We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
It should have included some reference to late Roman and later Catholic bans on cousin marriage, including second cousin. It was stated as "degrees of consanguinity," but it amounted to cousin ban. This had a huge affect on the population, and was an early deterrent to tribal social order.
I have been trying to pass this information along for years and am grateful to see it get wider play here. Specifically, the Hajnal Line is what you want to be looking at WRT Hajnal's research. A lot of western European history comes into sharp focus when you look at it. This has been well-known in the HBD community for years. It has likely even influenced genetics, increasing the preponderance of genes for trusting those beyond the immediate clan.
Brad's addition above is also important, and pushes the cultural changes back even farther. It's more like a thousand years than five hundred. The Catholic Church forbade cousin marriage everywhere, but somehow it was mostly only Northern Europe that obeyed it. Feudalism also reduced the power of clan ties, and was also a Northern European phenomenon.
Steven Pinker's The Better Angels of Our Nature does n excellent job of laying out all the information, but then he reverts to his priors in the end, saying that the greater cooperation was because of The Enlightenment. (Spoiler. It wasn't.)
Assistant Village Idiot
Everyone can search 'HBD Chick' for a lot more info about the Hajnal line.