked a little further into the sources of the genetic data that ralph and coop used for their ibd study [pdf]. the data came from popres (The Population Reference Sample), and afaict (correct me if i’m wrong) the european data in the popres collection came from two sources: the london-based Lolipop Study and the swiss-based CoLaus study.
the lolipop study surveyed both indian asian and white european individuals living in london [pdf] — i’m sure ralph and coop used only the white european individuals for their study of europe, of course. only europeans having four grandparents born in the u.k. were included, so i guess that must make them all british (english, welsh, scottish, northern irish) — but they could also be irish. the inclusion of welsh, scottish, and/or irish individuals could’ve skewed the ibd results. ralph and coop seem to have isolated some number of scottish and irish individuals (see their map above), but it’s not clear to me if those individuals were from the lolipop study or the colaus one.
the colaus study looked at caucasians living in lausanne, switzerland, who were either swiss or from another european(?) country. both the subjects’ parents and grandparents had to have been born in whatever country they were described as coming from. the researchers tried to further narrow down their ethnicity during a clinical visit. i presume it was from this study that ralph and coop drew the rest of their samples, including the data from: germany, france, belgium, the netherlands, norway, and sweden — possibly england, too. it’s difficult to know because they don’t spell it all out specifically. these data could be skewed, too, for my purposes — for example, hypothetically speaking, due to the presence of a lot of non-french, but still european, individuals in the set of samples of france. again, difficult to know.
finally, here are the numbers of individuals from each country sampled by ralph and coop. some of them are kinda low — like n=2 for norway:
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