Mapping the end of incest and the dawn of individualism. (Do not read the comments. Useless.) Glenn Reynolds commented "Hmm," an ambiguous response, but one that at minimum suggests he doesn't think much about this issue. It is well-known to those who dare to click over to those dangerous HBD sites. It's not his thing. The article very cautiously and wisely merely hints at reasons and results. I have mentioned the Hajnal Line here several times before, and contemplating these issues can be very informative about the last 1500 years of European history. It provides a surprising framework with some explanatory power.
Let me fill in some background which is not nailed down and could be modified when academics dare to study such things again, but for the moment might give you an "aha!" experience. The ban on cousin and other relatedness marriages by the Roman Catholic Church was not fully obeyed anywhere. The ban amounted to relative degrees of discouragement of such practice. Northern Europe embraced this more than any other region the Western, later RC, Church penetrated. I believe there is evidence that this was acceptable to those tribes because they already discouraged cousin, and certainly half- or step-sibling marriage prior to conversion. Women had higher status than elsewhere.
There is speculation that the Church pushed this solely to undermine the power-centers of intermarrying families preserving their lands and influence. It is also possible that monks, the carriers of observed and importantly written wisdom about stockbreeding, had noticed an increase in genetic problems from close interbreeding. The study authors make an additional suggestion. All quite fascinating and worth finding out. Yet the key fact is that it happened, and the loosened family ties created societies which were gradually more willing to think of themselves as parts of larger groups, not just their own tight cousinages. Ironically, this led to more voluntarily allegiances within tribes, and a slow increase in people viewing themselves as individuals. This expands in both directions, until you get Americans, a people who very much regard themselves as individuals, but also deeply identified as members of a nation of a third of a billion people. (India does not have that, and China has that in only an attenuated form.)
A thousand years later you get nations, and in that mix women, of all people, increasingly have rights to own property, inherit titles, enter guilds and professions, sue for divorce or take men to court. Next thing you know, they'll want to vote. Ridiculous, but it follows from the loosening of purely familial ties, so what are you going to do?
It didn't happen in other places.