Catholic Church's Cousin Marriage Ban Linked to 91% Reduction in Kin-Based Marriages, Fostering Market Growth

A compelling theory suggests that the Roman Catholic Church's historical prohibition on cousin marriage played a pivotal, though often underestimated, role in expanding social networks and stimulating the development of robust markets. Rachel Ferguson articulated this perspective, stating, "It's a little-known fact that the banning of cousin marriage by the Catholic Church is cited as a major cause of the network expansion required for fruitful markets - people had to go outside their 'tribe' to find spouses and found lots of other opportunities for cooperation too!"

Starting around 500 CE, the Catholic Church progressively implemented what scholars term a "Marriage and Family Program," systematically broadening the definition of prohibited degrees of kinship for marriage. This initiative, which eventually extended bans to distant cousins and even spiritual kin, aimed to dismantle the prevalent intensive kin-based structures across Europe. Research indicates that each additional 500 years under the Western Church's influence was associated with a 91 percent further reduction in cousin marriage rates.

Harvard Professor Joseph Henrich, a leading proponent of this theory, argues that by forcing individuals to seek spouses outside their immediate extended families, the Church inadvertently fostered a more individualistic psychology. This shift compelled people to form broader social ties, extending trust and cooperation beyond their immediate kin groups, a characteristic he describes as "WEIRD" (Western, Educated, Industrialized, Rich, and Democratic).

The dissolution of tightly-knit clans and the rise of nuclear families created a societal environment conducive to impersonal markets. Without the traditional safety nets and alliances provided by extensive kinship networks, individuals increasingly relied on broader community structures and formal institutions. This encouraged generalized trust and fairness with strangers, essential for complex economic interactions, trade, and the growth of capitalism.

Henrich's work, detailed in "The WEIRDest People in the World," posits that this cultural evolution laid the groundwork for modern Western societies. The long-term impact is observed in contemporary psychological traits and economic behaviors, such as higher rates of voluntary blood donation and greater trust in formal financial institutions, correlating with historical exposure to the Church's family policies. The Church's directives, therefore, are seen as a foundational, if unintended, catalyst for the economic and social characteristics defining much of the modern world.