Jayman writes in a comment to this post,
Hajnal line. It’s all about late vs early marriage. Northwestern Europeans married late (average age for women 23) for centuries. Eastern and Southern Europeans married much earlier (often in early teens for Russians, for example). This is a pattern that continues to this day, albeit at later ages across the board (the rank order remains fairly unchanged).
Right, and the Hajnal Line is now colloquially associated with differences in a bunch of other inter-european characteristics, such as clannishness, familism, nationalism, passivity, and fondness for watching from the corner stool. (Paradoxically, fertility rate is unusually low in Outer Hajnalia.) Anyhow, I wonder when the trend of Inner Hajnalia late marriages will bang up against the biological reality of the Wall. That is, given a choice, men aren’t really happy about marrying women with only a few years left of supple fuckability in them. Preference for younger virginal women is universal among men. And there are many biological reasons men prefer virgins over road worn sluts who’ve spent the better part of their salad decade warming up for their ludicrously ostentatious wedding day.
At some point the raw Darwinian calculus will re-emerge and there will be a pendulum swing back toward younger marriages with hotter, tighter babes who have oodles of residual reproductive value left.