Around 597 A.D., Pope Gregory I dispatched an expedition to England to convert the Anglo-Saxon king of Kent and his subjects. The leader of the mission, a monk named Augustine, had orders to shoehorn the new Christians into Church-sanctioned marriages. That meant quashing pagan practices such as polygamy, arranged marriages (Christian matrimony was notionally consensual, hence the formula “I do”), and above all, marriages between relatives, which the Church was redefining as incest. Augustine wasn’t sure who counted as a relative, so he wrote to Rome for clarification. A second cousin? A third cousin? Could a man marry his widowed stepmother?

He could not. Pope Gregory wrote back to rule out stepmothers and other close kin not related by blood—another example was brothers’ widows. He was lax about second and third cousins; only the children of aunts and uncles were off-limits. By the 11th century, however, you couldn’t get engaged until you’d counted back seven generations, lest you marry a sixth cousin. The taboo against consanguineous family had expanded to include “spiritual kin,” who were, mostly, godparents. (It went without saying that you had to marry a Christian.) Pope Gregory and Augustine’s letters document a moment in a prolonged process—begun in the fourth century—in which the Church clamped down, and intermittently loosened up, on who could marry whom. Not until 1983 did Pope John Paul II allow second cousins to wed.

You might assume that this curious story of how the Church narrowed the criteria for marriageability would be relegated to a footnote—a very interesting footnote, to be sure—but Joseph Henrich puts the tale at the center of his ambitious theory-of-everything book, The WEIRDest People in the World: How the West Became Psychologically Peculiar and Particularly Prosperous. Consider this the latest addition to the Big History category, popularized by best sellers such as Jared Diamond’s Guns, Germs, and Steel: The Fates of Human Societies and Yuval Noah Harari’s Sapiens: A Brief History of Humankind. The outstanding feature of the genre is that it wrangles all of human existence into a volume or two, starting with the first hominids to rise up on their hind legs and concluding with us, cyborg-ish occupants of a networked globe. Big History asks Big Questions and offers quasi-monocausal answers. Why and how did humans conquer the world? Harari asks. Cooperation. What explains differences and inequalities among civilizations? Diamond asks. Environment, which is to say, geography, climate, flora and fauna. Henrich also wants to explain variation among societies, in particular to account for the Western, prosperous kind.

WEIRD is an acronym for “Western, educated, industrialized, rich, democratic.” Read the whole thing, or the book itself.