“First, we hypothesize that, in adapting to the social worlds created by intensive kin-based institutions, human psychology shifts in ways that foster greater conformity, obedience and sensitivity to relational contexts but less individualism, analytic thinking and cooperation with strangers. Second, to account for part of the variation in kinship intensity, we hypothesize that Western Christianity, beginning around 500 CE, gradually implemented a set of policies about marriage and the family—the Marriage and Family Program (MFP),—that was a critical contributor to the eventual dissolution of the intensive kin-based institutions of Europe.

By 1500 CE, this left many regions of Western Europe dominated by independent, monogamous, nuclear families—a peculiar configuration called the European Marriage Pattern. This two-part theory implies that the Church, through the MFP, inadvertently contributed to what psychologists have termed WEIRD psychology.”